Nuclear Physics A566 (1994) 41 lc-414~

North-Holland, Amsterdam NUCLEAR PHYSICS A

Transverse Energy Production with Si and Au Beams at AGS Energy: Towards Hot and Dense Hadronic Matter

Jean Barrette

Foster Radiation Laboratory, McGill Univ., Mont,real, Canada H3A 2I32

For the E814/E877 Collaboration: BNL, GSI, LANL, McGill Univ., Univ of New Mt=xico,

Univ. of Pittsburg, Univ. of Sao PrZulo, SUNY-Qony Brook, Texas A&M Univ., Wayne

State Univ., Yale Univ.

1. Introduction

One of the main goals of t,he present. heavy-ion react,ion studies at. the AGS and

CERN is the understanding of the space-time evolut,ion of the part,icle and energy densit,y

produced in such collisions. These are necessary ingredients if one want,s t,o llnd~rst.and

quantitatively the properties of matt,er at high t,emperature and density. In t,ha.t. respect,

the study of inclusive global variables such as t.he transverse energy dist,ribut.ion gives

valuable information on the reaction dynamics and, a.lheit. indirect,ly, on t,he energy a.nd

baryon density reached in nucleus-nucleus collisions.

In the spring of 1992, the AGS produced t,he first, beam of very heavy ions, more

specifically a beam of 11.4 GeV/c per nucleon lg’Au. This is an important. st,ep for our

field since it gives a first glimpse as to how the reaction dynamics evolves a.s t,he mass a.nd

volume of colliding systems reach t~he la.rgest. value that, will be available in the fut.ure.

We have used part, of the E814 experimental &-up t,o st,ucly t.he transverse energy tlistri-

bution produced in Au induced collisions. ‘I’hesa clat,a can be compared to similar da.tta

obtained with lighter beams and thus provide for the first t.ime informat,ion on how t,he

ET distribution evolves with the ma.ss of the systprn as one reaches very la.rge systems.

Part of these data has recently appeared in print 111.

2. Experimental set-up

The E814/E877 experimental set-up is described in rcfs. [2-41. It is charact,eriz& by

it nearly 47r calorimetric coverage. The target calorimet,er (TCAL) covers t,he hackwa.rd

hemisphere (-0.5 < 71 < 0.8) while t,he part,icipant, ca.lorimet,er (PCAL) measures the energy

fIow in the forward hemisphere (0.83 < 71 <- 4.7). The target calorimeter is made of 992

NaI crystals, 5.3 radiation length deep. The part,icipant ca.lorimeter is a scint,illat,or tile

calorimeter with lead/iron absorber, 4 absnrpt,ion lengt,h deep. The analysis procedures

for TCAL are described in refs. [2,3]. Th e corrertion for the response of the detect,ors

and for leakage is obtained by tracking through t.he calorimeter events generat.ed using

0375-9474/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.

412~ J. Barrette I Transverse energy production with Si and Au beams at AGS energy

the code GEANT. For dat,a from PCAL, the tranverse energy distribution corrected for

leakage are obtained using a Monte-Carlo generat.ed response matrix method described

in more details in refs [1,5].

3. Results and Discussion

The measured distributions da/dET for Au+Au and Si+Au a.re shown in fig. 1 where

they are compared to the predictions of various models. For the comparison with TCAL

data, model calculations are filtered wit,h the det,ector response. Good agreement, is

observed with the predictions of the AR.C 1nodr1 [c] ( p o en squares). The Lund string

fragmentation model FRITIOF [i’] (dashed 1‘ mes) wit,h pammeters tuned to pp collisions

at AGS energies also reproduces rela.tively well t,he Au+Au c1at.a but, overpredict,s the maximun ET for Si+Al system. Finally t,he

1 14.8 * cov/c Sl + Al

F

.l

H .Ol

-5

b!i q 1000 not unlolded

3 100

0 10 ET

(%) 30 40

FIG. 1. Comparison of the measured to- tal ET dist,ributions (histograms) with the

predictions of various models (see text).

RQMD model [8] (close circles) overpredict,s

considerably the data for bot,h syst.ems in

t,he PCAL accept,ance while it underpredict.s

TCAL data by t,he roughly t,he same fra.ction.

FIG. 2. Experiment,al dr/dEr distri-

butions in the PCAL accept.ance. The dashed lines are the calculated contribu-

t,ions from collisions with h < 0.5 fm. The dot,ted line corresponds to t.he SitAl data

scale up by a f&or E$(Au)/EF( Si).

The abrupt fall-off at large ET in t,he Au-{-An tla.ta. shows t.hat, the flnct.uat.itrns in

heavier systems are considerably reduced. This is shown by the dott,ed histogram in Fig.

2 which was obtained by scaling up by a factor 7.9 the Si+ Al PCAL dat,a. Because of t,his

large difference in shape, the comparison of F~;T prodnct.ion for t,he t,wo syst,ems shonltl not.

be done at a fixed cross section level or a fixed fraction of the geomet,rical cross sect,ion. To

compare the two syst,ems we followed t,he procedure described in ref. [!Jj. We tlet,crmine

the mean transverse energy, ET, for collisions wit,h b<O..5 fm using different, models a.nd

then calculate the fraction of t,he geomet,ricnl cross section leading t,o ET > EF. Alt,hough

predicting different values of EF, all models give similar fract.ions of t,he cross sect.ion wit,h

J. Barrette I Transverse energy production with Si and Au beams at AGS energy 413c

an average value of u/uggeom of 1.5% for Si+Al a.nd 0.22% for Au+Au. When a.pplied to

our data, these fractions (which determine E$, the ET for an a.verage collision with h <

0.5 fm) give E$(Si)=40.2 GeV and E$(Au)=318.5 GeV (dashed lines in Fig.2).

