Paul Grannis

Stony Brook University

IBM Seminar Oct. 12, 2007

Developing the paradigm ‘Standard Model’

Where we are now

Future directions at the Terascale (1 TeV = 1012 eV)

Particle Physics at the

Crossroads

?

?

?

The scene in the ’60’s

EM – described by QED. Long range via massless photon exchange.

Strong force – attractive, short range; binds nuclei and hadrons. Exchanges of mesons etc.etc.

4 microscopic fundamental forces between matter particles, transmitted by boson. EM interaction (QED) is the prototype. Spin-1 photon couples to the electric charge of the matter particles.

Weak force – radioactivity; solar reactions; particle decays. Very short range. Heavy W bosons?

Gravity – very weak, long range; no quantum theory. Massless gravitons?

f1

boson exchange

coupling constant

f2

f3 f4

No connection or unification among these forces.

time

Hadrons = particles that feel the Strong int’n: Half integer spin baryons (p, n, etc.) and integer spin mesons like +, , K … )

Leptons = particles that feel the Weak interaction but not Strong. Both charged (e, ) and neutral (neutrinos, e , )

Particle classification

Q2 = momentum transfer2 carried by exchange quantum

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The scene in the ’60’sStarting from a few ‘elementary’ hadrons – proton, neutron, meson etc., hundreds more burst on the scene in the ’60’s.

The bewildering array of quantum numbers (charge, parity, spin etc.) and complex interactions defied easy explanation.

1 of many pages of particle

listings(p,n, , K, , , N*, , , , A1, A2, B, , , , … we ran out of Roman and Greek letters and still they came!)

Often particles come in multiplets with similar mass & properties except for charge ‘isotopic spin’ internal symmetry (I) with same SU(2) group properties as spin. Family with isospin I has (2I+1) members. e.g. I=1/2

Some particles were ‘strange’ – produced strongly in pairs but slow to decay (via weak int’n). Must be a new quantum number: strangeness, S, conserved in strong but not weak interactions.

nucleon: p has IZ = +½ , n has IZ = -½. Similarly, I= 3/2 : (++, + , 0 , ) and I=1 pions (+, 0, )

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Building the Standard Model – quarksIn early 60’s, Gell Mann and Zweig noted that observed hadrons could be built from smaller entities – quarks – to build the known hadrons. Three ‘flavors’ of quarks fill the fundamental representation of SU(3). Three antiquarks in conjugate representation.

Mesons made of quark and antiquark: eg = (du) . Baryons from 3 quarks: eg n = (ddu)

Quark model explained known (and missing) hadrons, but quarks not observed, so seemed like bookkeeping artiface. Some states [e.g. - (sss)] are fermions but with totally symmetric wavefns = statistics problem.

baryon octet

n p

S

IZ

S

IZIZ=-1/2, S=0 IZ=+1/2, S=0

IZ=0, S=-1

In 70’s SLAC expts on e p scattering at large Q2, found evidence for point-like objects within the proton, with charges 1/3e and 2/3e , just like the constituent quarks. These expts were a direct analogy with Rutherford -Au scattering.

So the quark hypothesis seemed to have physical validity after all. But why were there no free quarks observed?

e e

p q q

SSSS

q

d u

s

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Building the SM – QCDPuzzles resolved by development of QCD, a local gauge theory like QED but based upon a new SU(3) ‘color’ symmetry. Analog of is a set of 8 massless ‘gluons’ which couple to ‘color charge’ carried by quarks. Unlike QED, the gluons have color themselves, so couple also to other gluons. Each quark flavor (u,d,s) comes in 3 distinct colors (R, G, B). Observed mesons and baryons are color-less combinations with antisymmetric color wavefns (solves statistics problem). Compilation of

many experimentsS

√Q2

Key QCD property: coupling ‘constant’ S ∞ at small Q2 or long distance (infrared slavery) and logarithmically 0 at large Q2 or small distance (asymptotic freedom). So, quarks do not emerge freely, but fragment to ‘jet’ of colorless hadrons.

As Q2 increases (a more powerful microscope), see more resolved substructure within the proton. This causes a variation of the coupling as a function of Q2 and deviation from simple ‘Rutherford’ e p scattering – observed in expt.

qqqg g

SSSS

q

e e

p q q

SSSS

Quark and gluon dist’s in the proton have been mapped to Q2 ~106 GeV2 (down to ~1am).

1973: Gross, Politzer, Wilczek

(Q=momentum of ) 29

Building the SM – weak bosonsThe 4 fermion ‘charged current’ interaction of

Fermi with (V-A) currents qualitatively explained observed weak interactions.

