Transcript
Page 1: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

PHYSICAL REVIEW C VQLUME 48, NUMBER 6 DECEMBER 1993

Production of m+, K+, p, and pin relativistic Au + Pt, Si + Pt, and p + Pt collisions

G. E. Diebold, B. Bassalleck, " T. Burger, M. Burger, R. Chrien, H. En'yo,G. Franklin, J. Franz, T. Iijima, K. Imai, 3. Lowe, R. Magahiz,

A. Masaike, F. Merrill, J. M. Nelson, K. Okada, P. H. Pile, 2 B. Quinn,E. Rossle, A. Rusek, N. Saito, R. Sawafta, H. Schmitt, R. A. Schurnacher,

R. L. Stearns, R. Sukaton, R. Sutter, F. Takeutchi, D. M. Wolfe,S. Yokkaichi, V. Zeps, and R. Zybert

(E886 Collaboration)Department of Physics, University of Birmingham, Birmingham B15 2TT, United Kingdom

Brookhaven National Iaboratory, Upton, Negro York 11978Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15218

Fakultat fiir Physik, University of Freiburg, D 79104 F-reiburg, GermanyDepartment of Physics, Kyoto University, Sakyo Ku, Ky-oto 606, Japan

Faculty of Science, Kyoto Sangyo University, Kyoto 609, JapanDepartment of Physics, University of ¹wMexico, Albuquerque, New Mexico 87191

Department of Physics and Astronomy, Vassar College, Poughkeepsie, ¹wYork 12601Physics Deparment, Yale University, Ne'er Haven, Connecticut 06522

(Received 15 June 1993)

During the recent commissioning of Au beams at the Brookhaven Alternating Gradient Syn-chrotron facility, experiment 886 measured production cross sections for sr+, K+, p, and p in min-

imum bias Au + Pt collisions at 11.5A GeV/c. Invariant difFerential cross sections, F z s, weredp3 '

measured at several rigidities (p/Z ( 1.8 GeV/c) using a 5.7' (fixed-angle) focusing spectrometer.For comparison, particle production was measured in minimum bias Si + Pt collisions at 14.6AGeV/c using the same apparatus and in p + Pt collisions at 12.9 GeV/c using a similar spectrome-ter at KEK. When normalized to projectile mass, Ap, „, the measured sr+ and K+ cross sections arenearly equal for the p + Pt and Si + Pt reactions. In contrast to this behavior, the vr cross sectionmeasured in Au + Pt shows a signiIIicant excess beyond Ap, „scaling of the p + Pt measurement.This enhancement suggests collective phenomena contribute significantly to vr production in thelarger Au + Pt colliding system. For the Au + Pt reaction, the sr+ and K+ yields also exceed Ap, „.

scaling of p + Pt collisions. However, little significance can be attributed to these excesses due tolarger experimental uncertainties for the positive rigidity Au beam measurements. For antiprotons,the Si + Pt and Au + Pt cross sections fall well below Ap, ; scaling of the p + Pt yields indicatinga substantial fraction of the nuclear projectile is inefI'ective for p production. Comparing with p +Pt multiplicities, the Si + Pt and Au + Pt antiproton yields agree with that expected solely from"first" nucleon-nucleon collisions (i.e. , collisions between previously unstruck nucleons). In light ofexpected p annihilation in the colliding system, such projectile independence is unexpected withoutadditional (projectile dependent) sources of p production. In this case, the data indicate an ap-proximate balance exists between absorption and additional sources of antiprotons. This balance isremarkable given the wide range of projectile mass spanned by these measurements.

PACS number(s): 25.75.+r, 13.85.Ni, 25.40.Ve

I. INTR.ODU CTI(3N

The recent successful acceleration of Au ions to 11.5A GeV/c at the Brookhaven Alternating Gradient Syn-chrotron (AGS) represents nearly an order of magnitudeincrease in projectile mass for ions accelerated to highlyrelativistic velocities. This event marks the beginningof an era of truly heavy relativistic ion physics at theAGS. As part of this new era, experiment 886 (E886)conducted a study of positive and negative particle pro-duction in the collisions of Au ions with a similarlyheavy Pt target. Presented here are results of inclu-sive particle production measurements made during theerst Au beam run at the AGS. For immediate compar-

ison, particle production in Si + Pt and p + Pt colli-sions at similar beam momenta per nucleon (14.6A and12.9A GeV/c, respectively) were also measured and arereported here. The Si + Pt reaction was studied usingthe same apparatus as the Au + Pt measurements, whilethe p + Pt measurements were performed at KEK usinga similar spectrometer.

This paper will focus on the projectile dependence ofparticle production as demonstrated by the minimumbias reactions studied. Of particular interest is the pro-jectile dependence of the p multplicities. These are ex-pected to be sensitive probes of the hadronic systemforined in nucleus-nucleus (A + A) collisions owing toa potentially large annihilation probability in a dense,

0556-2813/93/48(6)/29844, '1 l)/$06. 00 1993 The American Physical Society

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48 PRODUCTION OF m+—

, K+, p, AND p IN RELATIVISTIC. . .

TABLE I. Acceptance characteristics of the D6 and T3 focusing spectrometers.

Spectrometer

D6 (AGS)T3 (KEK)

Productionangle (mrad)

99.099.0

~~enp

(msr %)

11.10.45

(%)+2.1+0.8

AO

(mrad)

+30+ 3.1

(mrad)

+ 4.6+8.9

baryon-rich environment [1]. However, despite severalmeasurements of antiproton production in Si + A col-lisions [2—4], the degree to which the collision. environ-ment affects p yields is not well established or under-stood. Aside &om the complex nature of A + A colli-sions, this pursuit is hampered by very small productioncross sections at AGS energies. As a consequence, ob-taining accurate and precise measurements of p produc-tion has proven to be a particularly challenging task. Forexample, although previous measurements in Si + A col-lisions at the AGS [2—4] difFer by factors of 2—4 [5], suchdifrerences are not grossly beyond the associated experi-mental (statistical + systematic) uncertainties. Further,as there are relatively limited p + A data at suitable ener-gies, these experiments have had to generally rely on ex-trapolations of higher-energy p + A data for comparisonpurposes. As a result, even a qualitative understandingof A + A antiproton yields relative to simple expectationshas not been firmly established (e.g. , relative to "first"collision scaling of p + p or p + A data; see Sec. IVD).

