Nuclear Physics A441 (1985)415c478c North-Holland, Amsterdam

415c

THE TRANSVERSE ENERGY DISTRIBUTION IN PROTON-LEAD COLLISIONS

T. Akeaaon”, Y. Cho?‘, P. Darn*). G. DiTorre*‘, C. Fabjan’), A. Franzn,

C. Grupen”, H. Kowalaki ‘), R. Kroege?‘, S. Lloyd”, D. Mellor4’, H.J. Meyer”,

U. Mjbrnmark”, R. Qdingen ‘I, L. Olsen”, Y. Oren%, U. Schiifern, A. Schlliaaer’), R. Wigmana*)and W.J. Willis*)

I) University of Bonn, Fed. Rep. Germany.

*) CERN, Geneva, Switzerland.

“DESY, Hamburg, Fed. Rep. Germany.

‘) University of Oxford, UK. ‘) University of Pittsburgh, PA, USA.

‘) Queen Mary College, London, UK.

‘) University of Siegen, Fed. Rep. Germany.

*) University of Syracuse, NY, USA.

9, University of Tel Aviv, Israel.

(Presented by Achim Franz, Department of Physics, University of Siegen, Fed. Rep. Germany)

The distribution in transverse energy, ET, has been measured in p-Pb collisions at 200 GeV/c

incident proton momentum. The data cover the pseudorapidity (7) range 0.6 to 2.4 in the

laboratory frame. The ET distribution extends to 50 GeV, which is nearly 2.5 times the kinematic

limit for the pp centre of mass. Furthermore, the distribution of ET in rj is found to shift towards low rapiditiea with increasing total ET.

1. INTRODUCTION

As part of a series of calorimeter studies in preparation for the HELIOS experiment fNA34) at the

CERN Super Proton Synchrotron f.SPSj, we have measured the transverse energy, ET, distribution in

p-Pb collisions at 200 GeV/c incident proton momentum.

2. THE EXPERIMENTAL SET-UP

The spectrometer was placed in the HE beam of the CERN SPS. It consisted of two blocks of

uranium-copper acintillator calorimeters (Fig. 1). One block was the ‘calorimeter wall’, 4X deep,

7 m2 in area, covering the pseudorapidity [v = -log [tan (8/2)1] range 0.6-2.4 in the lab. frame. It

was used to measure the transverse energy, by summing up the weighted energy content per

readout unit (tower). The very forward region was covered by the ‘beam-calorimeter’, 8X deep, to

monitor the beam energy. The-front face of the calorimeter wall with the calorimeter granularity of

- 20 x 20 cm* (one tower) is shown in Fig. 2; constant r) contours for a target placed 1 m away

from the wall are also shown.

The basic element of the calorimeter is the module (stack)’ shown in Fig. 3. The front

[electromagnetic fe.m.ll part of the stack is made of 2 mm uranium plates alternating with 2.5 mm

scintillator. The rear fhadronic) part is a mixture of 3 mm uranium with 5 mm copper in the ratio 2:l

and 2.5 mm acintillator after each plate. The acintillator is read out by wavelength shifters placed on

0375-9474/86/%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

416~ i? Akesson et al. / The transverse energy distribution

FIGURE 1

FIGURE 3

4-L-F ’ ‘1 i, i .I i i ‘: I

W‘\ ‘\i ‘\I i II

- 2c.m

FIGURE 2

FIGURE 4

both sides of the scintillator plates. The e.m. and hadronic parts are read out separately. The

resolution at the time of our test was olfi = 0.22 for e.m. and cl& = 0.52 for hadronic showers.

The trigger (Fig. 4) consisted of a pretrigger and a second-level ET trigger. The pretrigger

consisted of

i) a veto counter, to protect the calorimeter from beam halo,

ii) a beam telescope, to define the particles hitting the target,

iii) an interaction counter, to check on the event multiplicity.

A coincidence in the beam telescope measuring the number of incident beam particles on target was

used for the normalization of the ET distributions. The pretrigger was sensitive to - 50% of the total

T. Akesson et al. / The transverse energy distribution 477C

inelastic cross-section. For the second-level trigger we had an on-line summation of the weighted

anode signals from the calorimeter photomultipliers, giving an on-line measurement of ET.

3. RESULTS

The transverse energy distribution for 200 GeV/c protons on Pb targets is shown in Fig. 5. The

data were taken with different ET thresholds, and the data from different thresholds overlap

satisfactorily. We have observed events with a total ET up to 50 GeV and some events even higher.

The lines in Fig. 5 indicate different kinematic limits:

i)pp : 6 = 19.4GeV,

ii) p(6n): 6 = 47.8 GeV,

iii) pf8n): v’s = 55.3 GeV.

To explain the data by this kinematic argument, the proton has to interact with an object in the

nucleus having at least 6 or even 8 times the mass of a nucleon.

Two different target thicknesses, 5% and 1% of an interaction length, were used to check that

the high ET is not produced by multiple interactions in the target. There is no difference between the

two data samples over the whole ET range as indicated in Fig. 6.

Pile-up, i.e. two events produced by two beam particles taken as one, can fake a high ET event.

This was excluded by checking the total energy per event. The distributions have been corrected for

background, i.e. events that do not originate from the target, using data taken with no target in the

beam line.

The distribution of ET as a function of 7 in different bins of total ET is shown in Fig. 7. For the first

bins (ET < 15 GeV) most of the ET is observed in the forward region, while with increasing total Er

most of the energy is found at low 7.

FIGURE 5

47% T. Akesson et al. / The transverse energy distribution

~k-F%e-+Oke+-eY- g=-LOG(TAN(d/2.))

FIGURE 7

:”

FIGURE 8

4. MONTE CARLO

For the detector response we used a Monte Carlo program based on the event generator

HIJET ‘, and a simulation of the calorimeter response using a modified version of the shower

parametrization of Bock et al.‘. To get a true description of our data for the detector corrections, we

weighted the original Monte Carlo results to reproduce the slope of the ET distribution observed in

the data.

The HIJET model describes the proton-nucleus collisions as a sequence of independent nucleon-

nucleon interactions using the ISAJET model4 to treat the individual collisions. The only particles

that are allowed to reinteract after a collision in the target nucleus are the leading baryons. The ET

distribution produced by this model together with our data are shown in Fig. 8.

5. SUMMARY

We have measured the distribution of transverse energy in the interaction of 200 GeVlc protons

with Pb targets in the range 0.8 c q < 2.4 up to a maximum Er of 50 GeV. The Er distribution as a

function of q is moving to smaller e with increasing total ET.

REFERENCES

1) T. Akesson et al., Properties of a fine sampling uranium-copper scintillator hadron calorimeter,

preprint CERN-EP/85-80 (1983).

2) T. Ludlam, The HIJET event generator, these Proceedings.

3) R.K. Bock et al., Nucl. Instrum. Methods 186 (1983) 533.

4) F. Paige and S. Protopopescu, A Monte Carlo event generator for pp and 5p reactions, 8NL

31987 (1982) and 8NL 29777 (1981) with updates to version 4.0.