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A PERIODICAL OF PARTICLE PHYSICS

SUMMER1990 VOLUME 20, NUMBER 2

EditorMICHAEL RIORDAN

Associate EditorsRENE DONALDSON, BILL KIRK

Editorial Advisory BoardJAMES BJORKEN, JOHN MATTHEWS, MARTIN PERL

JOHN REES, RONALD RUTH

MARVIN WEINSTEIN

Photographic ServicesTOM NAKASHIMA

BETTE REED

IllustrationsTERRY ANDERSON, KEVIN JOHNSTON

SYLVIA MACBRIDE, JIM WAHL

DistributionCRYSTAL TILGHMAN

CONTENTS

FEATURES

1THE FUTURE OF ELECTRON PHYSICS

There are many promising directions that

high-energy physicists can pursue at

electron machines in the 1990s.

Burton Richter

10 THE NEXT LINEAR COLLIDER

Concrete designs for a next-generation linear

collider are close to becoming a reality.

Ronald Ruth

18 HIGH ALTITUDE PHYSICS

The Snowmass Summer Study charts

a course for particle physics in the 1990s.

Michael Riordan

DEPARTMENTS

The Beam Line is published quarterly by theStanford Linear Accelerator Center,P.O. Box 4349, Stanford, CA 94309.Telephone: (415) 926-2282.BITNET: BEAMLINE@SLACVMSLAC is operated by Stanford University undercontract with the U.S. Department of Energy.The opinions of the authors do not necessarilyreflect the policy of the Stanford LinearAccelerator Center.

COVER: Operators manning the controlpanels ofthe Stanford Linear Collider try to optmize itsperformance. (Photo by Tom Nakashima)

17 HISTORY NOTES/DATES TO REMEMBER

21 TOWARD THE NEXT LINEAR COLLIDER:

THE FINAL FOCUS TEST BEAM

David Burke

24 PEOPLE AND EVENTS

27 SLAC PUBLICATIONS

31 FROM THE EDITOR'S DESK

32 CONTRIBUTORS

.

T] IES

T'Iheje are any promising directionsthat ':hig :nerh ey physicists can pursue

at elec&'bn ~~chines in the 1990s.

ARTICLE PHYSICS

advances in fits and starts. Some-times a brilliant theoretical conceptintegrates data and ideas; sometimesexperiments find phenomena thatchange the direction of our thinking.I believe we are now waiting for ex-periment to lead the way to a deeperunderstanding of the world of ele-mentary particles.

Theoretical and experimentalbreakthroughs of the 1970s and 1980sled to the development of the Stan-dard Model, which seems capable ofaccounting for all the phenomenaaccessible to experiments with

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existing accelerators. Yet the Stan-dard Model is felt to be incompletefor two reasons-one esthetic and theother technical. The esthetic reasonis that it requires approximately 20apparently arbitrary constants(particle masses, coupling constants,mixing angles), and physicists believedeeply in reductionism. A systemwith a small number of apparentlyarbitrary parameters is regarded asbetter than one with a large number,and the Standard Model is thought tohave too many. The technical reasonis that this model, when used to cal-culate what might happen at ener-gies much higher than those nowaccessible, gives answers that are in-consistent with treasured conser-vation laws.

Many extensions and variants ofthe Standard Model have been devel-oped to try to resolve its problems;some of these go by such names assupersymmetry, technicolor, com-positeness, etc. Everyone waits forexperiments that will point outwhich of these, if any, is "the rightway." This is what accounts for the

of First Physics

desires of particle physicists for newaccelerators of higher energy, higherintensity, or novel collisions. Untilwe have those machines, however,we look for clues where we can-inthe pools of light under the lampposts represented by existing accel-erators at SLAC, Fermilab, Brook-haven, Cornell, DESY, CERN,Novosibirsk, Beijing, and KEK. If weknew exactly where to find what weare looking for, we could say whereto put the next lamp post, but wedon't know and therefore have toproceed on several different frontiers.

In the past, the light cast byexperiments at both electronmachines and proton machines hasbeen essential for developing ourunderstanding of the elementaryparticle world. In proton accelerators,it is clear where we and possibly theEuropeans are heading: in the UnitedStates toward the SSC, a 20x20 TeVproton collider, and in Europeprobably toward the LHC, an 8x8 TeVproton collider. These machines areunprecedented in their size and cost.The SSC will be some 54 miles in

The relationship between availablecollision energy in the constituentcenter-of-mass frame and the year ofinitial physics for hadron and e+e-colliders.

circumference and cost an estimated$8 billion. It represents a huge leapin accessible energies, and particlephysicists confidently believe thatexperiments with this machine willturn up the phenomena required toexplain the inconsistencies in thehigh energy behavior of the StandardModel. Because of the compositenature of the proton, the accessibleenergy in the SSC is something onthe order of a few TeV, which isthought to be enough. However, pro-ton machines do not and cannot doeverything.

The electron accelerators of theworld are, in a sense, complemen-tary to the proton accelerators intheir potential, as demonstrated bythe discovery at electron machinesof such things as the substructure ofthe proton, the psi family, charmparticles, jets, the tau lepton, B-Bmixing, the limit on the number ofquark-lepton families, etc. At presentthe big money is going into the nextgeneration of proton machines,which (with present technology) willbe able to cast a leaky net the farthestinto the sea of the unknown. Fishingcloser to shore with a net of tightermesh, the next generation of elec-tron machines now being discussedwill catch things that can get throughthe proton net. Eventually they willalso be able to throw their nets as faras the proton machines, for the SSCitself is roughly equivalent to only a3-4 TeV electron-positron collider.In the rest of this article, I describewhat is happening to bring a newgeneration of electron machines intobeing for high-energy physics stud-ies on three important frontiers: newperspectives, high intensities, andhigh energies.

2 SUMMER 1990

Aerial view of the DESY laboratory inHamburg, Germany, indicating thelocations of the HERA and PETRA tunnels.

T HE LATE 1960S MARKED thefirst attempt at inclusive in-

elastic electron-proton scatteringusing the then-new SLAC 20 GeVlinac. Almost immediately these ex-periments gave unambiguous evi-dence of substructure in the proton,and many other important resultsfollowed using neutrinos and muonsas probes in the center-of-mass en-ergy range from the 5 GeV of SLAC tothe 30-40 GeV of the high-energylepton beams. These important re-sults led physicists to consider howto produce much greater center-of-mass energies. The first serious de-sign of an electron-proton collidercame with the 1970 work by LBL andSLAC on the design of PEP (originallymeaning positron-electron-proton).But a proton ring was never built inPEP, for it was felt that the achiev-able center-of-mass energy of around100 GeV was not far enough abovewhat could be obtained by othermeans. (This shows how wrong onecan be by thinking conventionally-the energy was far enough above theW-mass to have produced these par-ticles well before they were actuallydiscovered.)

The electron-proton collider ideasat idle for nearly 15 years, until theDESY laboratory in Germany beganconstruction of HERA, with an elec-tron-proton center-of-mass energy of350 GeV. With its high-energy lepton-quark collisions, this facility will, Ibelieve, give us a different perspectiveon physics from that available in thelepton-lepton collisions of e+e-colliders and the quark-quark colli-sions of proton colliders.

HERA consists of a 5 km circulartunnel containing an 800 GeV su-perconducting proton ring and a 35

GeV conventional electron (orpositron) ring. In principle, the elec-trons can be polarized, allowing verysensitive tests of the weak interac-tions. Injection into this machine isfrom a chain of accelerators culmi-nating in the old PETRA storage ring,which has been modified to allowacceleration of protons to 40 GeV orelectrons to 15 GeV. The project ison schedule; the electron ring iscomplete and has been tested withcirculating beams, while the protonring installation is proceeding rap-idly with one octant already cooledto liquid-helium temperatures fortesting. The entire ring is scheduledfor cooldown in early 1991, and com-missioning should take place thatyear.

A follow-on project is already inthe thinking stages at CERN, but itsrealization depends on a decision toproceed with the LHC, whose protonrings will be in the same tunnel asthe LEP collider ring. Plans to makeelectron-proton or positron-protoncollisions available are under dis-cussion as part of the LHC complex.Collisions between the 100 GeVelectron beam in the LEP ring and oneof the 8 TeV proton beams of the LHCshould produce center-of-mass ener-gies of up to about 1.8 TeV in theelectron-proton system. Hard col-lisions of electrons with proton

constituents at center-of-massenergies up to nearly 1 TeV shouldbe possible. The schedule for thisproject depends, of course, on theschedule for the LHC, and theinterest in actually doing it willdepend on the results from HERA.

IGH-INTENSITYMACHINES giveus the opportunity to study rare

processes and look for new phenom-ena that occur only with very smallprobability. At present there is greatinterest in the development of veryhigh luminosity electron-positroncolliding beam machines (factories)for the study of the B-meson system,the tau/charm system, and the phisystem.

Projects now under design areaimed at providing sufficient lumi-nosity to study CP violation in the B-meson system. CP violation is one ofthe great mysteries of the StandardModel; it is allowed but not required,and the CP-violating phase in thequark mixing matrix is one of thearbitrary constants of the StandardModel. It has been observed up tonow only in the K-meson system, andits observation and study in the B-meson system may provide someclues about its origin as well as de-termine this important parameter tomuch higher precision.

BEAM LINE 3

Most of the design studies arefocusing on asymmetric systems,where a low energy beam of 3-4 GeVcollides with a high-energy beam of7-9 GeV with the center-of-massenergy adjusted to operate at theupsilon(4S) resonance. This asym-metric technique (first suggested byPier Oddone of LBL), which makes thecenter of mass move in the labora-tory, allows the study of the timeevolution of the B-B system, thusmaking it easier to determine the CPviolating parameters. The minimumluminosity required to study this ef-fect is estimated at 3 x 1033 cm-2s-1 , afactor of 30 above the highest lumi-nosity ever achieved in the morefamiliar symmetric electron-positroncolliders. Beam intensities in thesemachines are measured in amperes,requiring very large amounts of rfpower, generating unprecedentedamounts of synchrotron radiation,and demanding innovative solutionsfor the damping of multi-bunch in-stabilities.

Asymmetric machines requireseparate storage rings for the low-energy and high-energy beams, ofcourse, and one of the most difficultproblems in the design of these ma-chines is the job of bringing the beamstogether at the collision point with-out generating intolerable back-grounds in the detector.