In the results and discussion to follow, emphasis will beplaced on the measured p yields from the Au + Pt and Si+ Pt reactions in comparison with those measured in p +(A 200) collisions at similar hearn energies per nucleonand in similar regions of phase space. As will be seen,by focusing on p production within the narrow phasespace acceptance of these measurements (0.8 ( y & 1.4,p| ( 0.180 GeV/c) a quantitative comparison of p yieldsin p + A and A + A collisions can be made with min-imal extrapolation uncertainty. From such comparisonsone can begin to address the efI'ects of the collision envi-ronment on observed p yields. However, as is also true forprevious narrow-acceptance p measurements [3, 4], suchcomparisons cannot strictly distinguish difI'erences in pyields due to absorption from nonabsorptive aspects ofthe collision (such as phase space redistribution of an-tiprotons). Consequently, it is difficult to draw singularconclusions based solely on these comparisons.

II. EXPERIMENTAL APPARATUSAND TECHNIQUE

For each reaction, a thin Pt target of approximately6—7% of an interaction length was used together with a

5.7' (fixed-angle) beam-line spectrometer system. TheAu + Pt and Si + Pt reactions were studied at theAGS using the D6 secondary beam line [6] and associ-ated open-geometry dipole spectrometer constructed forthe E813/836 H particle search [7]. The p + Pt reactionswere studied at KEK using the T3 beam line. Ordinarily,these beam lines are used to produce secondary pion andkaon beams for a variety of intermediate-energy physicsexperiments. The characteristics of each beam line aresummarized in Table I. For the Au + Pt data, parti-cle production was measured for eight magnetic rigidities(p/Z = +0.8, +1.2, +1.5, +1.8 GeV/c) while only tworigidities (p/Z = +1.8 GeV/c) were studied for the Si +Pt reaction. The p + Pt measurements included a total ofsix rigidities (p/Z = +1.2, +1.5, +1.8 GeV/c). Table IIsummarizes the reaction, target thicknesses, beam mo-menta, and secondary magnetic rigidities for each mea-surement reported here.

A schematic diagram of the D6 beam line and E886 ex-perimental apparatus is shown in Fig. 1. For the presentmeasurements, the electrostatic separators were not used.Hence, all particle species produced within the momen-tum and solid angle acceptance of the beam line weretransported from the target to the final focus approxi-mately 31 m away. For each rigidity, particles were iden-tified by their time of Bight (TOF) measured betweentwo plastic scintillation counters, MTS and IE1, sepa-rated by approximately 15 m. The MTS counter waslocated just downstream of the erst mass slit while theIE1 counter was placed at the final focus of the beamline. As a hardware trigger, a delayed coincidence be-tween these counters selected particles by time of fIight.Additionally, an aerogel Cerenkov counter, IC (index ofrefraction, n =- 1.03), was used as a vr veto to enhance thesample of K and p triggers written to tape. Dead timein the data acquisition system was typically 20—40 %.

As an example of the clean particle identificationachieved, Fig. 2 shows the recorded MTS-IE1 time ofBight for the highest positive rigidity tune (1.8 GeV/c).For the Au beam data, this measurement alone was sufB-cient for efEcient particle identification. However, duringthe most intense Si beam runs, particle misidentificationdue to accidental hits in the MTS counter became sig-nificant ( 20—30%). In this case, a second TOF mea-

TABLE II. Summary of incident beam momenta and secondary rigidities studied for each reaction.

Reaction

Au+ PtSi+ Ptp+ pt

Targetthickness (cm)

0.150.280.50

Beam momentum(GeV/c per nucleon)

11.514.612.9

Secondaryrigidity (GeV/c)

+0.8, +1.2) +1.5) +1.8+1.8

+1.2, +1.5, +1.8

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2986 G. E. DIEBOLD et al. 48

Mass Slit 1

9~ME~ I

6 L'JELL'll UL'll

Separator B

MTS

10 Meters

FIG. 1. Schematic diagram of the D6 beam line and E886 experimental apparatus.

10 4

C3C3

10 3

U3 102

0O 10

Ill HII, I i, s I,IIT=

—2 0 2 4 6 8 10Time of Flight (ns)

FIG. 2. Time-of-flight difference spectra (MTS-IE1) of 1.8GeV/c positive particles produced in Au + Pt collisions at11.5A GeV/c. Plotted is the difference between the recordedBight times and that of a v = c particle.

surement was made in the E813/836 dipole spectrometerto identify and correct such rate-dependent identificationinc Kciencies.

To obtain invariant differential production cross sec-3

tions, Ed, , the identified particle yields were normalizeddp3 'to the integrated beam on the target and the differen-tial invariant phase space volume accepted by the beamline, d p/E. This phase space volume was determined byMonte Carlo simulation using the program DEGAY TUR-TLE [8]. Based on this simulation, the nominal accep-

tance of the D6 beam line was found to be —""dO (—:"P)p p'= 11.1 msr%. Depending on the precise position of theincident beam on the production target, the spectrom-eter acceptance may vary by as much as 10% from thisnominal value. In such a case, the variation in targetingproduces an observable change in the phase space distri-bution of particles transported by the beam line. Thisphase space distribution was monitored using three driftchambers located just upstream of the final focus. Tocompensate for this effect, the spectrometer acceptancewas calculated run by run assuming an incident beamposition on target which reproduced the observed phasespace distribution.

The beam intensity during the Si run was often as highas 10 ions per spill (spill length 1 s) while duringthe Au run the intensity reached as high as 10 ions

per spill. At such high rates, the integrated beam ontarget could not be accurately measured using pulse-counting techniques. Instead, the time-integrated chargefrom two ionization chambers was used. The first cham-ber was placed in the primary beam (in-beam chamber)and monitored the total beam delivered to the D targetarea. This chamber was calibrated in place using a plasticscintillation counter during periods of low beam intensity(( 5 x 10 per spill). The second chamber was placed out-of-beam near the production target (out-of-beam cham-ber) and monitored the average flux of produced parti-cles. Together, the ratio of the ionization collected bythese chambers provided a relative measure of incidentbeam actually on target. This ratio was used to correctfor variations throughout the runs due primarily to hor-izontal instability (i.e. , "sweeping") of the beam spot onthe target. The beam position was monitored. visuallyusing a fluorescent flag mounted 10 cm upsteam of thetarget. This ensured that the beam spot was centered onthe target during most of the data taking periods. Thesize of the beam was small compared to the target size,and so the maximum observed out-of-beam to in-beamchamber ratio should correspond to nearly all of the inci-dent beam on target. This maximum ratio provided theabsolute normalization, of the fractional beam on target.