JH

JL

n p

e

neutron decay

n p

eW+

s-wave unitarity violations, though delayed, still occur (e.g. in → W+W- at high energy).

Weinberg and Salam (1973) predicted weak neutral currents. These imply massive neutral Z0 boson. Such events seen.

pp

Z0

CC event

NC event

The W± and Z0 were discovered in 1983 (Rubbia, van der Meer) with about 100x proton mass (81 and 91 GeV/c2 respectively)

But the 4 fermion interaction violates unitarity for energies above about 600 GeV. In analogy with QED, postulate spin 1 boson carrier, W+. The W+ must be heavy to give the short range observed for the Weak Int’n, thus theory non-renormalizable.

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Building the SM – Electroweak Int’nPropose unification of Weak and EM

interactions. Postulate four fundamental massless spin 1 gauge

Unifies the EM and Weak int’n into Electroweak Force and fixes unitarity and renormalizability – but alas does not give massive W and Z bosons!So, introduce (ad hoc) a complex doublet of spin 0 Higgs fields – one pair (, *) is neutral and one (, ) is charged.

(isospin 0)

() (isospin 1)

bosons within a U(1)xSU(2) (both EM and Weak) group symmetry.

Glashow, Salam, t’Hooft, Weinberg, Veltmann,

= cosW + sinW Z = -sinW + cosW

The symmetric theory is spontaneously broken – the 0 and 0 states mix. In the process, the ± and 0 acquire mass by absorbing 3 of Higgs fields to become W± and Z, whereas one combination () remains massless. One Higgs field is left, giving a physical Higgs boson, not yet observed but constrained: 115< mH <150 GeV. Quarks and leptons also acquire their mass from Higgs coupling.

A familiar example: an external B field breaks the spatial symmetry in a ferromagnet.

The Higgs mechanism is a Rube Goldberg device? You bet – but many experiments agree with predictions!

B

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By 1973, 3 quarks (u,d,s) and 4 leptons (e,e,, ) had been seen. But theory diverges if Nq ≠ Nl1974: BNL & SLAC experiments see narrow

resonance at 3.1 GeV, bound state of charm quark-antiquark pair.1976: FNAL expt sees dimuon resonance at 9.5 GeV interpreted as bottom (b) quark - antibottom state.1976: SLAC finds lepton (1.8 GeV), partner to e, and infers the related (not seen directly until 2004)1995: FNAL experiments discover top quark at mass 175 GeV (~Au nucleus - but no substructure!).

Completing the quark & lepton lineup

1998 : Japanese exp’t shows that neutrinos have non-zero mass ( oscillates into e or ).

e+e→ hadrons

J/

pp→ +-

p p → t t

tdowngoing

upgoing

/e (obs/exp)no

oscillation

4 q, 4 l

5 q, 4 l

5 q, 6 l

6 q, 6 l

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The Standard Model

e

e

Z

SU(3)

Strong (QCD) Electroweak

12 fermion masses

3 lepton flavor isospin doublets (generations). Mass eigenstates are rotated from flavor states: transformation matrix VW.

W±

g

+ the other 2

color setsu

d s

c t

b

(3 generations is minimum needed for CP violation as seen in nature)

3 quark ‘flavor’ isospin doublets (generations). The weak int’n quark states ≠ strong states: Mixing matrix VQ connects.

Interactions/force carriers:

x SU(2) x U(1)

+ the other color gluon states

Fermion (‘charge’ carrying) matter particles

26 arbitrary parameters:8 mixing matrix parameters 3 force couplings2 EW boson masses

+ 1 strong CP phase

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etc. etc.

The Standard Model has withstood 1000’s of tests

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1. Why those 26 ad hoc parameters? (Arbitrary, but they matter.)

If m(u) > m(d), proton (uud) is heavier than the neutron (ddu) and thus proton decays : stars, cosmic microwave background, H atoms, CM physics and people don’t exist.

Why do fermion masses vary by 10 orders of magnitude? Lepton and Quark mixing matrices are very different.

So whats wrong with the Standard Model?

g1

g3

g2

close but no cigar

No unification

2. SM permits CP violation, but not enough to explain why there is the huge asymmetry between number of baryons and antibaryons in the universe. A new source of CP violation is needed.3. The Strong and EW interactions are just pasted together in SM. If extrapolate the three coupling constants to high energy, they come close to a common value at the grand unification scale ~ 1017 GeV

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So whats wrong?4. Higher order quantum corrections (loop diagrams) would

cause the Higgs, W, Z boson masses to diverge to Planck or grand unification scale unless there is some fantastic accidental tuning of couplings to keep these at TeV scale. (fine tuning / hierarchy problem)

6. Dark energy, pushing the universe apart in the present epoch, has no explanation in the SM.

7. The SM would give (energy density due to cosmological constant) be O(10120). One might understand some new symmetry causing it to be zero, but ~ 1. The biggest fine tuning problem of them all !8. Gravity is not included in SM

5. Galaxies show substantial dark matter, also evident in early galaxy formation. DM seems to be massive particles, left from the early universe. SM provides no candidate.

