Nuclear Physics A497 (1989) 483c-492~ North-Holland, Amsterdam

483~

STUDY OF MULTI-HADRON REACTIONS WITH MULTI GeV ELECTRON ACCELERATOR

Gabriel TAMAS

Service de Physique Nucl6aire - Haute Energie CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

The field covered in this paper will be restricted to fixed-target reac- tions, the probe being real or virtual photons. Multi GeV will mean here photon energy below 10 GeV, because above tine effect of the Lorentz boost will confirm all the particles in the forward direction and hadrons will develop hadronic showers in matter. This will imply a completely different design of a detector. Finaly multihadron will start at 2 particles. After some considerations on the physical motivation, I will discuss the main characteristics of the detectors, then I will give some examples of the de- tection set-up presently under development around electron accelerators and propose some concluding remarks.

1. THE PHYSICS DOMAIN

Up to now we have experimental informations on inclusive quantities such as

dTOT for real photons and inelastic electron scattering. We have few data for

more exclusive reaction with one hadron detected : (y,p), (v,n), (e,e'pl,

(e,e'n), and very few results for more than one hadron : (Y,pr), (Y,pnl,

(r,pp). These experiments were mostly performed with high luminosity, small

detection acceptance (AQ = 1-30 msr, Ap/p - lo-20 %) and a good energy resolu-

tion in the case of virtual photons.

A wide field of physics remains completely unexplored : the properties of a

nuclear system which has absorbed a large energy and decays by the emission of

several hadrons. Their detection is necessary to specify the state. Figure 1

shows that the multiplicity of charged hadrons increases with energy in photo-

absorption by l*C and *08Po in the A-resonance regionl.

Many examples can be given to illustrate this remark and were developed

elsewere extensively, see for instance the ELSA [ref.*] and CEHAF [ref.3] pro-

grams. I shall just mention very briefly a few points.

1.1 Photo-absorption mechanism

The understanding of the photo-absorption mechanism passes through the study

of the partial channels and consequently the detection of several particles.

For instance the absorption by lnucleon is mainly dominated by pion production

whereas (v,pn) reactions are mostly responsible for the two-nucleon absorption

and account for 10 % of the previous one. Three-nucleon processes play also a

role of the order of a few tenth of the two-nucleon absorption, in the case of

3He [ref.+]. A similar behaviour was found with pion induced reactions5. It

0375-9474/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

484c G. Tamas /Study of multi heron-reacting

would be very instructive to look at the Q* dependence of these cross sections

to learn the structure of the absorbing objects.

I.2 Nucleon resonances

b TOTAL "CHARGES" CARBON The study of their pro- trj$=i 1,1$=2 perties is the central part * lrp.3 h2)

300, of the ELSA program and is

0 included in the CEBAF con-

Z 00 % cerned. The EZ/Ml mixing of

5 the A-resonance, the Roper T ZOO- 0 b b resonances, the N* has to be

0 0 measured more precisely on

the free nucleon. But their

behaviour in light nuclei is

also of great interest : for

instance to learn about AN

or N*N interaction. Above

4 1OTAL"CHARtES" LEAD the two-pion production

threshold, it is possible to

tag the A++ by the n' with 300, t'ne elementary reaction y+p+.

si (pf~+)~++ + 71-. It is possi-

1 ble also to sign a resonance

4 ZOO- by its decay products, for & instance the n for N(15351,

N(16501, N(1710).

1.3 Production of heavy

mesons

I have already mentioned

the T-,. &ctor mesons which 100 200 300 400 500

$‘PleW play a very important role

in the nuclear force can be

FIGURE 1 photo-produced in the for-

ward direction where sha- dowing dominates, but also at large angle *here the diffractive amplitude be-

comes very small and can leave room for nuclear processes6.

I.4 Strangeness

It is a very interesting and exciting physics with a new flavour in the pro-

blem3. The K and z will be mainly produced in the forward direction. Beside the

hypernuclear physics, the A-N, n-nucleus interaction study will necessitate the

A decay product detection. This will automatically give the A polarization di-

rection (the same remark is true for the A, the p . ..I.

G. Tarnas /Study of multi hadron-reactions 4852

All these examples demonstrate the necessity of multi-hadron detection in a

very large kinematical domain. With conventional spectrometers, it is only

possible to explore a very limited part of the phase-space in the case of two

hadrons in the final state. But some major problems occur then : it is diffi-

cult to move easily more than two spectrometers around the same pivot. The out

of plane meastirements present some serious questions, only partially solved.