Because of the high beam intensities used, no attemptwas made to select the ion species incident on the tar-get. Consequently, the target interactions sampled willinclude collisions of projectile fragments resulting fromprimary beam interactions upstream of the target. ForE886, material upstream of the target amounts to about5% (1%) of an interaction length for the Au (Si) beam.This includes contributions from the vacuum window atthe end of the beam pipe, air, the target flag, and thebeam flux monitor. The resulting error in the cross sec-tion measurements is limited to the difference in the par-ticle yield from upstream fragmentation products hittingthe target versus that from a primary ion collision. Thisdifference is relatively small and in the worst case (Aubeam) introduces less than 2% error in the measuredcross sections.

In the T3 beam line at KEK, similar experimentaland analysis techniques were used to determine invari-ant cross sections for the p + Pt reaction. Additionaldetails of the T3 apparatus and data analysis for the p+ Pt measurements will be discussed in a forthcomingpaper [9].

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PRODUCTION OF m.+, E+, p, AND p IN RELATIVISTIC. . . 2987

810 ~I r r

r o' —(o)

10 6

10 5

104

10

102

0.5

OAl)

(UC3

A

E

C!p

U

104

I

pj's

X

+~ ~ 7T

A A Pe ~ K+

o o K

10 =

I r I I I I I I

1.5 2

I

I

I r I rI

I f I rI

r 108

ro'=— (r)

106

105

104

10

102

10r I I

0

AAA

x10 p

7T

ppp

~y

~ y& K'

opo K

rI

r I I II

r I I I

FIG. 3. Invariant differential productioncross sections for particles produced at 5.7in Au + Pt collisions (11.5A GeV/c) plottedas a function of (a) rigidity and (b) rapidity.Note that the m cross sections have beenmultiplied by a factor of 10.

Rigidity (GeV/c) Rapidity

III. B.ESUj TSd3The invariant differential cross sections, Ed, , for a+,

K+, p, and p production in minimum bias Au + Pt colli-sions are plotted in Fig. 3 as a function of rigidity and asa function of rapidity, y. Note the transverse momentumacceptance necessarily changes slightly with each rigid-ity tune. These cross sections are also listed in Table IIIalong with those measured in the Si + Pt and p + Pt re-actions. Statistical and relative systematic uncertaintiesare given in parentheses.

In all cases, decay corrections for finite w and K life-times have been applied. For the +0.8 GeV/c rigidity

tunes e—vr separation by TOF is suKcient to avoid con-tamination of the sr+ yields by e+. For the Si + Ptand Au + P t measurements at higher rigidities, the e+contamination of the sr+ yields was estimated from thee+/m+ ratio measured in the p + Pt reaction at KEKwhere a gas Cerenkov detector provided clean e+ identi-fication. For this estimation, it is assumed that the e+are primarily the result of p conversion in the target (e.g. ,~ 2p, p -+ e+e ) and that the ratio of vr+/p is thesame for each reaction. Based on this, the contaminationof the m+ yields from e+ was estimated to be 11%, 8%%uo,

and 5% for the 1.2, 1.5, and 1.8 GeV/c rigidity tunes and18%, 12%, and 9% for the —1.2, —1.5, and —1.8 GeV/c

TABLE III. Invariant differential production cross sections at 5.7' (laboratory angle) for minimum bias p + Pt, Si + Pt,and Au + Pt collisions at 12.9A, 14.6A, and 11.5A GeV/c, respectively. Cross sections are tabulated by species and rigidityof the produced particle. Also listed are the corresponding values of the central transverse momentum, pt, and rapidity foreach particle-rigidity combination. Shown in parentheses are statistical and relative systematic uncertainties, respectively.Additional uncertainty in the overall normalization is estimated to be about 20% for the positive rigidity Au data and about1070 for all other measurements.

p/Z(GeV/c)

1.81.51.20.81.81.51.20.81.8

1.20.8

—1.8—1.5—1.2—0.8—1.8—1.5—1.2—0.8—1.8—1.5—1.2—0.8

u

rapidity

2.772.692.572.301.941.791.591.251.391 ~ 231.060.772.772.692.572.301.941.791.591.251.391.231.060.77

pt(GeV/c)

0.1780.1480.1190.0790.1780.1480.1190.0790.1780.1480.1190.0790.1780.1480 ~ 1190.0790.1780.1480.1190.0790.1780.1480.1190.079

p+ Pt

7.21(0.03, 0.81) x 108.92(0.04, 1.01) x 101.14(0.01, 0.13) x 10

Au+ Pt2.05(0.05, 0.43) x 102.34(0.02, 0.49) x 103.33(0.07, 0.70) x 104.00(0.04, 0.64) x 102.10(0.07, 0.40) x 101.96(0.18, 0.37) x 102.14(0.18, 0.41) x 10

1.81(0.03, 0.20) x 10

1.97(0.22, 0.18) x 108.70(0.21, 1.06) x 101.06(0.03, 0.13) x 101.33(0.07, 0.18) x 10

5.61(0.02, 0.56) x 107.47(0.02, 0.75) x 10~1.13(0.00, 0.11) x 10

5.11(0.03, 0.58) x 106.91(0.04, 0.78) x 10~9.54(0.06, 1.09) x 10

2.66(0.07, 0.32) x 103.55(0.12, 0.45) x 103.96(0.25, 0.53) x 10

5.21(0.40, 0.52) x 105.37(0.44, 0.54) x 103.48(0.58, 0.35) x 10

8.16(0.16,0.41) x 10

1.53(0.01, 0.17) x 10

5.94(0.30, 0.53) x 10

8.73(1.40, 0.44) x 10

6.67(0.16, 1.00) x 106.24(0.11,0.94) x 107.29(0.17, 1.09) x 109.37(0.22| 1.41) x 102.14(0.02, 0.24) x 102.58(0.01, 0.28) x 103.84(0.01, 0.42) x 104.59(0.02, 0.50) x 104.04(0.12, 0.36) x 103.82(0.38, 0.34) x 104.31(0.56, 0.39) x 103.22(0.45, 0.52) x 101.75(0.23, 0.09) x 101.67(0.17,0.08) x 10'8.45(1.27, 0.42) x 104.44(1.24, 0.22) x 10

d3Invariant cross section, E „3 (mb GeV c )Si+ Pt

Page 5: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

2988 G. E. DIEBOLD et al.

tunes. A corresponding correction has been applied tothe vr cross sections presented here. The m yields mayalso be contaminated by decay product muons which re-main in the accepted phase space of the beam line. Basedon DECAY TURTLE simulations, this contamination is es-tirnated to be about 3%. A corresponding correction hasbeen made.