A year ago it was not clear thatmachines with the required charac-teristics could be built. This last yearhas been a period of intensive workby the SLAC/LBL/LLNL collaboration,Cornell, KEK, CERN, and Novosi-birsk; and physicists are now con-fident that suitable machines can bebuilt at a cost of around $200 million.The most advanced design studies

are probably those in the UnitedStates, primarily because of the closeand effective collaboration betweenaccelerator and detector designgroups in solving the very difficultproblems at the interaction region.These designs are all somewhatdifferent because of both site con-straints and taste. Physically, thelargest is the SLAC/LBL/LLNL design,which puts the machine in the 2.2 kmPEP tunnel and reuses many of itscomponents, while the smallest isthe Novosibirsk design at 650 m.Personally, I believe that a larger cir-cumference makes it easier to achievethe luminosity goals. Cornell, KEK,and SLAC/LBL/LLNL all aim at hav-ing detailed conceptual design reportsavailable early in 1991. It will beinteresting to see who, if anyone,gets the funding.

There are many interesting ques-tions still open in tau and charmphysics. Addressing them requires asymmetric e+e- collider having aluminosity in the 1032-1033 cm-2s- 1

range-roughly a factor of a hundredabove the luminosity of the Beijing

collider BEPC, which is the highestluminosity machine now operatingin this energy range. The newmachine must also be a two-ringfacility; the first design studies wereconducted by John Jowett and JasperKirkby at CERN. There has beenmuch discussion at various labora-tories on this kind of project and, atpresent, the most likely site is inSpain, where the government is seri-ously considering a tau/charmfactory as a major science facility. Adecision is expected in the next sixmonths or so; if it goes ahead, Spainwill almost certainly receive tech-nical assistance from both CERN andOrsay. There is considerable inter-national interest in participating inthe detector construction and physicsprogram of such a facility. While thismachine is a considerable extrapo-lation from existing e+e- colliders, itis not as difficult a project as a B-factory. There has been some talk inthe Soviet Union about tau/charmfactories, but I do not know the statusof these projects.

Phi factories would be small, high-luminosity machines operating atthe phi resonance (about 1 GeV inthe electron-positron system). Theyaim to study CP violation in the K-meson system by examining corre-lations in the decay p -- KO-K°, andin particular at measuring the CP-violating parameter e'/e to an accu-racy better than has been obtained inexperiments on proton machines atCERN and Fermilab. Luminosity re-quirements are at least 1032 cm-2s-1.Three projects have been understudy-at UCLA, Frascati, and Novo-sibirsk. As of this writing, onlyFrascati seems to have made thedecision to proceed.

4 SUMMER 1990

T HE PRIDE AND JOY of accelera-tor builders, experimenters and

theorists are the big machines thatopen up new energy regions for study.Here often relatively crude experi-ments can have a high discoverypotential, and theories can be testedin a regime different from the onethat generated the facts on whichthey were built. The two highestenergy electron-positron colliders arethe LEP storage ring at CERN and theSLC linear collider at SLAC. The LEPstorage ring, located underground ina 27 km tunnel on the Swiss-Frenchborder near Geneva, presently has amaximum center-of-mass energy of100 GeV and a luminosity of a fewx10 30 cm-2s-1; it should be capableof a luminosity about five times great-er as more experience is gained inoperating this large facility.

An improvement program is al-ready underway to increase themaximum energy of the LEP storagering to about 200 GeV in the centerof mass. This improvement involvesthe construction and installation ofa large number of superconducting rfcavities; it is scheduled to be com-pleted by the end of 1993, at whichtime LEP should have sufficient en-ergy to operate above the thresholdfor W -boson pairs. Superconductingrf technology is required because theenergy loss per turn of electrons instorage rings is proportional to thefourth power of the energy, and the16-fold increase in rf accelerationper turn required in LEP cannot beaccomplished with conventionalroom-temperature cavities withoutprohibitive costs.

The energy loss from synchro-tron radiation makes it exorbitantlyexpensive to build storage rings of

energies much higher than that ofLEP. A properly optimized electron-positron storage ring (LEP is so opti-mized) has a cost and circumferenceproportional to the square of thecenter-of-mass energy. Thus, an e+e -

storage ring with ten times LEP'senergy would have a hundred timesits circumference and cost more thanan order of magnitude above the $8billion SSC.

The linear collider concept wasinvented as a way to overcome theunfavorable scaling laws of storagerings; the first of these machines isthe SLC at SLAC. A true linear colliderwould consist of two linacs firingbeams of particles at each other.Because of the straight-line nature oflinear accelerators, there is no syn-chrotron radiation loss in the accel-eration process, and the cost of thesemachines is therefore proportionalto the first power of the energy. Athigh energies they will be much lessexpensive than a comparable storagering.

The SLC is a hybrid facility thatuses a single linac to accelerate bothelectrons and positrons, with sepa-rate beam-transport lines at the end

DELPHI, one of the four all-purpose particledetectors at LEP.

BEAM LINE 5

to swing electrons and positrons awayfrom each other and then bring theminto head-on collisions. The SLC hasa center-of-mass energy of up to 100GeV and, while its luminosity isconsiderably less than that of LEP, ithas produced experimental physicsresults. Perhaps more importantly,it has also taught the acceleratorcommunity a great deal about beamdynamics and other aspects of linearcolliders.

Groups all over the world areworking effectively together on abroad-based R&D program to developthe technology for very high energy

linear colliders. This collaborationinvolves not only the usual exchangeof people and information but alsojoint construction of test projects tovalidate new concepts and test outnew technology. The most advancedof these joint projects is the FinalFocus Test Beam (see article by DavidBurke on page 21 ) being built at SLACby a collaboration of Japanese, Rus-sian, European and American physi-cists. This project is aimed at testinga new concept for final focus systemsand has as its goal the production ofa beam spot of roughly 50 nm high by1 gLm wide (to be compared with the

Cutaway drawing of the SLC, illustratingits principal components.

6 SUMMER 1990

roughly 3-micron beam spot of theSLC).

A high-energy linear collider con-sists of three basic systems. The firstis a low-emittance source of elec-trons and positrons. Low emittanceis important because the smaller thebeam is at birth, the easier it will beto focus down to the truly tiny sizesthat are required at the collisionpoint. The invariant emittances re-quired are about 10- 6meter-rad in thehorizontal plane and 10- 8 meter-radin the vertical. These are about afactor of 20 below the emittancesachieved in the SLC sources, but thedesign of the strongly damped, few-GeV storage rings required to achievethem is thought to be in hand.

The second system is the high-energy linac that boosts the beamfrom a few GeV at the source to themany hundreds of GeV required atthe interaction point. There is generalagreement that the linear acceleratorshould be a higher frequency versionof the SLAC machine. (See article byRonald Ruth beginning on page 10.)Higher frequency is required toincrease the energy efficiency of thelinac and to achieve economicaloperations. The frequency willprobably be between 10 and 20 GHz,and the machine will probably bepowered by klystrons of 100-200 MWpeak power, supplemented by rf pulsecompression techniques to achieveeven higher peak power. Theaccelerating gradient will probablybe between 50 and 100 MV/m,considerably above the 20 MV/mroutinely used at SLAC. The linacswill most likely accelerate a train ofelectron and positron bunches andwill have to have some kind of highermode damping so that the early

bunches in the train do not disturbthe later ones.

The third system is the final focus,a complex, chromatically correctedtransport line that must producespots of a few by a few hundrednanometers at the collision point. Inaddition, the final focus system andthe experimental detector must bedesigned in an integrated fashion sothat backgrounds are sufficiently lowat the collision point to allow thedetector to function properly. Expe-rience with the first-generation finalfocus system of the SLC has led to thedevelopment of improved conceptsthat will be tested in the Final FocusTest Beam. Experimental physicistsare also working closely with theaccelerator designers on appropriatebeam-crossing geometries and de-tector shielding.

The aim of all of this effort is todesign a collider generating electron-positron collisions at 0.5 to 1.5 TeVin the center of mass with a lumi-nosity of 1033 to 1034 cm-2s- 1. Thethree most advanced studies are thoseat SLAC, KEK and Novosibirsk. TheSLAC and KEK approaches are verysimilar (no accident, since the ac-celerator physicists have been work-ing closely together for the past few

Artist's conception of the Japan LinearCollider (or JLC), a next-generationlinear collider being designed byphysicists at KEK.

BEAM LINE 7

years). This concept is for a linearcollider with a 500 GeV initial energyand an expansion capability thatwould allow improvements to bringthe energy up to 1.0 or 1.5 TeV. Therf frequency for the main acceleratorwould be 11.4 GHz, powered by kly-strons of 100-150 MW with binarypulse compression if required for highaccelerating gradients. The repetitionrate for the machine would be a fewhundred Hz, and the electron andpositron pulse trains would have 10to 20 bunches each. At Novosibirskthe basic idea is the same, but the rffrequency is somewhat higher whilethe number of bunches acceleratedper second is considerably lower. Thisleads to much higher intensities perbunch, with a concomitant tighten-ing of alignment tolerances but alower total power consumption.

At Cornell the superconductinglinear collider is under study. Such amachine would operate with a grossduty cycle of around 10% (for ex-ample, one second on and nine sec-onds off) to keep the power losses tothe cryogenic system from the cavi-ties within bounds. Within themacroscopic duty cycle, it is essen-tially a continuous-wave machine.The main problems with this ap-proach at present are the low acceler-ating gradient attainable in super-conducting cavities and the very highcost per unit length of superconduct-ing rf systems. If these problems canbe solved, this is a very promisingconcept.

In Europe two approaches are be-ing examined. At DESY a group hasproposed what I would call the "let'sget on with it" concept. This woulduse a SLAC-style linac at 3 GHz, andan accelerating gradient of 20 MV/m,

which would accelerate hundreds ofelectron and positron bunches in eachpulse of the machine. The problemsare the low gradient, which implies ahigher capital cost initially than thehigh gradient attainable in higherfrequency machines, and a very largenumber of bunches, which requiresmuch more attention to be paid totheir mutual interaction. The goal isa collider of 400-500 GeV in the centerof mass, but the entire structurewould have to be replaced to increasethe energy. It has the advantage thatthe technology is all in hand and onecould proceed rapidly, but the SLAGand KEK groups feel that the costwould be two to four times that of a"more modern" linac.

