Leading particle distributions in 200 GeV/c p+A interactions

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Leading particle distributions in 200 GeV/c p+A

interactions

K. Abe, G. Alexander, E.D. Alyea, M. Badiak, P. Beilliere, M. Bloomer, T.

Bober, J.E. Brau, D.H. Brick, E.B. Brucker, et al.

To cite this version:

LEADING PARTICLE DISTRIBUTIONS IN 200 GeV/c p+A INTERACTIONS

K. ABE", G. ALEXANDER b, E.D. ALYEA Jr. c, M. BADIAK ct, P. BEILLIERE C, M. BLOOMER r,

T. BOBER t, J.E. BRAU g, D.H. BRICK\ E.B. BRUCKER', W.M. BUGG g, W. BUSZA r, C. CASTOLDI J, B. COLE r, H.O. COHN k, G.T. CONDO g, R. DIMARCO ct, R. DOLFINI J, T.A. FRANK r, T.A. FUESS r, E. GALLIGARICH 1, N. GEL'FAND Q, L. GRODZINS r,

J. GRUNHAUS h, E.S. HAFEN r, T. HANDLER g, H.J. HARGIS g, P. HARIDAS r, E.L. HART g, K. HASEGAWA

a,

Y. HAYASHINO 3, M. HIGUCHI m, D. HUANG 1, H.Z. HUANG r,

R. HULSIZER r, L. INTROZZI J, P.F. JACQUES ct, M. KALELKAR ct, V. KISTIAKOWSKY r, T. KITAGAKI 3, E.L. KOLLER', R.J. LEDOUX r, A. LEVY h, T. LUDLAM 0, P. LUTZ e,

C. MILSTENE r, J.L. NARJOUX °, S. NOGUCHI r, S.H. OH r, Y. OTANI", R.J. PLANO ct,

I.A. PLESS r, S. RATTI J, A. RAFATIAN g' A.H. ROGERS g, M. SATO

m,

P.E. STAMER 0,

S.G. STEADMAN 1, R. STEINER 0, T.B. STOUGHTON r, V. SUCHOREBROW r, B. TAFT 1,

K. TAMAI

a,

S. TANAKA", S. TETHER r, P.C. TREPAGNIER r, B.F. WADSWORTH r, M. WIDGOFF h' Y. WU r, A. YAMAGUCHI a, R.K. YAMAMOTO rand H. YUTA

a

" Tohoku Universl/y, Sendai 980, Japan

0 Tel Aviv Un1vers1ty, 69978 Ramat-Av1v, Israel ' Indiana Universi/y, Bloomrngton, JN 47401, USA " Rutgers University, New Brunswick, NJ 08903, USA

° College de France, F-75231 Pans Cedex 05, France

' Department of Physics and the Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

• Un11•ers1ty of Tennessee, Knoxl'llle, TN 37016, USA

h Brown Universl/y, Providence, RI 02912, USA

' Stevens Institute of Technology, Hoboken, NJ 07030, USA ' University of Pavia and INFN, 1-27100 PaV1a, Italy ' Oak Ridge Natwnal Laboratory. Oak Ridge, TN 37830, USA ' Fermi/ab, Batana, IL 60510, USA

"' Tohoku Ga/..um Umi•ersily, Taga;yo, M1yag1, Japan " Yale Universlly, New Haven, CT 06520, USA

0 Seton Hall Univers/ly, South Orange, NJ 07079, USA

D1stnbut10ns of the leading positive particle have been measured in colhs1ons of a 200 Ge V /c proton beam with hydrogen, Mg, Ag and Au targets using the FNAL Hybrid Spectrometer Estimates of the rapidity loss of the projectile have been obtained as a function of the target mass, the multiplicity of negative particles and the number of recoil protons We obtained a lower hm1t of the rapidity loss of 2.0±0.2 for central p+Au colhs10ns. The impact parameter dependence of the momenta of the first few leading particles has also been investigated with important imphcat10ns to estimates of energy densities obtained in central collisions

I

lntroductwn.

There has been considerable prog­

ress m recent years m understanding the statistical properties of strongly mteractmg systems by means

baryon). Many authors have speculated that a quark -gluon plasma (QGP) may be formed and observed m high energy nucleus-nucleus (A+ A) colhs10ns. The advent of accelerators, at CERN and Brook­ haven Nat10nal Laboratory, which can now produce relativistic heavy 10ns with energies from 15 to 200 A Ge V should make possible the study of nuclear matter at high baryon and energy densities [ 3,4].