Taking the most conservative approach and sim-ply adding estimated systematic uncertainties from allsources, one obtains the relative systematic uncertaintieslisted with each measurement in Table III. For all butthe positive rigidity Au beam measurements, these weretypically 5—10% and. reflect observed run-to-run irrepro-ducibility (= 5%) as well as uncertainty in corrections forparticle decay and contamination. Overall normalizationuncertainty is estimated to be about 10% for these mea-surements. For most of the positive rigidity Au beamruns, the beam targeting was unstable. This resulted inadditional relative and overall normalization uncertaintywhich is estimated to be about 10% in each case. Thisraises the uncertainty for the positive rigidity Au beammeasurements to about 15—20% relative and 20% overall.

IV. DISCUSSION

A. Comparison with previous measurements

In light of the Iledgling state of the AGS Au program,there are little or no published data to compare withthe Au + Pt measurements reported here. On the otherhand, the production cross sections for Si + Pt can becompared with minimum bias Si + (A = 200) measure-ments of other AGS experiments, such as E802 (Si +Au), E814 (Si + Pb), and E858 (Si + Au). However, itshould be noted that (y, p&) kinematic overlap of all fourexperiments is small.

For Si + Pt, the present results are limited to rigidi-ties of + 1.8 GeV/c, which correspond to an average pqof 178 MeV/c, while E814 and E858 measure at lower p&

(& 50 MeV/c) and generally above the nucleon-nucleoncenter-of-mass rapidity, y~~ (= 1.72 for 14.6A GeV/c).E802 measures at higher pq () 300 MeV/c) and generallynear and below y~~. The largest overlap of all four ex-periments occurs for measurements of antiprotons [2—4]for which there has been an apparent disagreement be-tween the previous measurements of about a factor of2—4 [5]. The uncertainty implied by such disagreementimpairs the usefulness of antiprotons as probes of thecollision environment.

To make specific comparisons with the E886 Si + Ptantiproton cross sections (corresponding to y = 1.39,pq ——0.178 GeV/c), the E802, E814, and E858 mea-surements at y 1.4 were extrapolated to pq

——0.178GeV/c. For this an exponential in transverse mass, mq

(p2 + m2)1/2

0 —mt /I3dp3

was used with B = 0.141 GeV [+ 10% (statistical) + 10%(systematic)] as reported by E802 for target-averaged

O

C3

LLJ

ThisExpt

I I

E802 E814 E858 E886

AGS Experiment Number

FIG. 4. Comparison of invariant di6'erential antiprotonproduction cross sections measured in Si + A collisions at14.6A GeV/c. The E886 (Si + Pt) measurement is for 1.8GeV/c antiproton production at 5.7' which corresponds to y= 1.39, pq

——0.178 GeV/c. The E802 (Si + Au), E814 (Si +Pb), and E858 (Si + Au) measurements have been extrapo-lated to this y and pq (see text for details).

central Si + Al and Si + Au collisions over the kine-matic range (1.1 & y & 1.7) [2]. The extrapolated pmeasurements of E802, E814, and E858 are shown in Fig.4 for comparison with the E886 Si + Pt measurement.As needed, the A. + A inelastic reaction cross sections,0&&, of Table IV were used to convert invariant multi-

3plicities to invariant cross sections according to E& 3

d3E g&~ /+AA '

Since the average mq of the E814 and E858 invariantcross section measurements are close to that of E886,the above extrapolation is small (- 10% change in theirreported cross sections) and not very sensitive to uncer-tainty in the inverse slope parameter, B. Consequently,only the quoted E814 and E858 experimental uncertain-ties are shown (40% and 20%, respectively). On the otherhand, for minimum bias Si + Au collisions E802 only re-ports pq integrated antiproton yields, &

. These were

estimated from measurements with p& ) 0.3 GeV/c andthe assumed mq behavior of Eq. (1). As a result, the re-ported 10%%uo statistical and 10% systematic uncertainty inB introduces an additional uncertainty of approximately20% when inferring an invariant difFerential p cross sec-tion from the integrated & reported by E802. Thus, anadditional 20% uncertainty was added to the reported35% statistical and 15—20% normalization uncertaintiesof the E802 p measurements to arrive at a total uncer-tainty of 70% shown for the E802 extrapolation in Fig.4.

The distribution and uncertainties of the previous mea-surements shown in Fig. 4 are generally large and canaccommodate a range of values that nearly spans an or-der of magnitude. One Ands the E886 measurement liesabove the E814 and E858 extrapolations by about 0.6and 1.4 standard deviations, respectively. Comparingwith the E802 extrapolation, the E886 measurement isnearly a factor of 3 smaller. However, if one conserva-tively combines the statistical, systematic, and extrapo-

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PRODUCTION OF m+, E*,p, AND p IN RELATIVISTIC. . . 2989

lation uncertainties associated with the E802 data point(as has been done for Fig. 4), these measurements agreewithin one standard deviation.

The addition of the E886 Si + Pt measurement cer-tainly helps clarify the experimental situation. Thepresent result is in basic agreement with the previ-ous measurements and the overall uncertainty and rmsspread of the combined experimental data is reduced.It is worth noting that the E814, E858, and E886 datapoints in Fig. 4 all lie well below that of E802. This trendalso persists over other measured rapidities I5]. As a re-sult, it seems likely that the reported E802 antiprotoncross section may be biased by statistical fluctations to-wards higher yields and/or systematic exaggeration, atleast when extrapolated to low pq. Not excluded, ofcourse, is the possibility that p production deviates frommt, scaling below pq 300 MeV/c.

B. Projectile dependence of m and K cross sections

To illustrate the projectile dependence of the particleyields, the invariant differential cross sections measuredat 1.8 GeV/c are shown in Fig. 5 as a function of pro-jectile mass number, Ap j To facilitate comparison, thecross sections for each reaction have been divided by thecorresponding projectile mass number. The 7t+ and K+data are shown in Fig. 5(a) while the p and p data areshown in Fig. 5(b).

Before discussing the projectile dependence of Fig. 5,it is important to note that each reaction differs in pro-jectile isospin and beam momentum per nucleon. With-out accounting for these differences in detail, conclusionsabout the projectile dependence of the various yields aresomewhat limited. For particle production well abovethreshold (e.g. , sr+), the dependence of the productioncross section on beam momentum per nucleon shouldbe small and differences will be ignored in the presentcomparisons. For antiprotons this in not the case anda correction factor is required to compare the differentsystems in a meaningful way. Such a correction for thep yields has been determined using available p + (A200) data and will be discussed in the following subsec-tion. The dashed curve shown in Fig. 5(b) shows theprojectile dependence of the p yields after applying thiscorrection.