At CERN a "two beam" conceptknown as CLIC is being investigated.This design uses a low-gradient, high-current linac as a power source togenerate very high frequency rf (above30 GHz) to power a more conven-tional high-gradient accelerator par-allel to the power-generating linac.This two-beam concept shows greatpromise, but a considerable amountof R&D will be needed to assess itspracticality.

I believe that the next one to twoyears should see a convergence on asingle design for the Next LinearCollider (NLC). The scale of proto-type work needed is not yet clear.When and where the NLC will be builtis anybody's guess at present.Although the physics potential isvast, the machine will be in thebillion-dollar class, and money isshort all over the world. A buildabledesign will certainly be in hand inthe next few years, but we will haveto be creative to convince govern-ments to fund it.

T HE AGENDA FOR ELECTRONphysics in the 1990s is clear.

What is not so clear is how to do itand fund it all.

The electron-proton collider pro-gram is secure, for HERA is soon toturn on. The most important factordetermining the long-range future ofthis program is how interesting itturns out to be.

The "Factory" program is lesssecure. Only the Phi Factory in Italynow has funding. There are manystudies of B and Tau-Charm Facto-ries, but no authorization to proceedfor any of them. If we consider themfor what they really are, experiments(the total cost of a B Factory withdetector is about equal to one mid-sized SSC or LHC experiment), thenperhaps they will be easier to sell.

The program with the mostproblematic future is that of the highenergy electron-positron collider.TeV linear colliders will fall in thebillion-dollar class. The United Statesis now committed to the multi-billion dollar SSC, while Europeseems headed toward a commitmentto the LHC-a smaller project but stillin the several billion-dollar class.These two great proton machineswill leave little room in their respec-tive regions for a large electron-positron collider. Perhaps it is timeto go beyond talking about inter-regional action and finally do it!

Here are the opportunities thatare knocking. Is anyone listening?

0

8 SUMMER 1990

SLAC's Role

SLAC HAS BEEN A PIONEER inelectron machines and their

exploitation for physics. The two-mile linac was completed in1966, SPEAR in 1972, the PEP ringin 1980, and the SLC in 1988. It isnatural that we be heavilyinvolved in these advancedprograms. PEP makes an idealbase for a B Factory, and the SLCwill continue to be a mostimportant development tool forlinear colliders.

In collaboration with theLawrence Berkeley and LawrenceLivermore National Laboratories,SLAC will have a design studyand cost estimate for an asym-metric B Factory completed inearly 1991. There is a great dealof physics interest in the expments that can be done witha machine; if that were the o:criterion, construction couldaround the beginning of 1994be complete sometime in 19'While waiting for a decision

Artist's conception of what a sectthe SLAC B Factory might look likhigh-energy ring is built using exPEP magnets, while the low-enercring is made of completely newcomponents.

our federal authorities, work willcontinue with component R&D,and some of the concepts will betested on PEP.

Linear colliders were born atSLAC, and it is natural that thislaboratory play a major role in thedesign and the construction of theNLC, wherever it is built. As thefirst of the linear colliders, theSLC has a lot to teach us aboutthis new kind of facility. Thesuccess of this machine to datehas convinced the acceleratorcommunity that high-energylinear colliders are indeed practi-cal. The on-going work to increasethe luminosity of the SLC makesus and the rest of the groups

interested in this kind of machineconfront the problems that willhave to be faced in the next-generation collider as well as inthe present one. Our rf experts,accelerator designers and accel-erator theorists have played amajor role in developing theconcepts for the NLC, and weexpect to continue to do so in alarge-scale R&D program. SLAC'stradition of integrating theexperimental users with themachine designers is importantfor the development of usefulfuture facilities. We expect to bein the thick of things for sometime to come.

-Burton Richter

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I-- - -

The Next Linear Collider

RONALD RUT........

Concrete designs..... .....for a next-generation linear collider

are close to becoming a reality. ..........

................ .. ........

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IERE IS NOW BROA....................

agreement in the high energy phys-ics community that to continue ex-ploring the energy frontier in e+e-

interactions, we will have to aban-don circular colliders and adopt lin-ear colliders. This realization has ledto active research throughout theworld towards the next generation oflinear colliders. The past few yearshave seen great strides in our under-standing of both the acceleratorphysics and the technology of linearcolliders. We are now at the pointwhere we can discuss in fair detailthe design of such a "Next LinearCollider," or NLC.

where ....... .... dicu s .nfar. etithe. .......... a N e t i n aCollde , ... .....

10 SUMMER 1990

The two key design parameters ofthe NLC are its energy and luminos-ity. A broad consensus has emergedover the past couple of years that theenergy should be 0.5 TeV (total elec-tron plus positron energy), upgradableto at least 1.0 TeV. One reason forthis choice of energy range is thegreat potential of such a collider forsignificant high-energy physics re-search in the era of the SSC. Anotheris that this energy range is a naturalnext step; it is a factor of 5 to 10beyond that of the present StanfordLinear Collider (SLC). In order to ob-tain a sufficient event rate to per-form detailed measurements, the lu-minosity of the collider should in-crease with the square of its energy.For an NLC in the TeV energy range,a luminosity of 1033-1034 cm-2 S-1 isrequired.

A factor of 5-10 energy increasecan be obtained in two ways: byincreasing the collider length to 10-20 times that of the SLC (3 km), or byraising its accelerating field to 5-10times the SLC gradient (20 MV/m).The present consensus is that weshould first increase the acceleratingfield by about a factor of 5-up toabout 100 MV/m. To limit the rfpower required, this field should beprovided by structures similar tothose used in SLC but at a higher rffrequency of 10-30 GHz. At SLAC, thefrequency choice for the NLC is 11.4GHz, or 4 times the present SLC fre-quency. Of course the ultimate trade-off between length and acceleratingfield is governed by the overall costand the upgradability. A broad opti-mum occurs at the point where thelinear costs (accelerating structure,magnets, tunnel, etc.) equal the costof the rf power source.

The choice of luminosity rangealso greatly influences the design ofthe linear collider (see box on right).In principle, one could increase theluminosity simply by raising therepetition rate of the accelerator, butthe wall-plug power increases in di-rect proportion. In a reasonable de-sign, the wall-plug power should fallin the range 100-200 MW. Given thisconstraint, the best way to increasethe luminosity is to shrink the beamsize at the interaction point (IP). Inaddition, the beam cross section mustbe kept flat at the IP in order to mini-mize the amount of "beamstrahlung"radiation emitted as energetic elec-trons or positrons interact with theelectromagnetic field of the opposingbunch.

The luminosity can be further en-hanced by accelerating severalbunches on each machine cycle. Asingle bunch of particles can in prac-tice extract only a few percent of theenergy available in the acceleratingstructure. With additional buncheswe get both greater luminosity andhigher efficiency of energy transferto the beam. The number of particlesin each bunch, another factor thatdirectly affects the luminosity, islimited by the rf energy that can bestored in the accelerating structureand by the amount of beamstrahlungradiation that can be tolerated. Theobvious solution is to generate trainsof successive bunches, each withfairly moderate numbers of particles.

Given these goals and constraints,we can now sketch a rough design ofa linear collider able to achieve boththe desired energy and luminosity. Apossible layout is shown on the nextpage. There are two complete linearaccelerators, one for electrons and

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Schematic diagram of one possible NLCdesign. The various subsystems are notdrawn to scale, and the crossing angleof the two beams is exaggerated.

ee-

Wiggler

Bending andFocusing Magnets

-* 50 m ----

A possible NLC damping ring design.

the other for positrons. Each linac issupplied with particle beams by adamping ring followed by apreacceleration section consisting oftwo bunch compressors and a 16GeV linac. After passing through themain linacs and final focus system,the beams collide at a small crossingangle inside a large particle detectorlike the SLD.

To illustrate the basic features ofthe NLC operation, let's follow someelectron bunches through the col-lider. A sequence of 10 bunches or sois created at the source and acceler-ated up to about 1.8 GeV in a pre-accelerator. This "batch" of bunchesis then injected into a damping ringthat serves to reduce the transverseand longitudinal phase space occu-pied by the electrons in each bunch.At the proper moment, these bunchesare extracted from the ring and thencompressed along their direction ofmotion by a bunch compressor, afterwhich they are accelerated up toabout 16 GeV and compressed a sec-ond time just prior to injection intothe main, high-gradient linac. Theentire batch is carefully steered andfocused as the electrons are acceler-ated up to full energy in the linac.Precision magnets in the final focussystem squeeze the bunches downby about a factor of 300 just beforethey collide at the IP with similarbunches of positrons. Except for thefact that they were created differ-ently, from the shower of particlesthat occurs when a bunch of electronshits a metal target, these high-energypositron bunches have followed asimilar evolution. After the beamscollide, their debris is channeled outof the detector area and into shieldeddumps.

T O OBTAIN HIGH ENOUGH lumi-nosity for physics research, we

need very tiny beams at the IP.Typically, they must be a fewnanometers (10- 9 m) high and a fewhundred nanometers across. Such asmall size is achieved by deliveringsmall beams with low divergence tothe final focus system, where theyare demagnified much further.Making small, low-divergence beamsin the first place is the work of thedamping rings and bunch compres-sors.

The damping ring serves to re-duce the emittance of the bunches ofparticles in all three degrees of free-dom (see box on the next page). It isan electron storage ring similar in allessential features to the storage ringsused for colliding beams or synchro-tron light production. The particlesin an electron storage ring radiate asubstantial fraction of their energyon each turn-energy that is restoredby rf accelerating cavities. In the pro-cess of radiation, the particles loseenergy from all three degrees of free-dom, but it is restored only alongone, the direction of motion; the pro-per amount is supplied at a single rfphase for a particle with the designenergy, which leads to damping inall three dimensions. The fact thatradiation is emitted as discrete quan-ta, however, introduces stochasticnoise that causes diffusion of par-ticle trajectories.