There do not yet exist calculations of soft hadromc processes based on a fundamental theory such as QCD. The only estimates of the baryon and energy densities achievable m A+ A colhs10ns are based on phenomenological models which attempt to extrap­

olate from p +A to A+ A colhs10ns Most of these models assume that the leading baryon is the mam projectile fragment (i.e., carnes the largest fraction of the beam momentum and the baryon number of the beam) and that there exists a correlation be­ tween the rapidity loss of the proton in p +A colh­ s10ns and the baryon and energy densities that may be achieved in A+ A collis10ns.

The data available on leadmg baryon distributions m p +A collis10ns are very incomplete [ 5-8], con­ sisting primarily of proton distribut10ns measured over restncted kmematic ranges. with no mforma­ t10n on impact parameter. There exists an even more hmited set of mclusive neutron distributions [ 9]. This mcomplete body of data has led to an inabihty to differentiate between competmg models for p +A collis10ns and thus to ambigmties m estimates of properties of A+ A colhs10ns.

This paper reports on the results of 200 GeV/c p +A measurements made using the Fermilab Hy­ bnd Spectrometer [ I 0] The momenta of all charged particles can be measured and particle identificat10n obtamed for momenta less than 1.5 GeV/c. The rap­ idity distnbut10n of the leadmg positive particle has been obtained as a funct10n of the target mass (A), the multiphcity of produced particles and the num­ ber of slow protons (

np).

It is shown that these dis­ tnbutions set hmits on the rapidity loss of the proton m p +A collis10ns. The impact parameter depen­ dence of the first few leadmg particles is also inves­ tigated, with important implicat10ns to estimates of energy densities obtained m central colhs10ns.

2.

Experzmental procedure.

The Fermilab Hybnd

Spectrometer consists of a 30" hydrogen bubble

chamber m a 2 T magnetic field, followed by elec­ tromc trackmg chambers. Two sets of Mg, Ag, and Au plates were placed1ust mside the bubble chamber entrance, with enough space available for a 4n visual measurement of tracks. The tracking chambers pro­ vided improved momentum resolut10n for particles gomg forward in the center of mass. Particle iden­ tificat10n was obtained from measurements of the ionization m the bubble chamber and enabled n-p separation for tracks with momenta< 1.5 GeV/c.

The data reported m this paper consist of 312, 96, 23 7 and 180 events for the p, Mg, Ag, and Au targets, respectively. The total inelastic cross sect10ns for the nuclear targets [ 10], uncorrected for possible scan­ nmg inefficiencies for low multiplicity events, are 0.36, 1.05 and 1.37 b for the Mg, Ag and Au targets, respectively.

3.

Results and analyszs.

We begin by extractmg the

rapidity distributions of the leadmg positive particle as a funct10n of target mass A, the negative particle multiphcity, and the quantity

ii(np).

The A depen­ dence can be expressed m terms of the variable ii=A (

ab�ci /a;;'�1

), where ii is referred to as the "mean

number of colhs10ns" since its defimt10n is consis­ tent with takmg the mean path length through the target nucleus divided by the p+p mean free path. To obtam an event-by-event measure of the impact parameter [ 11] we use the quantity ii(

np)

= C

"�'

where

np

is the number of observed protons with p< 1.5 Ge Vic (i.e., the target protons) and C,.. is a constant chosen for each target such that (ii(

np))

=ii for that target. The values of C,.. ob­ tamed from the present set of measurements are 1.8, 1.8, and 1.9 for the Mg, Ag, and Au targets, respec­ tively. The average number of produced particles is lmear with ii(

np

), m agreement with previous data

[ 11 ] .

The leadmg positive particle rapidity distnbutions are shown m fig. 1 a for the four targets, and are re­ plotted in fig. 1 b for cuts on ii (

nP).

The p + p dis­ tnbution m fig. 1 b was gated on total multiphctties

08 06 04 02 00 0 8 06 04 0 2 00

r

0 8 -

c)

06 04 -4

gate

gate

2 -2 0 - 6 !Jy

gate

3 3 24 6 01 0

Fig I. Normalized rapidity loss distnbutlons for the leading positive particle (a) p, Mg, Ag and Au targets (b) p target and nuclear targets gated on iJ( np) ( c) Nuclear targets gated on iJ( np) and negative particle mult1phc1ty The leading positive particle is assigned the mass of a proton L'i.y;=y"'""'-Y1<adrngpamc1c• where Yhcam=6.06 for a 200 GeV/c proton beam All d1stnbutions are normalized to umty See table I for more informat10n

particle) was assigned the mass of the proton. Be­ cause of possible misidentifications, caut10n should be exercised m comparing these distnbutions with the results of mclusive distributions from spectrom­ eter expenments

[

5-8] which identify the leadmg proton. It is shown below , however, that important limits can still be set on the

mean

rapidity loss of the incident proton.