As can be seen in Fig. 5(a), when normalized to theprojectile mass number, the m+ and K+ cross sectionsare nearly equal for the p + Pt and Si + Pt reactions.Thus, the yields of these particles in the Si + Pt reactionare consistent with A~, j scaling of independent p + Ptcollisions. For the Au + Pt reaction, there is a substan-tial increase in the m cross section which exceeds A~, jscaling of p + Pt by a factor of 2.13 + 0.36 (relative) +0.33 (overall). The relative (statistical + systematic) andoverall uncertainties reflects those of the p + Pt and Au+ Pt measurements added in quadrature. No evidence ofa m excess is found for the Si+ Pt reaction. The sr+ andK+ cross sections shown in Fig. 5(a) for Au + Pt alsoexceed A~, j scaling of p + Pt but only by factors of 1.44+ 0.37 + 0.34 and 1.22 + 0.32 + 0.29, respectively. Un-fortunately, because of larger systematic uncertainty for

the positive rigidity Au beam measurements, little if anysignificance can be attributed to these relative increases.Corrections for differences in projectile isospin and beammomenta per nucleon would slightly increase the signifi-cance of any sr+ and K+ excess relative to Ap j scalingof the measured p + Pt cross sections. An enhancementrelative to such scaling is clearly not seen for K pro-duction.

In Au + Pt collisions, the observed increase in thecross section suggests that collective effects, seem-

ingly absent in the Si + Pt reaction, now contributesignificantly to vr production. A plausible explana-tion for this enhancement is a marked increase in (low-energy) rescattering interactions such as %K M N¹rand vrN ~ ¹rm. In this case, a similar enhancementis expected for m+ although this yield may be smallerthan vr due to the neutron excess in the Au + Pt sys-tem. The present sr+ measurements are consistent withsuch enhancement. The fact that the K yields clearlydo not show any enhancement relative to Ap, j scalingmay partially reflect the higher K production thresh-

104 (a)n x10

10'

10

C3

10

I

&C

CL

10'U

210

(b)

10I I I I I I I II I

10

I I I I I I III I

10

—1

10FI I IIIII I I I I IIIII I I I I IIIII I I I I IIIII

10 10Projectile Mass,

10

Aproj

FIG. 5. Projectile mass dependence of the invariant dif-ferential cross sections for 1.8 GeV/c particle production at5.?'. Note that the m cross sections have been multipliedby a factor of 10. For comparison purposes, all cross sectionshave been divided by the mass number of the projectile, A~, ;.The projectile momenta are 12.9A, 14.6A, and 11.5A GeV/cfor the p + Pt, Si + Pt, and Au + Pt reactions, respectively.Errors shown include relative (statistical + systematic) andoverall uncertainty. The lines are drawn as a guide for the eyewith the dashed curve representing p production after correc-tion for differences in beam momentum per nucleon (see textfor details).

Page 7: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

2990 G. E. DIEBOLD et aL 48

old, in which case fewer re-scattering interactions will beabove threshold. Further, as a result of K absorptionin nuclear matter, a decreasing fraction of produced Kmesons are expected to survive as the size of the systemincreases with projectile mass. It seems that either ofthese eKects could contribute to the slower increase ofthe K yields with projectile mass. In addition to thesepoints, as a result of the higher production threshold, thedependence of K production on the projectile momen-tum per nucleon may be non-negligible. Correcting forthis eÃect would reduce the Si + Pt yield and increasethe Au + Pt yield relative to the p + Pt data.

As a consequence of the observed projectile depen-dences, the K /K+ ratio decreases from 0.31 + 0.03and 0.30 + 0.06 in the p + Pt and Si + Pt reactionsto 0.1S + 0.04 in Au + Pt collisions. If this reflects thebehavior of total K+ yields, there must be a correspond-ing enhancement in strange baryon yields (neglecting sig-nificant abundances of antihyperons). Such an increasewould have favorable implications for strange quark mat-ter searches with Au beams at the AGS [10, 11].

C. Projectile dependence of proton and antiprotoncross sections

When normalized to the projectile mass, the protoncross sections for Si + Pt and Au + Pt collisions fall wellbelow that measured in p + Pt [Fig. 5(b)]. Since protonyields largely reflect initial abundances in the collidingsystem, the observed decrease for the A + Pt reactionsmostly reflects the proton mass fraction of the projectiles.

Turning to the projectile dependence of the antiprotonyields, corrections for the difFerences in beam momentaper nucleon will be significant since the nucleon-nucleon(N + N) center-of-mass energy (gs~~) for each reac-tion is not far above the p production threshold [~/'s~~= 3.8 GeV at threshold versus 5 GeV at (12—15)AGeV/c]. Consequently, account must be taken of the un-derlying energy dependence of p production before onecan qualitatively evaluate the projectile dependence ofthe p yields.

To characterize the ps~~ dependence of p productionat these energies, measurements in p + Ta [12] and p +Pb [13] reactions have been used along with the presentp + Pt measurements. These data are similar to thepresent A + Pt measurements both in target mass and(y, pq) kinematic acceptance, and span a range in V'sN~from 4.56 to 6.84 GeV (10.1 & pb, & 24 GeV/c). Fordirect comparison, the p + A data were extrapolated tothe (y, pq) of the present measurements at 5.7'.