The competition between thesedamping and diffusion effects leadsto an equilibrium value for the emit-tance of an electron storage ring.Damping rings are designed to en-hance the damping effects usingstrong magnetic fields (such as thosein wiggler magnets), while limiting

12 SUMMER 1990

the diffusion by the special design ofthe transverse focusing in the ring.In addition, there is a unique featureof electron storage rings that can beused to advantage. Due to the lack ofvertical bending, the vertical emit-tance of the beam is much smallerthan the horizontal-typically twoorders of magnitude smaller. Suchnaturally flat beams are a key featureof most NLC designs.

One possible design for a futuredamping ring (see schematic at bot-tom of page 12) is about a factor of 5larger and operates at an energy 50percent higher than that of the SLCdamping rings. The final emittanceof the beam is more than an order ofmagnitude smaller than that of theSLC beams, which leads to muchsmaller sizes. In fact, the verticalextent of a beam emerging from thisdamping ring would be a few microns,or about equal to the final spot size atthe SLC interaction point.

Another key difference is the si-multaneous damping of manybatches of bunches. In the SLC, atmost two bunches are damped si-multaneously, whereas this NLC ringwill damp 10 batches of 10 bunchesall at once. This feature allows alonger damping time for any givenbunch, because we can extract the"oldest" batch and inject a new"young" batch while leaving thosein their "adolescence" to continuedamping undisturbed.

Because the bunches forget theirorigins in the damping ring, theirconditions upon emerging are en-tirely determined by their behaviorin the damping ring. This places spe-cial emphasis on the stability of themagnets in the damping ring andextraction system.

BEAM LINE 13

Model of a damped accelerating struc-ture that might be used in the NLC. Theparticles would travel along the axis ofthe cylinder, while the waveguides ex-tending radially damp out undesirablewake fields.

LTHOUGH THE LONGITUDINALemittance obtained in the damp-

ing ring is small enough, the bunchis still much too long for accelera-tion in a linac. In the SLC and NLC,this problem is solved by a techniquecalled bunch compression, whichshortens the bunch while increasingits energy spread. Each bunch passesthrough an rf accelerating structurephased so that the trailing particlesemerge with lower energy than theleading particles. Then the bunchpasses through a sequence of mag-nets that disperses the beam so thatparticles of different momenta travelon different paths. Particles withhigher momentum (at the head ofthe bunch) travel a longer path thanthose of lower momentum (at thetail). The tail of the bunch can there-fore catch up with the head, produc-ing a shorter bunch -but at the costof a greater energy spread.

This type of bunch compressionhas been used routinely in the SLC,where bunches 5 mm long are com-pressed to 0.5 mm for acceleration inthe linac. Much shorter bunches willbe required in the NLC. Shortbunches will suffer less from trans-verse wake fields in the linac, andthey permit a smaller depth of focusat the IP (about 100 microns for theNLC). In principle, another order ofmagnitude in compression could beobtained in a single stage; in prac-tice, however, this approach wouldlead to other deleterious effects dueto the large energy spread that wouldbe induced in the beam. For thisreason, the extra compression isprovided by a second bunch com-pressor operating at a higher energy.

In the NLC, the bunch is firstcompressed as in the SLC to 0.5 mm

in length, after which the beam isaccelerated to about 16 GeV. Thelongitudinal spread of the beam isunaffected by this acceleration, butthe relative energy spread decreaseslinearly with energy (see box onpage 13). The compression is thenrepeated, resulting in a bunch lengthof about 50 microns. By separatingthe compression process into twodiscrete steps, we can keep the rela-tive energy spread small throughout.

A T THE HEART OF THE LINEARcollider is the high-gradient

linac, where we accelerate thebunches to very high energy withoutincreasing their emittance. This featis accomplished with oscillatingelectric fields in traveling wavestructures. The present SLAC accel-erating structure, which operates at2.8 GHz, consists of a long coppercylinder periodically interrupted bydisks with holes along the centerline. The structure is designed sothat a speed-of-light particle passesthrough the holes at the proper rfphase for maximum acceleration.Every so often, this structure is in-terrupted by a feed for fresh rf powerand a load to remove upstream power.

Although similar in spirit to thepresent SLAC waveguide, the struc-tures envisioned for the NLC aresomewhat different in detail. Thereare two key differences: the struc-tures are much smaller because theywill operate at higher frequency ( 11.4GHz, for example), and they must bedesigned to damp modes of rf oscil-lation that differ from the funda-mental. Such damping is necessaryto isolate the bunches within onebatch from each other, so that the

14 SUMMER 1990

additional electromagnetic fieldsinduced by a bunch (wake fields) donot affect succeeding bunches. Amodel of such a structure is shownon page 14. In this structure, outgoingwaveguides coupled to each celldamp the unwanted wake fields.

The characteristics of the powersource for an NLC are substantiallydifferent from those in the SLC. At theSLC, klystrons provide up to 65 MWof peak power for pulses of about 2.5microseconds, which is then com-pressed into a pulse with higher peakpower but for a shorter duration,typically 1 microsecond. In order toachieve a gradient of 100 MV/m at11.4 GHz in the NLC, we must sup-ply about 220 MW to each meter ofaccelerating structure in pulses witha duration of only 100 nanosecondsor so. The problem is not the totalenergy content of the pulse, but therequirement that the rf come in shortbursts of very high peak power. Thereare basically two ways to achievethis goal.

In the first, called "rf pulse com-pression," a modulated pulse of dcpower is converted to a long rf pulseby some power source such as a kly-stron. This pulse is then compressedto the proper duration with a corre-sponding increase in the peak power.The first part of the pulse is stored ina delay line or cavity, while the latterportions catch up. After combina-tion, the resulting pulse has almostthe same energy content but is higherand shorter. Such a method is usedat the SLC to boost the peak powerfrom about 60 MW to 180 MW. Aprototype system operating at 11.4GHz, just completed at SLAC, hasalready achieved a factor of 5.5 gainin peak power.

In the other approach (called"magnetic pulse compression"), wefirst compress the energy in the dcpulse into a shorter interval; thispulse is then used to power severaldevices that produce short, intensepulses of rf power. One example ofsuch a device is the relativistic klys-tron, which has generated 330 MWpeak power in a pulse only 20 nano-seconds long.

There are extensive research pro-grams at SLAC, KEK and INP(Novosibirsk) on these and other ad-vanced rf power sources. All threeprograms emphasize the develop-ment of high-power klystrons withabout 100 MW peak power, whichare used with rf pulse compressionto amplify the peak power by factorsof 5 to 10. If a 100 MW pulse is am-plified by a factor of 10, for example,such a system could achieve a totalacceleration of 500 MeV in a struc-ture 5 meters long.

The primary purpose of the mainlinac is to accelerate the particles totheir full energy. Along with this,however, comes the capacity to de-stroy the luminosity, because a linacis a rather hostile environment for avery tiny particle bunch. Longitudi-nal and transverse wake fieldsthreaten to increase its energy spreadand tear it apart. As these bunchesare only 2 microns high, 20 micronswide and 50 microns long, specialcare must be taken to preserve them.

As the particles pass through theaccelerating structure, the wakefields they induce act back on thefollowing particles. The head of agiven bunch can deflect the tailtransversely, spoiling its delicatealignment. And a single bunch canexcite wake fields that affect trailing

RFpulse compression (top) and magneticpulse compression (bottom).

Prototype SLAC klystron designed to gen-erate 100 MW peak power.

BEAM LINE 15

bunches-either deflecting or accel-erating them in undesirable ways.Such effects have led to the develop-ment of the new generation of accel-erating structures (described above),which damp out higher-order modesof oscillation that can affect subse-quent bunches.

Even after these wake-field ef-fects are controlled, we must stillprovide very careful alignment ofthe magnetic focusing structurealong the linac. Special techniquesfor the correction of the bunch tra-jectory will probably be necessary;they will require high-precision de-vices to monitor both the positionand size of the beam.

Although these problems arechallenging, much experience is be-ing gained at the SLC. By combiningthis experience with simulations ofNLC performance, we should be ableto obtain definitive answers in thenear future.

T THE END OF EACH LINEAR ac-celerator is a final focus system

whose purpose is to compress thetiny bunches to sub-micron dimen-sions. To obtain the luminosity de-sired, the cross-sectional area of eachbunch must be only few hundredsquare nanometers. In addition, wemust focus it to the shape of a flatribbon (rather than a string) in orderto minimize the radiation emitted asthe particles in the bunch encounterthe intense electromagnetic field ofthe opposing bunch. These goals areaccomplished by the use of a com-plex magnetic focusing systemanalogous (in reverse) to an opticaltelescope used to magnify distantobjects. This system uses quadru-pole magnets as focusing elements

in a unique combination that pro-vides a very large demagnification.

A major problem is the so-called"chromatic" effect of the final qua-drupole magnets. Two parallel elec-tron beams with different momentaentering a perfect quadrupole mag-net are brought to a focus at slightlydifferent longitudinal positions be-cause the lower energy beam is bentslightly more than the higher energybeam by the magnetic field. For itnot to affect the spot size, this shiftof focal point must be smaller thanthe depth of focus of the beam, writ-ten as 3* (see box on page 13). Due tothe requirement of flat, or ribbon-shaped beams, this depth of focus isabout 100 microns in the verticaldimension.

Such a small depth of focus makesthe chromatic effects particularlyserious. The chromatic correctionof the final quadrupoles is in fact thekey to the final focus. Upstream ofthese quadrupoles, a combination ofbending magnets that disperse thebeam combined with nonlinearsextupole magnets ensures thathigher energy particles get a bit morefocusing than lower energy particles.When a bunch arrives at the lastquadrupole, the chromatic effect ofthe magnetic field upon it is exactlycanceled.

The basic principles of the chro-matic correction for particle beamshave been known for about 30 years.Their first application in a linearcollider was in the SLC, where thebeams are demagnified by about afactor of 30, yielding spot sizes ofabout 3 microns. Because thedemagnification necessary in theNLC is about a factor of 300, how-ever, the design of its final focus

system will be substantially differ-ent from that of the SLC.

In order to test such a next-generation final focus experimental-ly, an international collaborationincluding SLAC, INP, KEK, Orsay, andDESY has been formed to design andconstruct a Final Focus Test Beam atSLAC (see article by David Burke, p.21). This facility will use the SLCbeam emerging straight ahead fromthe linac as its source of electronbunches. The goal is to producebunches with transverse dimensionsof 60 nanometers high by 1 micronwide. The collaboration will also usethis facility to test the alignment,stability and instrumentationrequirements needed to achieve suchsmall spots. The Final Focus TestBeam is a key component of theworldwide research effort towardsthe NLC.