Figs. la and 1 b reveal that the distributions shift

Table I

Gating regions used in generating fig I Also included are the measured < iJ( n")), <negative particle mult1phc1ty), and mean rap1d1ty shift (Lly= y""""' - y, y''"""' = 6 06) of the leading positive particle [+ 1s the ratw of the number of events in which the leading charged particle 1s pos1t1ve to the total number of events

Figure Target Events iJ(n") Negative multiphcJty l a p 312 all all Mg 96 all all Ag 237 all all Au 180 all all l b p 201 all total mult;.3 plates 228 0-2 0 all plates 153 2.0-4 5 all plates 132 4 5-10 0 all le plates 143 0-2 0 0-4 plates 90 2 0-4 5 5-11 plates 63 4 5-10 0 ;:> 12

v( np)

and high mult1plic1ty better selects central col­

lis10ns than a cut on

v ( np)

alone.

The mean rapidity shifts for different cuts on the data are listed in table 1. Mean rap1d1ty shifts of ap­ proximately

2.4

units are obtained for events gated on large

v(np)

and high multiplic1t1es. However, be­ fore a lower hmit can be set on the mean rapidity loss of the incident proton, correct10ns must be made for possible mlSldentificat10ns in which the leading positive particle is not a proton.

If the leading particle is a 7t + and the leading bar­ yon is a proton, this leads to an underestimate of the rap1d1ty loss of the leading baryon. Of greater con­ cern for obtaining a lower hm1t of the rap1d1ty loss in central p +A colhs10ns is the mlSldentificat10n of a 7t + as a leading proton, for events in which the leading baryon is an unobserved neutron. The cor­ rect10n to our estimate depends on the fraction a of the total events in which the leading baryon is a neu­ tron This fraction is hkely to be a function of

v(np),

with estimates in the range between

0.25

and

0 5

for central colhsions [ 8,

12].

The uncertainty in the mean rapidity loss of the leading proton can be estimated from the express10n

( Y > I pos = ( 1

-

Cl'.)

< Y)

I

prot

+ ll'

( Y)

Irr+ , ( 1)

where

<Y> lpos

is the measured mean rap1d1ty of the leading positive particle assuming it has the mass of

(ii{np) ) ( mult1phcity)

_J+

(Lly) n 'In -I 4 5 ill 9 0 85 I 4 20 12 1/5 l 0 74 l 8 3 I 16 617 2 0 69 20 3 6 18 818 2 0 71 20 I 3 7 3/2 9 0 82 I 6 I 3 JO 3/4 3 0 80 I 6 3 2 16 217.2 0 62 2 2 6 0 27 71120 0 64 2 3 l 2 6 7/2 6 0 85 1 5 3 3 16 417 3 0 66 2 I 6 4 36 9116 3 0 52 2 4

a proton. Here,

<Y>

1

prot

is the unknown mean rap­ idity of the leading proton, and

<Y>

irr+ is the mean rapidity of a leading 7t + which is mISidentified as a leading proton for events in which a neutron is the leading baryon. An estimate of

< y) 1"

+ can be ob­

tained by gating on events in which the leading par­ ticle is positive and generating the second leading particle rapidity d1stnbut10n assuming the mass of a proton for all charged particles. If the proton and neutron d1stribut10ns are identical, then this distri­ bution should be an accurate representation of the leading 7t + distribution from events in which a neu­ tron is the leading baryon. The value of

<Y>

1"+ thus attained is 3.3

± 0

.3 for central colhs10ns. Solving (1)

with

a= 0.5,

which will result in a lower hmit of the rapidity loss, yields

<

.1y)

1

prot = 2.0 ± 0.2.

These results can be compared to analyses of p +A--> p + X data. The Busza and Goldhaber [ 13] analysis of the data of Barton et al. [ 7] yields an ex­ trapolated value of

<'1Y)centra1=2.5±0.5.

Other au­ thors have analyzed the same data and obtained similar results [ 14] . J ezabek and Rozanska [ 12] have analyzed the data of Bailey et al. [ 8] in the context of the dual parton model and obtained

<'1Y)ccntraI� 1.5.

An intermediate value was ob­

tained by Date et al. [

15],

using the multi-chain model. The present set of data is inconsistent with the results of Jezabek and Rozanska [

12].