W, Z, H masses are hereBut would tend to here

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First we need to find what plays the role of the Higgs boson to break EW symmetry. Moreover, to solve the SM defects (fine tuning of Higgs mass, provide dark matter particle, unify the forces … ) there needs to be new physics at few 100 to 1000 GeV – the Terascale.

The new theory must reproduce the successes of the SM while adding new ingredients – much as Quantum Mechanics gives Classical Mechanics in the correspondence limit.

There are several classes of theoretical models suggested for the new paradigm: New symmetries of nature New forces and particles New kinds of space dimensions

Each model class has many variants

So, despite the SM successes we strongly believe it must be superceded

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And there are two demonstrated new accelerator colliding beam facilities that will give a complementary view of the new terrain:

The Large Hadron Collider (LHC), will start in 2008 at CERN: proton-proton collisions at ECM = 14 TeV

The International Linear Collider (ILC) being designed by global international collaboration: e+e- collisions at ECM = 0.5 – 1 TeV.

An experimentalist’s dream !

Go there and find out!

We know there is a new playing field at the Terascale, but have no idea who the players are, or what the rules of the game might be.

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ILC

proton proton

e- e+

High energy reach Broad range of quark/gluon energies simultaneously (ECM not fixed) Large event rate Large QCD backgrounds Don’t know initial state quantum #s Event pileup – spectator quarks & other pp collisions Radiation damage issues

Known initial quantum state Well-defined ECM and pol’zn low bkgd → allows ambitious experimental techniques Event rates low; need sequential runs at different ECM and polarization Complex machine detector interface; need exquisite control of beam optics

LHC & ILC collider characteristics are highly complementary

Colliders for the energy frontier

LHC

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The LHC

The 14 TeV (ECM), 27 km circumference Large Hadron Collider (proton-proton) at CERN on the Swiss-French border – complete in 2008. The LHC will be the highest energy accelerator for many years.

Mt. Blanc

Lake Geneva

But …

The protons are bags of many quarks and gluons (partons) which share the proton beam momentum. Parton collisions have a wide range of energies – up to ~2000 GeV. Initial quantum state is not fixed.

airport

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The International Linear Collider (in planning)

main linacbunchcompressor

dampingring

source

pre-accelerator

collimation

final focus

IP

extraction& dump

KeV

few GeV

few GeVfew GeV

250-500 GeV

Linear to beat synchrotron radiation (~E4/R). Just one pass of beams (but electrons are cheap).

Collide beams with energy tuneable up to Ecm = 500 GeV (upgrade to Ecm = 1000 GeV). Two identical linear 11 (20) km long accelerators, bringing beams to head-on collision in 6 nm high spot.

Damping rings (wigglers) cool transverse phase space; bunch compressor squeezes longitudinal bunches.

Superconducting rf acceleration (35 MV/m) at f=1.3 GHz in main linacs. Layout of electron arm

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1. Find the agent for Electroweak symmetry breaking – in the SM, the Higgs boson.

The ILC will tell us if what LHC sees is the SM Higgs or some surrogate. It can detect the ‘Higgs’ even if it decays into invisible particles. It can tell us the Higgs quantum numbers, and its couplings to different particles.

collision energy

inte

ract

ion r

ate

Curves denote different Higgs boson spins; ILC data cleanly discriminate.

The LHC should discover the Higgs if it exists up to >1 TeV (10 times higher than SM-indicated value). Fermilab still has a shot!

The physics program for the LHC and ILC

Higgs mass →

signifi

cance

e+

e-

,Z Z

H

16

The Higgs couplings to other particles distinguishes different models.

In the SM, Higgs couplings to fermions, W/Z are directly proportional to mass; they differ in other models. Measuring these couplings to few % level is a sensitive test of whether we have the SM or some new physics. LHC can only get to few 10’s%, for only some couplings. ILC can do to few %. Particle mass →

Yu

kaw

a

couplin

g

Extra dimension model New symmetries model

SM

value

2. Mapping the Higgs boson

LHC/ILC Physics

Points for different H decay modes, relative to SM.