When the two oppositely-chargedbunches collide at the IP, the intenseelectromagnetic fields of one bunchfocus the other bunch. This pinchingeffect enhances the luminosity,because the beam cross section be-comes smaller. In an NLC the com-bination of very high electromagneticfields and high particle energy yieldsubstantial amounts of synchrotronradiation known as beamstrahlung.The average energy loss due to beam-strahlung ranges from 1 to 30 percentin various NLC designs. In extremecases, many of these photons cansubsequently generate electron-positron pairs in the intense electro-magnetic fields present. The radiatedphotons or charged particles canstrike detector components, causingundesirable backgrounds.

In practice, these beam-beam ef-fects are what impose the ultimate

16 SUMMER 1990

limits on the possible charge perbunch-and thus on the luminosity.In the design described above, theluminosity limit is bypassed by us-ing a short train of bunches, eachwith moderate total charge. This ap-proach allows us to maintain thedesired luminosity while keepingbeam-beam effects under control.

URING THE PAST SEVERAL years,there has been marked progress

towards the design of the Next Lin-ear Collider. We have moved frombroad general outlines to the detailedfeatures of such a machine. This ar-ticle has only touched on its majoraspects, but they will be discussed inmore detail in future issues of theBeam Line-in the column "Towardthe Next Linear Collider." We beginthis process with a discussion of theFinal Focus Test Beam on page 21 ofthis issue.

o

DATES TO REMEMBEROct 23-26

Nov 7-14

Jan 9-12, 1991

Mar 11-15

May 6-9

1990 Nuclear Science Symposium, Arlington, VA (for fur-ther information contact K. W. Kraner, InstrumentationDivision, Brookhaven National Laboratory, Upton, NY11973)

Joint US-CERN Particle Accelerator School: Frontiers ofParticle Beams: Intensity Limitations, Hilton Head, SC (forfurther information contact S. von Wartburg, LEP Division,CERN, 1211 Geneva 23, Switzerland)

Workshop on Mark II Physics at PEP, SLAC (for further in-formation contact Karl Van Bibber, SLAC, Bin 43, P.O. Box4349, Stanford, CA 94309, or BITNET PEGASYS@SLACVM)

9th International Conference on Computing in High En-ergy Physics, Tsukuba, Japan (for further information con-tact Yoshiyuki Watase, KEK National Laboratory for HighEnergy Physics, Computer Center, Oho, Tsukuba, Ibaraki305, Japan)

Particle Accelerator Conference, Sheraton Palace Hotel,San Francisco (for further information contact ReneDonaldson, SLAC, BITNET RENED@SLACVM)

BEAM LINE 17

High Altit lsilCS

18 SUMMER 1990

B Factory Studies at Snowmass

about 200 GeV, Fermilab physicistswere looking forward to a series ofluminosity upgrades on the Tevatron,leading eventually to the proposedMain Injector which they hope willcome on line in 1995. Even beforethat, noted Mark Strovink of LBL ina summary talk, they will have tocontend with 36-bunch operation ofthe collider. With only 400 nanosec-onds between bunch crossings, theCDF and DO detectors will experi-ence major impacts, and how to dealwith them was a major topic atSnowmass.

Those who prefer to work in thecleaner environments of electron-positron colliders seem to havereached a general consensus that ahigh-luminosity B factory (see box atright) is the next logical machine tobuild. In a summary of acceleratorphysics work at Snowmass, BobSiemann of Cornell reported thatseveral groups are confident they canbuild an asymmetric collider (i.e.,with unequal electron and positronenergies) having a luminosity ofabout 3xl033 cm-2s-

1 needed to makedetailed studies of CP violation in theB meson system. Feasible solutionsnow seem to exist for one of the keyremaining problems, detector back-grounds due to synchrotron radia-tion and "lost particles" from beam-gas interactions occurring upstream.

On the subject of a next-generation linear collider, or NLC,Siemann noted that the theoreticalfoundations for such a machine arein good shape, and that there hadbeen substantial technical progressin the past few years. RF powersystems able to generate acceleratinggradients of 50-100 MV/m are closeto the testing stage. At Snowmass

THE RECENT SNOWMASS SUMMER STUDY witnessed a lot of activity relatedto B factories and physics. There were active groups working on acceleratorproblems, detector design questions and many aspects of B physics, especially inthe area of CP violation.

The emphasis in the accelerator realm was on the machine/detector interface.This is a crucial area, as the high currents, individual beam pipes and asymmetricenergies of an e~e B Factory pose substantial new challenges. Comparison of thework done at SLAC and Cornell in the design of masking schemes against synchro-tron radiation and electron backgrounds showed impressive gains in our under-standing and progress toward practical designs. Much effort also went into definingcriteria for the scattered particle levels that are acceptable in individual detectorsubsystems.

The principal motivation for building a high-luminosity asymmetric ete- B fac-tory is the desire to study the phenomenon of CP violation, for the light it can shedon the consistency of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. A large groupat Snowmass examined our current knowledge of the nine CKM matrix elements,with an eye toward making improved measurements that might be done withexisting facilities in the next several years. This study found that within the next fiveyears or so, a series of specific measurements could improve the experimentalsituation to the point where our knowledge of these matrix elements is dominatedby theoretical uncertainties, rather than by experimental errors. As measurementsget better, the limitation on our knowledge of the CKM matrix elements will then stemfrom uncertainties in the charmed quark mass and in the normalization of semileptonicform factors.

With the matrix elements determined as well as is currently practical, we willthen need measurements of CP-violating quantities in the B meson system in orderto ascertain whether or not the CKM phase angle is the source of CP violation. Asthese measurementswill require large numbers of B mesons, much ofthe Snowmasseffort was aimed at determining whether experiments other than the "classical"ones involving B - tK and B0 4ir can be brought to bear on the question,both to reduce the number of B mesons required to provide an answer and to allowmore detailed tests of underlying assumptions.

The1se additional modes are ofseveral distinct types. The first class consists ofexclusive decays of charged B mesons, which can produce a measurable CP-violatin g asymmetry without requiring a tag. Unfortunately, such asymmetries arelikely tobe difficultto measure, either because they are small or they occur in modesw ith small branching ratios. In either case, there are also substantial uncertaintiesin relating measured asymmetries to CKM matrix elements. Much more promising

duce measurable and readily interpre table OP-violating asymmetries. It is evenpossible to make sensitive measurements using decays to final states that are notOP eigenstates but are composed of a OP self-conjugate collection of quarks, such

By employing these and other decay modes, it may be possible tomake measurements of P vviolation in the B meson system with substantially lowerintegrated luminosity than was ori ginally contem plated.

Many other questions bearing on the physics of the CKM matrix were discussedat Snowmass. Among these were: measurements of B° and B0 mixing, the use ofrare Kmeson decays and CP-violating asymmetries in K decay to determine CKMmatrix elements, determination of pseudoscalar meson decay constants (both froman experimental and theoretical perspective), and the use of CP asymmetrymeasurements to provide a window beyond the Standard Model. The Snowmassstudies have firmly established the central role that a B factory will play in ourattempts either to establish the consistency of the Standard Model or to explorespecific alternatives in the coming decade.

: 0: : ^: i ' : :: ::: : l . :0 0 : 0 S : : - David Hitlin

BEAM LINE 19

! 7 r f !? 7? four r 7 r Fr r F r 7 7 7 r 7 r 7 :

Physicists participating intensely in oneof the Snowmass workshops.

KEK physicists presented the firstdetailed studies of the maskingaround the interaction pointnecessary to eliminate the great bulkof electron-positron pairs created bythe clashing beams. Powerfulsolenoidal magnetic fields help toconfine these low-energy particlesalong the beam line and keep themaway from the detector.

There has been much progressrecently in the area of non-acceleratorparticle physics, and great stridesforward are anticipated during the1990s. Summarizing work on low-background instruments aimed atdirect detection of dark matter(Weakly Interacting MassiveParticles, or WIMPs) in the Milky Wayhalo, Michael Witherell of SantaBarbara suggested that the time hasnow arrived to begin the design of afull-scale cryogenic WIMP detector.First-generation detectors based onroom-temperature germanium have(together with the SLC/LEP limits onthe number of neutrinos kinds)helped rule out a fourth, heavyneutrino as a possible source of thedark matter. Small-scale cryogenicdetectors are the next logical step inthis effort, Witherell noted, followedby a full-scale experiment at one ormore of the existing deep under-ground facilities.

The largest contingent of physi-cists in attendance were interestedin how to design and build detectorsfor the SSC. "Whatever mechanismNature uses for generating mass, itwill occur at the SSC," observedGordon Kane of Michigan in report-ing the theoretical work on symme-try breaking done at Snowmass. "Theonly question is, 'Will we be able todetect it?"' Nearly 300 physicists

participated in the workshops de-voted to SSC detectors, and most ofthe 14 designs proposed so far werepresented in evening sessions.

In summarizing all this workFrank Paige of Brookhaven voiced agrowing belief among high-energyphysicists that the purported Higgsphenomena are likely to be heavy,i.e., occur at masses of 250 GeV ormore. So detectors are being designedaccordingly, to pull the signal from

HO - ZZ - C--> f+C-

decays out of the enormous back-grounds that will be present. For apossible intermediate mass Higgsboson (with a mass below 150 GeV),physicists at Snowmass began toconsider the decay H0 - yy, whichshould occur less than 1 percent ofthe time, but might be distinctiveenough to detect. Extracting this sig-nal would take advantage of the factthat such a lower mass Higgs wouldshow up as a narrow peak in aninvariant mass plot. Resolution ofabout 1% in momentum and energywould be necessary, as well as anability to reject a background of 104hadron jets for every photon.