These data were generally found to be well representedby a Gaussian distribution in rapidity,

(2)

while the pz dependence was well described by an ex-ponential in mq [Eq. (1)]. These functional forms wereused for extrapolation with the parameters o.„and B de-termined from Bts to the p + A data. All reactions wereconsistent with cry = 0.4 to 0.5 units while B varied from

0.0012

U

0.001 0

Q3

0.0008C3

I I I

e Au+pt~ Si + Pt This ExperirrlentEj P+ Pt

p+Ta~ p+Pb

0.0006

0.0004CL

0.0002

0.00003.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

N + N Center of Mass Energy (C~eV)

I IG. 6. Comparison of invariant differential antiprotonmultiplicity per first collision, E" 3/N~, as a function ofdp3colliding nucleon-nucleon center-of-mass energy, gsiviv (notethat N~ = 1 for p + A collisions). The present p + Pt, Si +Pt, and Au + Pt yields were measured at 5.7' with p/Z = 1.8GeV/c (corresponding to y = 1.39, pz ——0.178 GeV/c). Theremaining p + A yields are extrapolations of data from Refs.[12] and [13] to y = 1.39, pq ——0.178 GeV/c (see text fordetails). Errors shown for the A + Pt data reflect relative(statistical + systematic) uncertainty only; those shown forthe p + A data include relative and overall uncertainty. Thecurve is spline 6t of the extrapolated p + A data drawn toguide the eye.

about 0.02 GeV for the go~~ = 4.56 GeV data to 0.14—0.18 GeV for the ps~~ = 6.84 GeV data. Wheneverpossible, measurements at the same p& and/or same y(or y —y~~) were used, thereby minimizing the extrap-olation in one or both variables (i.e. , y and/or pq). Typi-cally, extrapolations were relatively small and on averageonly spanned 0.1 units in y —y~~ and/or 0.009 GeV inmg

The p + (A = 200) extrapolations to the (y, pq) ofthe present 1.8 GeV/c measurements are shown in Fig.6. Plotted is the invariant p multiplicity per collision,

d3 d3E &, (= E &, /o&&), versus gsN ~. The inelastic reac-

tion cross sections, o.&&', of Table IV were used to con-vert invariant cross sections to invariant multiplicities.The curve drawn through the p + A points is a spline fitto guide the eye. These data provide a means by whichthe energy dependence of p production can be estimatedbased on actual measurements in the range of interestand should be more accurate than extrapolations basedsolely on higher-energy data (+8~~ ) 6.84 GeV) as havebeen used elsewhere (e.g. , Refs. [2, 3]).

The energy dependence of the p + (A = 200) datashown in Fig. 6 has been used to compensate theprojectile-normalized p cross sections of Fig. 5(b) for dif-ferences in beam momenta per nucleon. Even after thiscorrection, the A + Pt antiproton cross sections fall wellbelow independent Az, j scaling of the p + Pt yields asshown by the dashed curve in Fig. 5(b). As mentioned

Page 8: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

48 PRODUCTION OF m*, E+, p, AND p IN RELATIVISTIC. . . 2991

TABI E IV. The number of first collisions, NF and NF", and inelastic reaction cross sections, o&z, determined by MonteCarlo simulation of A + A collisions (see text for details). Listed for comparison are several measured inelastic cross sections.

Reaction

p+ Bep+ Tap+ Ptp+ PbSi+ PtSi+ AuSi+ PbAu+ Pt

1.01.01.01.06.06.06.0

12.0

First collisionsNinelF0.920.800.800.804.44 44 48.8

Simulationo~~ (mb)

1991606168117584025404941236941

Measuredo~~ (mb)

199 (Ref. [15))

1708 (Ref. [15])1770 (Ref. [15])

3835 (Ref. [16])

in connection with K production, collective effects suchas rescattering and secondary N + N interactions maynot contribute significantly to p production due to a highproduction threshold. Additionally, the relatively largeabsorption probability of antiprotons in nuclear mattermill reduce the observed yield. In comparison with theK mesons, each of these effects should be more pro-nounced for antiprotons and indeed the p yield shows alarger production deficit relative to Ap j scaling of p +Pt cross sections.

To quantify the projectile dependence of the p crosssections, one can discuss an effective number of projec-tile nucleons contributing to p production, A,&. This isdefined by the ratio of invariant production cross sectionsin A + Pt and p + Pt collisions,

o;„(A+Pt)~;„„(p+Pt) '

3where o;„=E

& 3. Implicit in this definition is the as-ap

sumption that the shape of p transverse momentum dis-tributions is the same in each reaction. Using the presentA + Pt antiproton cross sections at 1.8 GeV/c along withthe gsiv~ dependence of p production in Fig. 6, one findsA"& 12 and 61 for the Si + Pt and Au + Pt reactions,respectively. These values, which are well below the pro-jectile mass number for each reaction, are plotted in Fig.7 as a function of projectile mass. Fitting this depen-dence as A"+ Ap j one finds a = 0.77 + 0.03. Itis interesting to note that this behavior lies somewhatabove pure surface production (a = 2/3) but well be-low total volume production (n = 1). In fact, A",& isapproximately equal to one-half the number of projectilenucleons residing within 2 fm of the nuclear surface asshown by the solid curve in Fig. 7.

At least two simple scenarios could lead to this half-shell scaling behavior which seems to characterize thepresent measurements of p production. The first sce-nario is one in which p production is dominated by thatin "first" nucleon-nucleon collisions (i.e. , nucleon-nucleoncollisions between previously unstruck nucleons; see Sec.IV D below). Since the nucleon mean free path in nuclearmatter is about 2 fm, such N + N collisions will primar-ily involve projectile nucleons on or near the leading sur-face of the projectile. This would result in the observedscaling provided there is relatively little p absorption byother projectile nucleons (absorption by target nucleons

D. First collision comparison of antiproton yields

A common basis for comparing p production in A +A and p + A collisions is the p multiplicity per "first"

31 0 I I I I III4-

D

C

b

CL

10 2

10

I I I I I IIII

10I I I I IIIII I I I

10I I I II

10

Projectile Mass Number, A„.,

FIG. 7. Projectile mass dependence of the number of in-teracting projectile nucleons efFectively contributing to p pro-duction, A",&, [defined as o;„(A + Pt)/o; «(p + Pt) where

o;„„=R& 3 from the 1.8 GeV/c antiproton measurements].dp

The errors shown reflect relative (statistical + systematic) un-certainty only; the overall systematic uncertainty is approxi-Inately 20+0. As a reference, the dashed curve is the line A ff= A~, ;. The solid curve is one-half of the number of nucle-ons within 2 fm of the projectile surface assuming a uniformnuclear density.

is presumably the same in the A + Pt and p + Pt cases).Alternatively, if collective effects significantly contributeto antiproton production, the total number of interacting(projectile) nucleons may be a more appropriate mea-sure of initial y production. Such production togetherwith p annihilation by trailing projectile nucleons wouldyield a scenario in which only antiprotons produced inN + N collisions suKciently close to the trailing projec-tile surface (i.e. , "last" collisions) would avoid subsequentannihilation (again, absorption by target nucleons is pre-sumably the same in the A + Pt and p + Pt cases).For the relative momenta of the present measurements,the absorption length of antiprotons in nuclear matteris about 1.5—2 fm, thus yielding half-shell scaling of theapproximate size observed.