ILC precision

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3. New SymmetriesSupersymmetry (SUSY) introduces new fermionic space-time

coordinates, resulting in a new boson for every existing SM fermion and vice versa. (Partner of the spin ½ electron is a spin 0 selectron, etc.). In unbroken SUSY, selectron mass = electron mass etc. We know this is not true, so SUSY is a broken symmetry. All the other properties of the selectron are like the electron (charge, couplings). There are many model variants, and many parameters, so it will be difficult to unscramble.

SUSY boson and fermion contributions to Higgs mass cancel those from SM particles, so the hierarchy / fine tuning problem is solved.

SUSY has a natural dark matter candidate.

SUSY could provide the CP violation needed.

SUSY modifications to SM predictions are small, so not in conflict with data.

LHC/ILC Physics

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The LHC and ILC have complementary strengths in mapping the SUSY spectrum – LHC sees quark and gluon partners; ILC sees lepton and W/Z/Higgs partners. Together they can extrapolate to the scale where SUSY is broken and tell us how that happens.

SUSY provides a good candidate for DM (lightest SUSY particle). LHC and particularly ILC can determine its mass and density.

Mass and coupling unification pattern deduced from ILC & LHC reveals how SUSY broken. (Plots show two model possibilities).

energy →

4. Learning about Supersymmetry

LHC/ILC Physics

LHC

ILC

Compare with cosmic microwave bknd, underground DM experiments to see if the picture is consistent.

DM mass

DM

cosm

ic d

ensi

ty

13

5. New forces

New forces and the particles they introduce provide a new energy scale. This would stabilize the hierarchy problem of the SM.

The prototype candidate was a new interaction similar to QCD (“Technicolor”) with new particles at O(few TeV). The simplest of these models would however produce deviations from the SM that are not seen, but many more complex variants exist.

These models give new quarks, bosons, ‘leptoquarks’, etc. that would be seen at LHC and ILC.

dimuon mass

pro

duct

ion r

ate

An example: a new higher mass Z boson seen at LHC

LHC/ILC Physics

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6. Hidden dimensions

String theory requires at least 6 extra spatial dimensions (beyond the 3 we already know). The extra dimensions are curled up like spirals on a mailing tube. If their radius is ‘large’ (~1 attometer = billionth of an atomic diameter) or larger, they could lower the effective Planck mass, eliminate the hierarchy problem and unify all forces (including gravity?) at the new Planck scale.

If a particle created in an energetic collision goes off into the extra dimensions, it becomes invisible in our world and the event shows missing energy and total momentum imbalance.

e.g. e+ e + ‘nothing’ at ILC

LHC/ILC Physics

11

Combination of data from LHC and ILC allow the determination of the reduced Planck scale and the number of extra dimensions.

dimuon mass

pro

duct

ion r

ate

Wavefunctions trapped inside a ‘box’ of extra dimensions yields a series of resonance states (like new heavy Z bosons). At LHC, these are indistinguishable from other possible sources.

ILC measurements of the couplings (vector and axial vector) allow us to distinguish what new physics is operating. Different models have quite different couplings.

7. Untangling New Dimensions

LHC/ILC Physics

This?

or this?

or … ??

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4 ways to produce a ‘signal’ with same final objects: jet (quark), 2 leptons & missing energya) & b) SUSY with different choices of

dark matter particle (lightest SUSY particle) = partner of photon or of neutrino.

c) & d) Extra Dimensions models with different character of excited Z.

These all look the same at LHC.

At ILC, the cross-sections and angular distributions for specified initial state polarizations tell us which is happening.

This information can in turn be used by LHC to deduce the heavy particle masses.

Observed final particles

Example of ILC and LHC complementarity

Crudely, LHC discovers and ILC discoverss what the discovery was. 9

Detectors at LHC and ILC

Conceptually “simple” – charged particle tracking, surrounded by calorimeters (particle energies), surrounded by muon detectors.

In detail, very large (to contain the high energy particles), and complex (to identify and measure objects precisely). Detector collaborations of >1000 people from many nations.

at LHC

8

CMS detector at LHC

The important things to note about all the postulated models of new physics:

All known models have observable new phenomena within reach at the LHC and ILC.

“Pardon me, I thought you were much farther away”

Each model class has many variants, each with a large degree of freedom of parameters. The LHC and ILC are needed to give a complementary, binocular view of new phenomena. Together, they will tell us much more than either alone.

Is there a discovery guarantee?

But of course Nature could be more cunning than we, so we eagerly await the early results from LHC to help certify the case for the ILC.