Compared to previous SnowmassSummer Studies, this one was arather humdrum affair, however,with few completely new directionsbeing charted. Most of the resultsthat came out of the many work-shops were refinements of existingmethods and techniques rather thancompletely new approaches. Con-solidation seemed to be the watch-word, perhaps due to the growingrealization that funding for new ideasis going become increasingly scarceduring the 1990s. O

20 SUMMER 1990

TOWARD THE NEXT LINEAR COLLIDER

The Final Focus Test Beamby DAVID BURKE

O NE OF THE GREATEST CHALLENGESwe face on the road to the Next Linear Collider (NLC) isto make particle beams with extremely small sizes.Whereas the particle bunches in the SLC are millimeter-long needles 4-5 microns across, those in the NLC will haveto be ten times shorter and up to a thousand times nar-rower. Such tiny beams are needed to produce luminositiesof 1033-1034 cm-2 s- 1 that will be necessary to generatesufficient numbers of events as the center-of-mass energyclimbs toward 1 TeV-and the cross sections for interest-ing physical processes drop toward 10-3 7cm-2.

Other ways to increase the luminosity, e.g., by raisingthe number of particles per bunch and the machinerepetition rate, are limited by the available ac power andby interactions of the bunches with each other and withthe accelerating structure (see article by Ron Ruth onpage 10). Therefore, achieving spot sizes that are a hun-dred times smaller than the wavelength of visible lightwill be one of the chief goals in the development of theNLC.

The part of a linear ee- collider that reduces the beamsizes and maintains the beams in collision is called thefinal focus. Its magnetic elements act much like thelenses of a fine optical telescope to collect the particlesproduced by the linear accelerator and focus them to a

BEAM LINE 21

spot with small cross-sectional area.To produce tightly focused beamsand maintain them in collision re-quires careful control and stabiliza-tion of these magnets, and placesconsiderable emphasis on accuratemeasurement of the properties of thebeam itself. We have learned a great

30 10 deal from operation of the SLC, butsuccessful implementation of future

the FFTB machines at higher energies will de-of the mand that even tighter mechanicalDotted line and electrical tolerances be respected.the solid It will also require greater measure-

? spot that ment precision and the development3hromatic of tuning mechanisms and tech-repre- . .rof the niques considerably more powerful

st Beam. than those presently in use.In collaboration with teams of

physicists and engineers from theSoviet Union, Germany, France, andJapan, we have recently begun tobuild and instrument a prototypemagnetic system capable of produc-ing the small beam spots required forthe NLC. This Final Focus Test Beam(FFTB) will occupy some 180 metersin the straight-ahead channel at theend of the SLAC linac, with the finalelements extending onto the tarmacof the Research Yard. As input it willuse the unique SLC electron (orpositron) beam with its very smallemittance. The optics of this beamwill be corrected to third order forgeometric and chromatic aberrationsto produce a focal point at which thebeam height will be demagnified bya factor of 300-to a size smallerthan 100 nanometers. Just such acompression factor will be requiredfor the final focus of a TeV-scalelinear collider. To attain it we willhave to address most of the critical

issues inherent in their design, con-struction and implementation.

The central problem in the designof the final focus for a linear collideris to achieve a large geometric de-magnification of the beam while con-trolling chromatic aberrations intro-duced into the beam envelope at thefocal point by the final focus ele-ments themselves. In a simple tele-scope the geometric demagnificationof particles of fixed momentum issimply the ratio of the focal length ofthe image lens to that of the objectlens, M = fi/fo. But the position of thefocal point along the axis of the sys-tem depends upon momentum, sothe transverse size of the beam at thenominal focal point will be diluted ifthe beam is not monochromatic. Thisdilution, or chromatic aberration,becomes more severe at higherdemagnification (smaller M) and asthe momentum spread of the beamincreases. Because the beam from ahigh-gradient linac is not monochro-matic, these aberrations can domi-nate the problem of creating smallspot sizes.

Chromatic aberrations can becontrolled by the use of sextupolemagnets, which have quadrupolemoments that vary linearly withtransverse position in their aperture.When paired with bending magnetsthat disperse the beam, they can beused to introduce momentum-de-pendent focusing into the beam line.Such a correction is shown for theFFTB design in the graph above. Ourinitial goal is to engineer, constructand operate this test beam to verifythe theoretical behavior of these op-tics.

22 SUMMER 1990

1I

0.

0.01000 300 100

M-1

Beam height a produced at ifocal point at various settingslattice demagnification M-1. ris for a monochromatic beam;curve shows the dilution of thewould occur if there were no ccorrection. The dashed curvesents the design performancefully corrected Final Focus Te

. I I . t

The FFTB is truly an internationaleffort. The optical design principleswere developed by accelerator physi-cists from around the world, andhave been reviewed at major inter-national workshops on linear col-liders. Detailed design, error analy-sis, and development of precise tun-ing procedures for these optics havebeen carried out by a group consist-ing of physicists from KEK, LAL/Orsay, and SLAC. The magnets weredesigned and are being fabricated atINP/Novosibirsk in the Soviet Union,and those for the final lens pair arebeing designed there and in Japan.State-of-the-art mechanical stabili-zation of these critical componentswill be done by the group at KEK.

Precise mechanical alignment andstabilization are necessary also forthe remainder of the magnetic ele-ments and electronic monitors. Theymust be aligned to better than 30microns along the entire test beam,and their position must be stable to1-2 microns in order to maintain thefinal spot size. Groups from DESYand SLAC are studying how to achievethese goals.

Improvements, and in some casescompletely new directions, in beamdiagnostic instrumentation areneeded to measure the properties ofthe beam accurately throughout thesystem. Measurement of the finalspot size is a particularly challengingproblem, as the tightly-focused beamwill destroy any material in its path.Physicists and engineers at LAL/Orsay and SLAC are currently attack-ing these problems. Beam positionmonitors and associated electronicsare being designed to provide

resolutions of 1-2 microns. Wirescanners will be used to measurebeam profiles at locations other thanthe focal point of the system, andtechniques are being developed tomeasure the focused spot by ob-serving the interaction of the beamwith gas-jet or renewable solidtargets.

The Final Focus Test Beam is wellunderway. All participating groupshave received the necessary funding,and the U.S. Department of Energyhas given the go-ahead for its con-struction at SLAC. Completion ofconstruction is expected in late 1992,and successful commissioning of thetest beam will be a major step in theworld-wide effort to design and builda TeV-scale linear collider. Layout of the principal elements of the

0 Final Focus Test Beam, which willextend about 180 meters from the BeamSwitchyard into the Research Yard.

BEAM LINE 23

PEOPLE AND EVENTS

Witherp,e, Win, Pannfskv Prize

MICHAEL WITHERELL, PROFESSOR OF PHYSICS at the University ofCalifornia, Santa Barbara, has been named winner of the 1990 W. K. H.Panofsky Prize for his outstanding contributions to the field of elementaryparticle physics. Awarded by the American Physical Society Division ofParticles and Fields, this year's prize went to Witherell for his pioneeringstudies of charmed particles.

Together with a team of physicists working on Fermilab Experiment691, he developed experimental techniques based on silicon-microstripvertex detectors to extract an unprecedented number of charmed particlesfrom photon-nucleus collisions. Upwards of 100 million charmed particleswere detected, enabling the team to determine their lifetimes with greataccuracy.

Following graduate study at Wisconsin, where he received his Ph.D. in1973, Witherell became a postdoc and later assistant professor at Princeton.In 1981 he moved west to Santa Barbara, where he has been professor of phys-ics since 1 986 In addition to his Fermilab research on charmed narticles. he.......... U I / V V ·-- XJ. nn ..- ....- r .-- IL - - -...- u -.--.. -- --- -...- ---- -- - - I ---

is involved at SLAC with the SLD collaboration and is a member of the groupstudying the possibility of building an asymmetric B factory at PEP.

Fermmnn Cn.ps tn Hnrvnrd

THIS AUTUMN GARY FELDMAN, a longstanding veteran of SLAC, re-turns to academe. He will step in as professor of physics at Harvard Univer-sity, where he did his Ph.D. thesis in 1971. Following that he came to SLAC,joining Experimental Group E as a postdoctoral research associate underMartin Perl, and working initially on inelastic electron-nucleon scattering.

Feldman teamed with Perl and others in the famous Mark I collabora-tion to search for heavy leptons in electron-positron collisions. By 1975 theywere finding evidence of "anomalous u-e events," which Feldman presentedat the Lepton-Photon Sympoisum at Stanford that summer. This was theearliest hint of the tau lepton, one of the great discoveries of the mid-1970sthat helped to put SLAC on the map of high-energy physics.

Feldman was named associate professor at SLAC in 1979 and becamefull professor in 1983. More recently, he served as a co-spokesman of the

- Mark II collaboration at the SLC, which produced the initial results on Z| particle production and decay in electron-positron collisions.

24 SUMMER 1990

Tigner to Receive Lawrence Award

MAURY TIGNER OF CORNELL UNIVERSITY has been named one of thesix recipients of the 1990 Ernest O. Lawrence Awards, conferred annually bythe Department of Energy for outstanding contributions in the field ofatomic energy. Formerly the Director of the SSC Central Design Group, hewas recognized for his many contributions to accelerator physics andtechnology. Among these are his work on the Cornell Electron Storage RingCESR, the development of superconducting rf cavities, and the conceptualdesign of the SSC.

In 1963 Tigner received his Ph.D. in physics from Cornell, where heworked as a research assistant to Robert R. Wilson. Following that heremained at Cornell and became Director of Accelerator Operations, play-ing a major role in the design and construction of CESR.

When the Superconducting Super Collider became a national priorityfnr the h i h-enpergv nhvTicr conmmrnunitv he tnnlo a lpeacing rnlp in m alincr it

a reality, heading the design group that was headquartered at LawrenceBerkeley Laboratory from 1984 to 1989. According to the Department ofEnergy, "Tigner's vision and unquestioned creativity have in large part beenresponsible for the technical maturity of this project."

SLAC Summer Institute

WITH NEW RESULTS ROLLING IN from proton-antiproton and electron-positron colliders, the theme for this year's SLAC Summer Institute was"Gauge Bosons and Heavy Quarks." An international group of physicists,ranging from graduate students to professors and representing 10 countries,gathered at SLAC from July 16-27 for the 18th meeting of this Institute. Outof the 269 participants, 125 were representatives of SLAC and the local usercommunity, while others came from as far away as Europe, Japan and Brazil.

The Institute was divided into two separate parts, a seven-day schoolof a generally pedagogical nature followed by a three-day Topical Confer-ence. There were lectures each morning of the first part, followed byorganized discussion sessions with the speakers during the afternoons. TheTopical Conference consisted of 27 invited talks on various experimentaland theoretical results of current interest in high-energy physics. A high-light of this conference was the detailed presentations of new results fromthe four LEP experiments at CERN.