And the counting rate varies as (E AQ/~x)~ where E is the detection efficiency,

ASZ the solid angle and N the number of hadrons. Obviously we lose interesting

information (for example the A or A decay plane is related to the direction of

the polarization). For more than two hadrons, it is quite impossible to use

conventional spectrometers. But nevertheless a very interesting physics lies

there, in inclusive informations obtained from exclusive measurements by inte-

gration over kinematical variables (angles, mass . ..I. The conclusion seems

obvious : we need large acceptance detection devices.

2. QUALITIES REQUIRED FOR A DETECTOR

The overall structure is schematized on figure 2. The central part is a

vertex detector determining the emission angle of the charged reaction products

surrounded by a volume used to measure the momentum of the charged particles

and their identification. At the outside a calorimeter detects the neutral par-

ticules (angle, energy). The detector should be provided with a hole in the

beam direction to avoid the effects of the electromagnetic shower.

Calorimeter neutral I

Hole for EM shower

domentum energy charged

FIGURE 2

It is impossible to give in these few pages a complete discussion and I will

just present a list of the main parameters entering in the design of a detector.

1) Accuracy of the localization

The determination of the trajectory constraints strongly the kinematics.

Depending on the technique (+X3, MWPC, drift chambers, scintillating fibers)

the spatial resolution varies fro111 a few micrometers to a few millimeters.

486~ G. Tamas / Study of multi hadron-reactions

2) Energy and mo~ntum determination for charged particles

i) Without magnetic field : total absorption or range measurement. The reso-

lution is rather poor lop/p > 5 %I limited to low energy because of nuclear

reactions.

ii) With magnetic field :

- Dipole transverse : dead angle in the pole direction. The charged component

of the electromagnetic shower is brought back in the detector.

- Solenoidal configuration with field parallel to the beam.

- The toroidal configuration has nice features : magnetic field is zero, the

trajectories remain in 6, = cte plane, the resolution is independent of e and

the overall weight is lighter than in the

solenoidal case.

3) Charged particle identification

- At low energy E x dE/dX or p x dE/dX.

- At higher energies time of flight (TOF)

and momentum.

- Relativistic particles : Gas ?erenkov

counters, ring ?erenkov imaging.

- For stopped unstable particles : detec-

tion of the decay products.

4) Neutral particles

- For neutrons ; plastic scintillator +

TOF.

- For y (mostly from 7~~ decay), high Z

scintillators, lead glass scintillators.

Also shower counters associated with 1ocaA

lization detectors allow precise angular

determination and consequently z" energy

measurement.

5) Multiplicity of detected particles

determines the granulometry of the detec-

tor (figure 3).

6) Limitation of the dead angle in the

forward direction where high energy parti-

cles are emitted because of the Lorentz

boost. Association with a dedicated detec-

tor for these small angles.

7) Blarization : how it is possible to

include a polarized target in the set up?

GRA#ULOMETRY

104

N

1 0 0.5 1

P(N,P)

FSGURE 3 - P(N P) = ',Bp. Proba- bility to find all 9 particles (uncorrelated) in different cells for a division of the detector in

N independent cells.

G. Tamas /Study of multi hadron-reactions 487c

8) Luminosity

- For tagged real photon beam (- 107/s), no problem.

- For virtual photons : is it possible to send an electron beam through the

detector and to reach a luminosity of 1033-1034 s-l?

9) Complexity

The detector will deliver a tremendous quantity of data. It is then necessa-

ry to keep the useful information by the selectivity of the trigger (various

levels) : for small cross section, rejection of the major dominant contribu-

tions. The data acquisition will be controlled by dedicated microprocessors for

each part and will necessitate an on-line software reduction of the data.

Complexity means also man-power.

10) The cost depends on the qualities required. Put all together it can be

very expensible.

We see that these two last points will bring serious limitations to the

realization of an universal detector, a compromise will certainly be necessary.

It have to be adjusted to the experimental program.

3. DETECTORS UNDER DEVELOPMENT

3.1 Without magnetic field

They are low energy detectors, the momentum measurement and the particle

identification are given by total energy measurement or TOF with dYdX determi-

nation.

A) Partial solid angle coverage (around the horizontal plane)

They are limited to very specific reactions with two detected particles :

- PHOENICS (Bonn)2 : study of nucleon resonances on IH, 2H.

- A2 collaboration (Mainz) : (y,y), (y,n”).