Page 9: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

2992 G. E. DIEBOLD et al. 48

nucleon-nucleon collision introduced by E802 [2]. Fol-lowing their definition, a "first' nucleon-nucleon collision(hereafter referred to simply as "first collision" ) is definedas one in which neither of two colliding nucleons has pre-viously undergone an interaction in the A + A collisionprocess. Since such N + N collisions occur at the highesti/s~~, they should have the greatest chance of produc-ing antiprotons given the relatively large pp productionthreshold at these energies.

To make such a first collision comparison, the invariant3

difFerential p multiplicity per A + A collision, E &, (=dpd3Ed, /iT&&i), is compared with that from p + A after scal-

ing by the average number of first collisions occurring inthe A + A reaction. A simple Monte Carlo simulation ofthe A + A collision was used to determine values for O.

A&and the average number of first collisions, %~. These arelisted in Table IV. The simulation used a Woods-Saxondistribution to describe the nuclear density and definedcolliding nucleons in a Glauber model, that is, as con-stituents which pass within a distance v cr~~/7r of eachother, where o~~ is the total K + N cross secti.on (=40 mb). The determination of iT&&' required. at least oneinelastic % + N scatter in the A + A collision. The sim-ulation agrees with the number of first collisions cited byE802 in Si + A collisions [2] and reproduces measured in-elastic reaction cross sections [14, 15] for various systemsunder discussion.

As mentioned above, owing to the relatively large ppproduction threshold, one might expect that p produc-tion will be dominated by two-body collision processesin which the available center-of-mass energy is maximum.In this case, direct p production in first collisions wouldbe the dominant production mechanism in A + A col-lisions at AGS energies. However, given a considerablenumber of secondary interactions (especially for a largesystem such as Au + Pt as suggested by the vr enhance-ment noted previously), additional p production fromsuch sources may be significant. Nonetheless, first col-lision yields should roughly correspond to a lower limitof the expected p production and provides a simple basisof comparison for observed yields.

Within this framework, relative to a first collision scal-ing of p + A yields, an observed p enhancement wouldsuggest significant production due to collective eff'ects

(e.g. , secondary interactions) while an observed p sup-pression would indicate antiproton absorption due to theprojectile. It is also possible that secondary interac-tions, collective efFects, and annihilation may conspire toyield little or no deviation from first collision predictions.Thus, the most one can conclude from such a compari-son is the net dominance of additional production mech-anisms or annihilation processes at work in the collision.

The invariant antiproton multiplicities per first colli-3

sion, E&, /K~, are shown in Fig. 6 for the Si + Pt and

Au + Pt reactions. These multiplicities were Ineasuredat a rigidity of 1.8 GeV/c and can be directly comparedwith the p + (A 200) multiplicities shown (for which%~ = 1). Both the Si + Pt and Au + Pt first collisionyields are consistent with the p + (A = 200) behavior.Similar agreement with first collision estimates is also

0.004O

0.003

0.002

v~ 0001

——(p + Pt)+N

e Au+ Pt

I I]

i I I I

iI i I I

0 I I I I I I I

0.6 0.8I I I I I I I I I

1.2 1.4 1.6

Rapidity

FIG. 8. The invariant difFerential antiproton multiplicityat 5.7 versus rapidity for Au + Pt collisions. The dashedcurve is a spline fit of the expected yield from erst collisionscaling of p + (A —200) data (see text for details). Er-rors shown reflect the relative (statistical + systematic) un-certainty.

found for the Au + Pt antiproton yields measured atlower rapidities. Figure 8 shows the invariant 5.7 an-

d3tiproton yields, Ed, , versus rapidity for the Au + Ptreaction. Also plotted is the expected yield from firstcollision scaling of p + (A = 200) collisions at ~s~~ =4.83 GeV (corresponding to a beam momentum per nu-cleon of 11.5A GeV/c). The p + (A = 200) estimate isbased on linear interpolation between the present p + Ptdata (gs~~ = 5.10 GeV) and p + Ta data of Ref. [12](gsiviv = 4.56 GeV).

As evident from Fig. 8, the measured Au + Pt an-tiproton yields are in good agreement with first collisionscaling of the p + (A 200) data. The ratio of the Au +Pt yields to the first collision scaling of the p + (A —200)data is 1.04 + 0.14 (relative) + 0.16 (overall). This ratiofor the Si + Pt data at 1.8 GeV/c is 0.77 + 0.21 (relative)6 0.12 (overall). The relative (statistical + systematic)and overall uncertainties reffects those of the p + A andA + Pt measurements added in quadrature.

Recently, E802 has published measurements of p pro-duction in p + A collisions at 14.6 GeV/c [16]. Basedon these measurements, they conclude their Si + Au an-tiproton yield [2] is twice the first collision scaling of theirp + Au yields. This divers from the present Si + Pt com-parison by a factor of 2.6 (= 2/0. 77) which is consistentwith the aforementioned diff'erence between the two ex-periments in absolute yield (Fig. 4). This indicates the p+ A comparison data used in each case are in agreement.

If first collisions were the sole source of p production inA + A collisions, the similarity of the Si + Pt and Au +Pt antiproton yields and first collision scaling of the p +(A = 200) data would be rather surprising. One wouldexpect the local baryon density and/or longitudinal ex-tent of the A + Pt systems would be greater than in thep + (A = 200) system. This should increase p absorptionin the A + Pt systems and result in lower first collisionyields (barring additional production mechanisms). Thepresent results seem to disagree with these simple expec-

Page 10: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

48 PRODUCTION OF m, K*,p, AND p IN RELATIVISTIC. . . 2993

tations, although the uncertainty of the data [includingthat of the p + (A 200) comparison data] cannot ex-clude deviations from first collision yields at the 20—30%level.

For completeness, it should be noted that scaling thep + A yields by the number of first collisions as definedabove (i.e. , including elastic first collisions in the tally)will actually overestimate the minimum p yield in thespirit of first collision production. To bracket this over-estimation, one can define a number of inelastic first colli-sions, N&, by excluding from the erst collision tally anynucleons which elastically scatter. These are also listedin Table IV. For p + p, p + (A = 200), Si + Pt, and Au+ Pt reactions, N+ is smaller than N~ by a factor of1.0, 0.80, 0.73, and 0.73, respectively. If one were scalinga very light system such as p + p, the overestimation maybe as high as a factor of 1.37 (=1/0.73). However, for thepresent p + (A —200) comparison this overestimation islimited to 10%. The ratio of the Si + Pt and Au + Ptyields to K&'~ scaling of the p + (A —200) data is 0.84+ 0.23 + 0.13 and 1.13 + 0.15 + 0.18, respectively.