BEAM LINE 25

U)(aC)

Y)cl'o

"BeeJay" Bjorken on the mound.

Burt "Big Daddy" Richter serves upanother pitch to a Theory batter.

Unified Force Stumps Experiment, 10-9

IT WAS A DARK AND STORMY DAY on the SLAC diamond. With thun-

derclouds threatening on Saturday, May 19, the Experiment team wentdown to an unprecedented second straight defeat at the hands of Theory inthe annual softball game, losing by one measly run, 10-9. By combiningsteady pitching, sparkling fielding and timely hitting, the third-floor crowdmanaged to tame the booming Experiment bats and eke out a close victory.

As the game began, the revenge-hungry empiricists were rubbing theirhands with anticipation because ace hurler Lefty O'Drell, who had silencedtheir bats the previous year, was absent from the Theory lineup. But his oldsidekick and fellow southpaw, Jim "BeeJay" Bjorken stepped in and keptthem confused with his split-finger quarkball until the final inning.

The Theorists jumped out to an early 2-0 lead in the top of the first, onan inside-the-park homer by Howie Haber, who had just been brought upfrom their Santa Cruz farm club for this game. The Experimenters battledback, evening the score at 2-2 in the third. With runners at first and secondand only one out, it looked like a big inning for them, but BeeJay got Burt "BigDaddy" Richter to ground into a double play to end the threat.

With two out in the top of the fourth, four Theorists crossed the plate,after starting pitcher Richter stopped a wicked grounder with his nose andhad to leave the game. By the time relief pitcher Bill "Fireman" Kirk hadretired the side, the score stood at 6-2.

Theory scored four more in the sixth, as Lance Dixon looped a lazy flyball into Panofsky Grove for a three-run homer, making it 10-2. Solo homersby Ron Cassell and Mike Woods made things a bit more respectable for theempiricists, but the score was still 10-4 heading into the last of the ninth.

Needing only one more out to end the game, however, Bjorken finallybegan to tire. Successive home runs by Cassell, Mark Petree and GaryGladding made it 10-9, as the partisan crowd erupted in gleeful expectationof a thrilling come-from-behind Experiment victory. But Dixon made ashoestring catch in deep center field to end the game, preserving Theory'shard-fought win-only their 5th in 32 years.

As everybody departed for the traditional post-game beers, it hadbecome obvious that last year's 7-4 beating was not just a statisticalfluctuation, after all. In prior years, almost any ball hit to the Theory outfieldhad usually been good for two bases, if not more, but this Saturday they werejust long outs. With excellent coaching and lots of diligent practice, theTheory team has molded itself into a unified force that Experiment will haveto reckon with in the coming years.

26 SUMMER 1990

SLAC PUBLICATIONS

Experimental High-Energy Physics

Crystal Ball Collaboration: D. Antre-asyan, et al., Observation of the Ex-clusive Decay B - evD* and Searchfor B - ev7fw, (SLAC-PUB-5250, May1990).

Crystal Ball Collaboration: D. Antre-asyan, et al., First Observation ofthe Reaction yy - r2 --> f(ooo,(SLAC-PUB-5254, May 1990).

D. Aston, et al., A Summary of the Re-sults from LASS and the Future ofStrange Quark Spectroscopy, (SLAC-PUB-5236, May 1990; Invited talkgiven at 15th APS Div. of Particlesand Fields General Mtg., Houston,TX, Jan 3-6, 1990).

E. C. Brennan, Ed., Physics at the 100-GeV Mass Scale: Proceedings,(SLAC-0361, Jan 1990; Proc. of the17th SLAC Summer Institute onParticle Physics, Stanford, CA, Jul 10-21, 1989).

SLD Collaboration: P.N. Burrows,Physics with Polarization at the SLD,(SLAC-PUB-5264, May 1990; Pre-sented at Rencontre de Moriond,Les Arcs, France, Mar 4-11, 1990).

MARK II Collaboration: S. Komamiya,Determination of as from a Differ-ential Jet Multiplicity DistributionatSLCandPEP, (SLAC-PUB-5247, Apr1990; Presented at 15th APS Div. ofParticle and Fields General Mtg.,Houston, TX, Jan 3-6, 1990).

MARK II Collaboration: K.F. O'Shaugh-nessy, Charged Particle InclusiveDistributions from Hadronic Z°Decays, (SLAC-PUB-5216, May 1990;Presented at 15th APS Div. of Par-ticles and Fields General Mtg.,Houston, TX, Jan 3-6, 1990).

K.F. O'Shaughnessy, Properties ofHadronic Decays of the Z Boson,

(SLAC-0360, Jun 1990; Ph.D. The-sis).

E. Soderstrom, et al., A Search for PairProduction of HeavyStable ChargedParticles in Z Decays, (SLAC-PUB-5192, Feb 1990; Submitted to Phys.Rev. Lett.).

M. L. Swartz, Measurement of the W+W-Threshold, (SLAC-PUB-5258, Jun1990; Submitted to Nucl. Phys. B).

M. L. Swartz, Precision Experiments inElectroweak Interactions, (SLAC-PUB-5219, Mar 1990; Presented at17th SLAC Summer Inst.: Physics atthe 100-GeV Mass Scale, Stanford,CA, Jul 10-21, 1989).

R. Van Kooten, Searches for NewQuarks and Leptons in Z BosonDecays, (SLAC-0367, Jun 1990; Ph.D.Thesis).

MARK II Collabration: C. Von Zanthier,et al., Measurement of the TotalHadronic Cross Section in e+e-Annihilation at s = 29 GeV, (SLAC-PUB-5213, Mar 1990; Submitted toPhys. Rev. D).

L.W. Whitlow, Deep Inelastic StructureFunctions from Electron Scatteringon Hydrogen, Deuterium, and Ironat 0.6 GeV 2 < Q2 < 30 GeV2, (SLAC-0357, Mar 1990; Ph.D. Thesis).

L.W. Whitlow, et al., A Precise Extrac-tion of R = CL/CT from a GlobalAnalysis of the SLAC Deep Inelastice-p and e-d Scattering Cross Sec-tions, (SLAC-PUB-5284, Jun 1990; Tobe published in Phys. Lett.)

BEAM LINE 27

Theoretical Physics

J.D. Bjorken, New Symmetries in HeavyFlavor Physics, (SLAC-PUB-5278, Jun1990; Invited talk given at LesRencontres de la Valle d'Aoste, LaThuile, Italy, Mar 18-24, 1990).

R. Brooks and D. Kastor, MorphismsBetween Supersymmetric and To-pological Quantum Field Theories,(SLAC-PUB-5249, May 1990; Submit-ted to Phys. Lett. B).

A.R. Cooper, Loops in 2-D QuantumGravity, (SLAC-PUB-5286, May 1990;Submitted to Nucl. Phys. B).

C. Dib, Rare K Meson Decays in theCase of a Heavy Top Quark, (SLAC-0364, Apr 1990; Ph.D. Thesis).

L. J. Dixon, Supersymmetry Breaking inString Theory, (SLAC-PUB-5229, Apr1990; Invited talk given at 15th APSDiv. of Particles and Fields GeneralMtg., Houston,TX, Jan 3-6, 1990).

L. J. Dixon, et al., Moduli Dependence ofString Loop Corrections to GaugeCoupling Constants, (SLAC-PUB-5138, May 1990; Submitted to Nucl.Phys. B).

I. Dunietz and A. Snyder, Additional BdDecays with Large CP Violation andNo Final State Phase Ambiguities,(SLAC-PUB-5234, Apr 1990; Submit-ted to Phys. Rev. D).

D. Fryberger, A Reply to: A Comment onGeneralized Electromagnetism andDirac Algebra, (SLAC-PUB-5233, Apr1990; Submitted to Found. Phys.Lett.).

B.L. Ioffe and M. Karliner, How Can theStrangeness Content of the ProtonBe Both Large and Small? (SLAC-PUB-5235, Apr 1990; Submitted to Phys.Lett.).

T. Jaroszewicz and S.J. Brodsky, Z Dia-grams of Composite Objects, (SLAC-

PUB-5227, Apr 1990; Submitted toPhys. Rev. C).

S. B. Menahem, Two and Three PointFunctions in the D 1 Matrix Model,(SLAC-PUB-5262, May 1990; Submit-ted to Nucl. Phys. B).

Y. Nir and H. R. Quinn, LearningAboutthe CKM Matrix from CP Asymme-tries in B0 Decays, (SLAC-PUB-5223,Apr 1990; Submitted to Phys. Rev.D).

Y. Nir and D.J. Silverman, Z MediatedFlavor Changing Neutral Currentsand their Implications for CPAsymmetries in B° Decays, (SLAC-PUB-5245, May 1990; Submitted toPhys. Rev. D).

M.E. Peskin and T. Takeuchi, A NewConstraint on a Strongly Interact-ing Higgs Sector, (SLAC-PUB-5272,Jun 1990; Submitted to Phys. Rev.Lett.).

R.L. Singleton, et al., Do Weak Inter-actions Become Strong at 10 TeV?(SLAC-PUB-5225, Apr 1990; Submit-ted to Nucl. Phys. B).

A. Szczepaniak, et al., Perturbative QCDEffects in Heavy Meson Decays,(SLAC-PUB-5228, Mar 1990; Submit-ted to Phys. Lett. B).

A.C. Tang, Discretized Light ConeQuantization: Application to Quan-tum Electrodynamics, (SLAC-0351,Jun 1990; Ph.D. Thesis).

28 SUMMER 1990

_

_

Accelerator Physics

M.A. Allen, et al., RFPower Sources forLinear Colliders, (SLAC-PUB-5274,Jun 1990; Contributed to 2nd Euro-pean Particle Accelerator Conf., Nice,France, Jun 12-16, 1889).

K.L. Bane, et al., Measurements of Lon-gitudinal Phase Space in the SLCLinac, (SLAC-PUB-5255, May 1990;Contributed to 2nd European ParticleAccelerator Conf., Nice, France, Jun12-16, 1990).

B. Bell, et al., Datum Definition Prob-lems in Accelerator Alignment,(SLAC-PUB-5226, Apr 1990; Presentedat ACSM-ASPRS Spring Convention,Denver, CO, Mar 19-23, 1990).