- LEGS (BNL) : (v,PP), (v,npl.

These set-up are simple, easy to reconfigurate and to insert polarized target.

B) 4Tc-E

1) For neutral particles (~lO,v) crystal ball : (9Fbme-Genova-Frascati colla-

boration) constituted of 480 BGO pieces (22 radiation length) with a 6 20 cm

hole for the beam.

2) For low range particles (BonnI : a high pressure gass target surrounded

with cathode read-out drift chambers and a trigger of plastic scintillators. It

is very sensitive to the low energy particle emitted after photon absorption.

3) At Saclay we have developed a large acceptance detector7 consisting in 3

parts : a vertex detector (three cylindrical cathode readout MWPC) for charged

particles, a ExdY dX detector (three cylindrical coaxial plastic scintillators

of respectively 10, 100 and 5 mn) and a photon (no) detector (a cylindrical

488~ G. Tamas ,I Study of multi ha&on-reactions

lead convector and two 5 mm cylindrical plastic scintillators separated by a 6

mm aluminum absorber (figure 41.

Hadron Detector with a Large Acceptance (94% of 4n)

==iZ=& 2’_ _L.lc_ Wire Chambers -c;;

Sci

Longitudinal Cross-Section 30 cm Transversal Cross-Secti

-I

Perspective View

Drawings made with the Programme "@ANT"

FIGURE 4

on

3.2 With a magnetic field : closer to spectrometers

- SAPHIR (Bonn)2 has a dipole magnetic field which deflects the E.M. shower

inside the detector which implies a limitation of the luminosity. But ELSA is a

low intensity accelerator.

G. Tames /Study of multi hadron-reactions 489c

- TOROIDAL SPECTROMETER8 (figure 51 : With no field on the beam axis and at the

target (polarized targets can be set there), it has focusing properties and its

focal plane is not seen directly from the target. This would allow a high lumi-

nosity.

0.5BGeV 0.75GeV 1GeV

2.5E3

2.OE3

becteur

eau -

0.5E3 l.OE3 1.5E3 2.OE3 2.5E3 3.OE3 3.5E3 -

X

t

O=Zm centrifuge ~=O.XSSQ

Y

FIGURE 5

- LAS (CEBAF)3'g is a superconducting toroid (6 coils). The tracking of the

trajectories is provided by drift-chambers and the particle identification by

TOF and momentum determination. It presents a very large space around the tar-

get without magnetic field allowing the insertion of polarized target. By mo-

ving the target position (figure 6) it improves focusing properties. The toroid

will be also surrounded by shower counters.

4. CONCLUDING REMARKS

It is very difficult to find an universal solution for a detector able to

measure all kind of reactions, because of the complexity and the cost. More,

for a given problem, the useful information will be swamped by relevant data.

So we have to be specific and conceive detectors, well fitted to the beam cha-

racteristics, with definite goals, with of course a wide range of possible

applications.

49oc G. Tamas / Study of multi hadron-reactions

FIGURE 6

It would help to design a detector as flexible as possible : modular, easy to

reconfigurate "A la carte" to do your own application, and perhaps associated

with other detectors as schematized on figure 7.

-----_ ---

\

Neutron

I 1111 I 11 II1

FIGURE 7

REFERENCES 1) M.L. Ghedira, These d'Etat, Universitg de Paris-Sud (1984).

21 G. Anton et al., BONN-IR-87-30 (1987).

3) See the CEBAF publications : CEBAF Workshops, CEBAF Summer Study Group, Physics Program at CEBAF.

G. Tamas /Study of multi hadron-reactions 491c

4) N. D'Hose, These d'Etat, IJniversitG de Paris-Sud (1988) ; N. D'Hose et al., to be published.

5) J. Bockenstoss et al., Phys. Rev. Lett. 55 (1985) 2782 and 59 (1987) 767 ;

K.A. Ariol et al., Phys. Rev. C33 (1986) 1714.

6) G. Tamas, Journges de Physique Nucleaire sur la photoproduction et l'r?lec-

troproduction de mesons sur le nucleon et les noyaux, Universits de Clermont

Ferrand, November 1986, p. 90 ; Rapport DPhN/Saclay 2419 (1987).

7) J. Martin, Communication to Perspective in Nuclear Physics at Intermediate

Energies, Trieste (Italy), June 18-22, 1988.

8) P. Vernin, private communication.

9) V. Surkert et al., States of the design of the CEBAF large acceptance spec-

trometer, October 1987.