FIG. 3. Parameters of the Gaussian func-

tions describing the PCAL dET/ds dis-

tributions as function of ET/E;. Re-

sults from ARC model calculations are also included: Si+Al (solid lines), Au+Au

(dashed lines).

Fig.3 gives a comprehensive picture of the evolution of the ET distribut,ions a.s a func-

t,ion of centrality for four systems of different, size. It shows t,he evolut,ion of the pa.rarnet,ers

(amplitude B, width CT~, and centroid 70) of the t:aussian t,hat best described the dET/dT

distribution vs ET/E;. One observes first that,, for all systems, the peak va.lue B of t,he

dE,/dq distribution increases linearly wit,h i.he produced tot.al t,ra.nsvFrse energy. This in-

crease is correlated with a sizable decrease in t,he widt,h of t.he ET distribut.ion by roughly

0.4 unit of rapidity (or zz 30%) between periphera,l and the more cent,ral collisions. A sim-

ilar effect has been observed in t,he charged part.icle mult,iplicit,y distributions measured

with Si projectiles [4]. As stated above, one observes also that t,he widt,h get,s na.rrower

as the mass of the system increases. Finally, for light, projectiles on heavy t,argets t,he

distributions move backward in 17 as funct,ion of cent,ralit,y while the inverse is observed

for reactions of heavy projectiles on light, t,argets. This effect can be underst,ood as heing

due to the evolution with centrality of t,he rapidit,y of the center of mass of the participant

region. Both the ARC and RQMD models reproduce well the decrea.se of t,he widt,h with

the mass of the system and with centralit,y. However, for symmetric systems bot,h models

give centroids which are shifted forward by roughly 0.3 unit. of pseudorapidity rela.tive to

t,he data.

414c J. Barrette 1 Transverse energy production with Si and Au beams at AGS energy

These rest&s allows to characterize t.he evolution wit.h the mass number of t.he Er production. E$ increases by a fa.ct,or of 7.9 when going from Si t,o Au projectiles (see fig. 2). At mid-rapidity ET increases even more, by a fact.or of 8.8, because of the nar- rowing of t,he dEr/dq distribution for heavier beams. iMa,ny aspects of heavy-ion d&a obtained with light projectiles are explained q~lalit,at‘ively wit,hin a geomet.rica,l picture based on the number of nucleons in the overlap volume of t,he target, and projectile [9]. In this picture, ET production is expect,ed t,o be proportional to the available energy in t,he center-of-massy, E*. For symmetric A+A collisions at. impact. paramet,er b=O, E‘ rea,ches

a rn~irIlurn given by E’ = m,vA( dz,iA 71t,t~ .. 2) which, at, t,he present. hea.m ener-

gies, results in a ratio of available energy for centra.1 collisions of E’(Au)/E’(Si)--5.9. To describe the evolution of ET wit,h the mass of t,he syst,em we inbroducecl t,he para.met,riza- tion ET cc E’A” where a nonzero value of Q would indicate a depa.rture from t,his simple picture. The observed increase of a f&or 8.8 in E% a.t miclrapiciit~y corresponds to a value cy= 0.20 & 0.02. This large value of N implies a.n increase in ET procluct,ion of a fact,or of = 1.5 (i.e (197/27)*) for the Au+Au syst,em relative t,o the %+A1 syst,em as compared t,o what is expected for independent nucleonnucleon collisions. Such a relative increa.se is in agreement with the prediction of models where it ta.n be associa.ted t,o an increase in the energy and baryon density a.chieve in AufAu collisions. For example t,he ARC! model, which reproduce our data, predicts for that. syst,cm a. maximum baryon density *% 10.5 t.imes normal nuclear matter densit,y. This model also predict,s t,hat, a large t1ensit.y (,)/pa > 5 ) is reached over a volume of z 41) fm”, much la.rgcr t.ha,n the volume of rna~ximum baryon density in %+A1 collisions ( N 5 fm').

4. Conclusion

In conclusion, the first result,s obtained wit,h very heavy beams are very encouraging and give all indications that with t,hese baarns one reaches new and indeed extreme val- ues of energy and baryon density. Furthermore t.he larger volume and longer period over which t.hese are produced should facilit‘ate t.he cha,racterization of t,his hot, nuclear mat,t.er and thus allow to evidence more easily possible new phenomena, like part,ial deconfinement.

References

1. J.Barrette et ul., E814 Collaboration, Phys. Rev. Lett. 70, 2996 (1993). 2. J.Barrette et al., E814 Collaborat,ion, Phys. Rev. Let,t. 64, 1219 (1990). 3. J.Barrette et al., E814 Collaboration, Phys. Rev. C! 45, 819 (1992). 4. J.Barrette et af., E814 Collaborat,ion, Phys. Rev. C: 46, 312 (1992). 5. 2. Zhang, Ph.D. thesis, Univ. of Pit,tsburg, 1993 (to be published) 6. Y. Pang. T.J. Schlagel, and S.H. khma., Phys. Rev. Lett.. 68, 2743 (1992). 7. B. Anderson, G. Gustafson, and B. Nilss~)n-AlIl~clvist., Nucl.Phys. B281, 289 (1987) 8. H. Sorge et al., Nucl. Phys. A525, 95~ (1991). 9. J. Stachel and G.R. Young, Ann. Rev. Nucl. Part,. Sci. 42, 237 (1992).