F.J. Decker and R.K. Jobe, Phase Gradi-ents in Acceleration Structures,(SLAC-PUB-5271, May 1990; Contrib-uted to 2nd European Particle Accel-erator Conf., Nice, France, Jun 12-16,1990).

H. Deruyter, et al., DampedAcceleratorStructures, (SLAC-PUB-5263, Jun1990; Contributed to 2nd EuropeanParticle Accelerator Conf., Nice,France, Jun 12-16, 1990).

R.C. Field, et al., A Compact BeamProfile Probe Using Carbon Fibres,(SLAC-PUB-5253, May 1990; Submit-ted to Nucl. Instrum. Methods).

G.E. Fischer, SLAC Site Geology, GroundMotion and Some Effects of the Oc-tober 17, 1989, Earthquake, (SLAC-0358, Dec 1989).

W.B. Herrmannsfeldt, et al., High Reso-lution Simulation of Field Emission,(SLAC-PUB-5217, Mar 1990; Contrib-uted to Int. Conf. on Charged ParticleBeams, Toulouse, France, Apr 24-27,1990).

A. Hutton and M. Zisman, An Asym-metricB FactoryBasedon PEP, (SLAC-

PUB-5268, Jun 1990; Contributed to2nd European Particle AcceleratorConf., Nice, France, Jun 12-16,1990).

M.B. James, et al., A New Target Designand Capture Strategy for High YieldPositron Production in ElectronLinear Colliders, (SLAC-PUB-5215,Apr 1990; Submitted to Nucl.Instrum. Methods).

T.L. Lavine, et al., Binary RF PulseCompression Experiment at SLAC,(SLAC-PUB-5277, Jun 1990; Presentedat 2nd European Particle AcceleratorConf., Nice, France, Jun 12-16,1990).

L. Merminga and R.D. Ruth, DynamicCollimation for Linear Colliders,(SLAC-PUB-5265, Jun 1990; Contrib-uted to 2nd European Particle Accel-erator Conf., Nice, France, Jun 12-16,1990).

Y. Otake, et al., Cavity Combiner for SBand Solid State Amplifier for theHigh PowerKlystron atSLAC, (SLAC-PUB-5179, Mar 1990).

R.B. Palmer, Prospects for High-Energye+e- Linear Colliders, (SLAC-PUB-5195, Mar 1990; Submitted to Ann.Rev. Nucl. Part. Sci.).

T. Raubenheimer and R.D. Ruth, A NewTrajectory Correction Technique forLinacs, (SLAC-PUB-5279, Jun 1990;Presented at 2nd European ParticleAccelerator Conf., Nice, France, Jun12-16, 1990).

J.T. Seeman and L. Merminga, MutualCompensation of Wake Field andChromatic Effects of Intense LinacBunches, (SLAC-PUB-5220, May 1990;Contribution to Linac Conf., Albu-querque, NM, Sep 10-14, 1990).

R.L. Warnock and R.D. Ruth, Long TermStability of Orbits in Storage Rings,(SLAC-PUB-5273, Jun 1990; Contrib-

uted to 2nd European Particle Accel-erator Conf., Nice, France, Jun 12-16,1990).

BEAM LINE 29

_ __ _ _

_ __

Instrumentation and Techniques

CRID Group: K. Abe, et al., ElectrostaticDesign of the Barrel CRID and As-sociated Measurements, (SLAC-PUB-5214, Apr 1990; Contributed to Int.Conf. on Instrumentation for Collid-ing Beam Physics, Novosibirsk, USSR,Mar 15-21, 1990).

SLD-CRID Collaboration: K. Abe, et al.,Production of 400 Mirrors with HighVUV Reflectivity for Use in the SLDCerenkov Ring Imaging Detector,(SLAC-PUB-5199, Apr 1990; Submit-ted to Nucl. Instrum. Methods).

W.B. Atwood, et al., The REASONProject,(SLAC-PUB-5242, Apr 1990; Contrib-uted to 8th Conf. on Computing inHigh Energy Physics, Santa Fe, NM,Apr 9-13, 1990).

D. Durrett, et al., Calibration and Per-formance of the MARK II DriftChamber Vertex Detector, (SLAC-PUB-5259, May 1990; Invited talkgiven at 5th Int. Conf. on Instrumen-tation for Colliding Beam Physics,Novosibirsk, USSR, Mar 15-21, 1990).

D. Gustavson, Applications for theScalable Coherent Interface, (SLAC-PUB-5244, Apr 1990; Invited talk givenat 8th Computing in High EnergyPhysics Conf., Santa Fe, NM, Apr 9-13, 1990).

R. Jacobsen, et al., DetectorBackgroundConditions at Linear Colliders,(SLAC-PUB-5205, Apr 1990; Presentedat 5th Int. Conf. on Instrumentationfor Colliding Beam Physics, Novo-sibirsk, USSR, Mar 15-21, 1990).

R. Jacobsen, et al., The Silicon StripDetector at the MARK II, (SLAC-PUB-5224, Apr 1990; Invited paper givenat 5th Int. Conf. on Instrumentationfor Colliding Beam Physics, Novo-sibirsk, USSR, Mar 15-21, 1990).

J.G. Jernigan, et al., Performance Meas-urements of Hybrid Pin DiodeArrays, (SLAC-PUB-5211, May 1990;Invited talk at Int. Industrial Symp.on the Super Collider, Miami Beach,FL, Mar 14-19, 1990).

A.S. Johnson, et al., JAZELLE: An En-hanced Data Management Systemfor High-Energy Physics, (SLAC-PUB-5231, Apr 1990; Presented at 8thComputing in High Energy PhysicsConf., Santa Fe, NM, Apr 9-13, 1990).

A.S. Johnson, et al., DUCS: A Fully Au-tomated Code and DocumentationDistribution System, (SLAC-PUB-5230, Apr 1990; Presented at Com-puting in High Energy Physics Conf.,Santa Fe, NM, Apr 9-13, 1990).

R. F. Koontz, et al., High CurentDensityPulsed Cathode Experiments atSLAC,(SLAC-PUB-5257, Jun 1990; Contrib-uted to Workshop on Short-PulseHigh-Current Cathodes, Bendor,France, Jun 18-22, 1990).

P. F. Kunz, Object Oriented Program-ming, (SLAC-PUB-5241, Apr 1990;Invited paper given at 8th Conf. onComputing in High Energy Physics,Santa Fe, NM, Apr 9-13, 1990).

A.J. Lankford, Computing and DataHandling Requirements for SSC andLHC Experiments, (SLAC-PUB-5243,May 1990; Invited talk given at 8thConf. on Computing in High EnergyPhysics, Santa Fe, NM, Apr 9-13,1990).

F.L. Palmer, et al., Oxide Overlayers andthe Superconducting RF Propertiesof Yttrium Processed High PurityNb, (SLAC-PUB-4945, May 1990; Sub-mitted to J. Appl. Phys.).

S. Shapiro, et al., Progress Report on theUse of Hybrid Silicon Pin Diode Ar-

rays in High-Energy Physics, (SLAC-PUB-5212, May 1990; Invited talkgiven at 5th Int. Conf. on Instrumen-tation for Colliding Beam Physics,Novosibirsk, USSR, Mar 15-21, 1990).

30 SUMMER 1990

FROM THE EDITOR'S DESK

\UJN THE PREVIOUS ISSUE OF THE Beam Line, we tried for ahome run and at least managed to hit for extra bases. The striking new designand expanded coverage of high-energy physics have been extremely wellreceived in almost every quarter. We thank everybody for your compliments.

For those who wondered aloud whether we could keep it up, here isyour answer. The emphasis of this Summer 1990 issue is on the constructionand physics of electron machines, a subject dear to the collective heart ofSLAC. The lead article by Burton Richter summarizes the future prospectsfor these kinds of machines; it is followed by Ronald Ruth's excellent surveyof the design issues involved in the Next Linear Collider.

In forthcoming issues we plan to continue our broad editorial coverageof particle physics in general. Feature articles are already underway or havebeen commissioned on such far-ranging topics as dark matter, SLAC's newdetector the SLD, and the Tevatron Main Injector. Other articles currentlybeing contemplated include the commissioning of HERA and a survey of theoptions for B factories.

We are actively seeking authors and contributors from throughout theparticle physics community, both in the U.S. and abroad. If you haven't beencontacted yet, don't worry. You probably will be soon. Or if you can't waitfor the phone to ring, please write me a letter describing the article you mightlike to contribute.

With this issue we are also expanding the Beam Line circulation by atleast a factor of 2, to a total of about 5000 copies. Individual particlephysicists throughout the world will be pleasantly surprised to find thecurrent issue in their mail this month. We hope you all enjoy reading it.

/W'^<^ / ,7^-l_;

BEAM LINE 31

CONTRIB UTORSI \T r- 7,1 rl-F(~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

DAVID BURKE is head of SLAC's Experimental Group I, which has beenestablished to play a major role in the construction and instrumentation ofthe Final Focus Test Beam. David came to SLAC in 1978 and distinguishedhimself in the commissioning of the SLC and as a key member of the Mark IICollaboration. As one of the new directors of the SLAC Summer Institute,he helped to organize this year's program and lectured on "ExperimentalStudies of Electroweak Gauge Bosons."

BURTON RICHTER has been the Director of the Stanford Linear Accel-erator Center since 1984. He has been active in all phases of the developmentand experimental use of colliding-beam accelerators at Stanford, startingwith the joint Princeton-Stanford electron storage rings at the High EnergyPhysics Lab in the late 1950s, through the SPEAR and PEP electron-positronstorage rings, and culminating in the Stanford Linear Collider at SLAC. In1976 he shared the Nobel Prize in Physics with Samuel Ting for thediscovery of the J/If particle.

RONALD RUTH is head of SLAC's Accelerator Theory and Special ProjectsDepartment. His research interests are quite varied and include nonlineardynamics, plasma acceleration, new ideas for high-power rf sources, coher-ent instabilities, and beam dynamics in linear colliders. A frequent lecturerat the U.S. Particle Accelerator School, he is interested in encouraging andeducating the next generation of accelerator physicists. Ron recently lec-tured at the SLAC Summer Institute on "A Design for a 1/2 TeV LinearCollider. "

32 SUMMER 1990