V. CONCLUSIONS

We have reported measurements of inclusive particleproduction at 5.7 in relativistic collisions between veryheavy nuclei (Aqs&, A~, ~& = 200). Production cross sec-tions for minimum bias Au + Pt collisions have beencompared with those from p + Pt and Si + Pt collisionsat similar (but not equal) beam energy per nucleon.

For the Si + Pt reactions, the m+ and K yields areconsistent with Apzzj scaling of the p + Pt cross sections.In stark contrast to this behavior, the vr cross sectionfor Au + Pt shows a significant enhancement and exceedsAp j scaling of independent p + Pt collisions. This ob-servation suggests that collective production mechanisms(such as reseat tering of produced particles and secondaryN + N collisions) are relatively abundant in the Au + Ptreaction and play a important role in vr production inthis case. The absence of a similar enhancement in the Si+ Pt system suggests that these collective eKects occurless frequently in the smaller Si + Pt system. The ~+and K+ yields from Au + Pt also show an excess beyondAp j scaling of p + Pt collisions; however, the presentsigni6. cance of this excess is very small. The K yield inthe Au + Pt reaction continues to nearly scale with pro-jectile mass relative to the measured p + Pt cross section.Correction for difFerences in beam momenta per nucleon(expected to be small for 7r+ and K+ production) wouldincrease the noted enhancements in the Au + Pt crosssections.

The difFerences in the +shiv of each reaction are cer-tainly significant for p production. Based on available p+ A data, a suitable characterization of the +8iviv de-pendence of p production was developed for comparisonwith the measured p cross sections in the A + Pt reac-tions. Not surprisingly, the p cross sections increase evenmore slowly than the K yields with projectile mass. Bycomparing the A + Pt antiproton cross sections with thatfrom p + (A —200) reactions at the same gs~~, one canassess an efFective number of nucleons, A &, contributing

to p production in these collisions. For the present Si +Pt and Au + Pt data, A"& varies with projectile massas A, + . For the Si and Au projectiles, such depen-dence is very close to half the number of projectile nu-cleons within 2 fm of the projectile surface. The physicalorigin of this Ap, j dependence is unclear and its rangeof validity remains to be explored experimentally.

To further elucidate the plight of antiprotons, thepresent A + Pt yields were compared with first collisionscaling of p + (A = 200) data. While there has been dis-agreement in overall p yield, all experiments find that therelative yield for difFerent colliding systems (e.g. , Si + Alversus Si + Pb) is consistent with a constant p multiplic-ity per first collision. Aside from disagreement in totalyield, diA'ering estimates of antiproton production in p +A reactions have led to disparate conclusions of the ob-served p multiplicity per first collision. The present workhas attempted to improve this situation in several ways.First, the reported Au + Pt measurements extend thestudy of antiprotons to virtually the heaviest A + A sys-tem possible. Second, the present Si + Pt measurementcan be weighed with previous results to reine the bestestimate of p yields in such intermediate mass reactions.And Anally, for comparison with the E886 measurements,an estimate of p production in p + (A —200) has been de-veloped which is well constrained by data near the +shivand phase space acceptance of the present measurements.As a result of this e8'ort, the measured Si + Pt and Au+ Pt antiproton multiplicities are found to be consistentwith first collision scaling of p + (A = 200). To withinabout 20—30% uncertainty, the observed p multiplicityper first collision appears to be independent of projectilemass for Ap, j

——1, 28, and 197.Altogether, it is remarkable that over such a wide

range in projectile mass the antiproton yields do not de-viate substantially from first collision scaling. As an-tiproton absorption is expected to increase as the sizeof the colliding system increases, it is dificult to recon-cile the observed first collision scaling with entirely firstcollision production. Since additional production fromsecondary interactions is expected at some level, one isled to the conclusion that additional production must be(very nearly) balanced by additional absorption indepen-dent of the system size. The data cannot constrain thesize of these competing e8'ects.

While the exact mechanism and details of this balanceremains elusive, it should be noted that some types ofcollective effects (which can lead to additional p produc-tion) will scale with the number of first collisions. Forexample, according to the simple geometric model of A+ A collisions used to estimate erst collisions, the numberof W + N collisions between scattered projectile (target)nucleons and previously unstruck target (projectile) nu-cleons is found to scale with the number of erst collisionsindependently of the projectile (target) mass. Thus asignificant fraction of p production which originates fromsuch non6. rst collisions will nonetheless scale with firstcollisions. Further, antiproton production from nonfirstcollisions which deviate from (e.g. , exceeds) first colli-sion scaling will arguably be subject to a diferent (e.g. ,higher) absorption probability. Thus, it is plausible that

Page 11: ,               p               , and               p               ¯ in relativistic Au+Pt, Si+Pt, and               p               +Pt collisions

G. E. DIEBOI D et al.

difI'erences in p production due to sources which deviateRom erst collision scaling are efFectively (and specifically)reduced by absorption. The "last" collision scenario dis-cussed in connection with A"& is a specific example.

Undoubtedly, a more sophisticated treatment of an-tiproton production and absorption is required to quan-titatively understand the observed p yields and the (semi-empirical) scaling relations noted. For this reason, spe-cific comparisons of the present data with state-of-the-artcollision siinulation models such as ARC [17] and RQMD[18] are warranted.

Finally, it should be noted that additional experimen-tal p data spanning large regions of the available param-eter space (e.g. , rapidity, transverse momentum, beamenergy, centrality, projectile, and target mass) wouldgreatly improve the current experimental situation. Sincepotentially interesting eÃects may be subtle, reducing

the statistical and systematic uncertainties of subsequentmeasurements would be very beneficial.

ACKNOWLEDGMENTS

The authors would like to thank the BNL accelera-tor and support stafI' for their efforts during these runsespecially for the successful commissioning of the Au pro-gram. Thanks are also extended to Sean Gavin, Sid Ka-hana, Yang Pang, Tom Schlagel, Ekkard Schnedermann,and B. Shiva Kumar for stimulating discussions. Thiswork is supported in part by the U. S. Departmentof Energy under Contracts Nos. DE-FG02-91ER40609,DE-AC02-76H00016, and DE-FG04-88ER40396, by theGerman Federal Minister for Research and Technology(BMFT) under Contract No. 06 FR 652, by the UnitedKingdom SERC, and by the Japanese Society for thePromotion of Science.

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