0 20 x 5 0 10 "' ::;;; 0 05 i/(np) £5 I 31 i/(np)£5 324 v(np) £5 6 01 002 '---'---'��-'-��-'-��...__��-'---'"----' 0 2 3 4 5 6

Leading Particle Number

Fig 2 Mean momentum fraction x( =Piab/Pbeam) for the first five leading particles from nuclear target events gated on v(np). The gates are 0 ,,;;v(np) < 2, 2,,;; v( n") < 4 5, and 4.5,,;; v(np) < 10

the leading baryon goes. The exact nature of the rap­ idity distribution of produced particles is very im­ portant in estimating the maximum energy densities that may be achieved in A+ A collisions. Of partic­ ular interest would be the correlation of the energy of the leading baryon and that of leading pions. Without particle identification for leadmg particles this comparison cannot be done. However, some im­ portant observations can be obtained by examining the energy contamed in leading particles without re­ gard to particle identification.

Fig. 2 displays the mean momentum fraction,

X=P101a/Pbeam

(this is approximately Feynman

x)

for the nth leadmg particles gated on v(np). The de­ crease in

x

of the first leadmg particle is consistent with the results shown in table 1 for

<�Y)

of the leadmg positive particle. We emphasize that parti­ cles of both signs except for identified slow protons have been included in fig. 2.

The impact parameter dependence of the leadmg particle is stronger than that of the second leadmg particle. The

< x)

for the third leading particle is 0.05 (10 GeV/c) and has no impact parameter depen­ dence within the present statistics. The

<x>

for the 4th and 5th particles appears to be mcreasmg with

v

( nP). Approximately 3 5% of the total beam energy is observed in charged particles with X> 0.05 m cen­ tral collisions. These results suggest that even m cen­ tral collisions there is a significant amount of energy possessed by the three leadmg charged particles. This

energy is not hkely to be available for producing lo­ cal regions of high energy density and may in fact represent the fragments of the incident proton.

The present set of data 1s consistent with a picture in which the incident proton suffers a rapidity loss of approximately 2 units in central p +A collisions. Using simple hydrodynamical arguments and as­ suming that

�Y

is energy mdependent [2,1 3, 1 5,16] this result suggests that maximum baryon densities of approximately 7 times that of ground state nu­ clear matter may be achieved m 30 A GeV fixed tar­ get of 4 A Ge V collider A+ A collisions. The above results also suggest that it may be more difficult to estimate energy densities directly from the rapidity loss of the beam, since 35% of the total energy is ac­ counted for m just 3 fast particles in central p +A collisions. The inclusion of neutral particles would presumably increase this fraction further. Therefore, it will be necessary to make detailed measurements of the energy distribution of produced particles be­ fore reliable estimates of energy densities in p +A and A+ A collisions can be made.

One of the authors ( RJL) would like to thank M. Gyulassy and G. Stephans for their many helpful dis­ cussions. We thank Fermilab and the 30" bubble chamber crew for their support and for providing the pictures for this experiment. We also thank the scan­ ning and measuring groups at our mstitutions for theu professional services which made this expen­ ment possible. This work was supported in part by funds provided by the US Department of Energy, the National Science Foundation, the US-Israel Bina­ t1onal Science Foundation, the Italian Institute Na­ zionale di Fis1ca Nucleare and the Japan Society for the Promotion of Science.

References

[ l] J Cleymans et al , Phys Rep 130 (1986) 217, B . Svetttsky, Nucl. Phys A 461 ( 1987) 71

[2] L S Schroeder and M Gyulass1, eds., Proc Fifth Intern. Conf on Ultra-relattv1sttc nucleus-nucleus col11S1ons (Quark Matter '86) (Asilomar, Apnl 1986), Nucl Phys A

461(1987)1.

[

3] R J Ledoux, Proc. XXIII Intern Conf. on High energy physics (Berkeley ,1986) ed. S C Loken, p 1407

[ 5] J V Alla by et al , CERN Report 70-12. [ 6] T Eich ten et al , Nucl Phys. B 44 (1972) 333 [ 7] D S Barton et al , Phys Rev D 27 (1983) 2580 [8]R Ba1ley et al ,Z Phys C 29(1985) I

(9] P Forrest et al , private commumcat10n from K Heller, unpublished

[ 10] R Di Marco, Ph D TheSJS, Rutgers University (1985), unpublished

[ 11) W.Q Chao et al , Nucl Phys A 395 (1983) 482, and refer­ ences therein

[ 12] J Jezabek and J Rozanska, Z Phys C 29 ( 1986) 55

[ 13] W Busza and A S Goldhaber, Phys Lett 139 B (1984) 235

[ 14] RC. Hwa and M S Zahtr, Phys Rev D 31 (1985) 499, A Klar and J Hufner, Phys Rev. D 31 (1985) 491, C Y Wong, Phys Rev Lett 52 (1984) 1393,

L.P Cserna1andJ. Kapusta, Phys. Rev D 31 (1985) 2795, J Bowling and A.S. Goldhaber, Phys Rev. D 34 ( 1986) 778,