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Politics of big science: Planning the ILC The most important step was working out the detailed physics case along the lines I’ve outlined. The ILC has been driven by a science-based consensus from the field (not as for ITER, top down from government).

1. Exploit LHC opportunity2. Plan program to become world-leading center for R&D for ILC,

and do what is necessary to mount a compelling bid to host in US. 2003 & 2007: top of the list of DOE Office of Science new intermediate term initiatives

Similar exercises in Asia and Europe

Internationalize from the start: Asia, Europe, Americas are equal partners. Critical decision on technology (SC vs. room temp. rf) made by 2004 international panel. Oversight Steering Committee equally represented by all regions. (Learn the lessons from the SSC failure.)

US

Canada

FranceGermany

ItalyPolandRussiaSpainSuisse

UKetc

China

India

Japan

Korea

6

2006 US National Academy Report (members from industry, academe, other sciences, other physics disciplines):

Obtain high priority from high-level advisory panels.

Planning the ILC Formed international team “Global Design Effort” in 2005 to guide R&D and design effort. 65 members equally from all regions. No central home – a virtual lab with constant collaboration and a well defined project management.

Value estimate in international units: cost of delivered materials, services, average civil cost for 3 sample sites, manpower. No contingency, escalation, taxes, detectors, operations costs as national accounting practices vary widely on these.

4.80x109 ILCU technical systems

1.82x109 ILCU site-specific costs

14.1K man yrs

(1 ILCU = $1 F07)

Expect Host region to pay ~1/2; other two regions ~1/4 each. Need process for selecting site to proceed in parallel with Eng Design. R&D expenditures ~equal in 3 regions ($60M in US in FY08)

Ref. Design is basis for developing a full Engineering Design (2010), guide R&D to prove principles, industrialize key technologies. GDE is reorganizing for the Eng. Des. phase, assigning project responsibilities, ‘projectizing’ the work plan.

Ref. Design/Value Est formally received by ICFA (subcommittee of IUPAP) and international funding agencies group.

Or herding cats …

Feb. 2007: Deliver Reference Design Report (detailed conceptual design), Value Cost Estimate, physics program, detector conceptual designs and a generally accessible brochure.

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http://www.linearcollider.org

Technical challenges & payoffs

35 MV/m (Q>1010) cavities have been achieved, but not reliably. Processing the Nb surface is a black art, not yet reliably industrialized. Much of R&D will be toward high yield industrial fabrication. Damping rings prepare very low emittance beams (synchrotron radiation in wigglers) needed to achieve the small collision spot (6 nm). Challenges for controlling instabilities (e-cloud, fast ion

Polarized positron source (helical undulator, e+e, remote handling rotating target, e+ magnetic capture device)

High power high rep rate laser for polarized e

Dynamic feedback on nm scale for beam position control, ground motion correctors

Final focus optics, beam dumps, experiment interface, high precision and energy, luminosity, polarization measurements bunch by bunch

instability), low emittance tuning, and fast kicker magnets (3ns rise/fall time) for extraction.

electropolished

4

Wider impacts - is it worth it?The primary motivation for the LHC and ILC is the large scientific discovery potential.

But, there are scientific benefits outside particle physics. Accelerators are now basic tools for material science, nuclear physics, chemistry, environmental science, structural biology, plasma physics … Techniques developed for LHC & ILC will enable a new generation of light sources, XFELs, ERLs, rare ion accelerators and neutron sources.

Such large projects are magnets for drawing young people into science.The impact on the broader society are much harder to predict.

"I think there is a world market for maybe five computers." – Thomas J. Watson (1943)

Gladstone, Chancellor of the Exchequer, asking about Faraday’s discoveries of electric induction:“But after all, what use is it?” Faraday: “I do not know sir, but soon you will be able to tax it.”

But one can see potential wider applications – high power monochromatic X ray sources (Compton backscatter e from lasers), medical diagnostics, isotope production, nano-lithography, container scanning, shipboard defense, linac-based fusion and waste transmutation …

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The outlook

The Standard Model and measurements in hand provide a vista of new unity and interconnectedness within the microscopic world.

go here with LHC/ILC sense whats happening here

The experimental tools to take us there are in hand. LHC will start next year. The ILC prospects have improved steadily but the project depends on what we find at LHC and has yet to be approved by world governments. ILC is expensive and must be a fully international collaboration. 2

Over the course of 40 years, our understanding of the fundamental forces and constituents of matter has been revolutionized.

The SM paradigm is about to be broken in ways that we cannot predict. The next generation of experiments will tell us a fascinating new story.

A truly exciting time for particle physics !

Summary