BNL-65375
1 .' C£3
Centrality and Collision System Dependence of Antiproton Production^,
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from p4-A to Au-fAu Collisions at AGS Energies
H. Sako, for the E802 Collaboration
BNL-UCBerkeley-UCRiverside-Columbia-INS(Tokyo)-Kyoto-LLNL-Maryland-MITTokyo-Tsukuba-Yonsei
L. Able7, Y. Akiba5, K. Ashktorab1, M. E». Baker7, D. Beavis1, H. C. Britt6, J. Chang3,
C. Chasman1, Z. Chen1, Y. Y. Chu1, V. Cianciolo13, B. A. Cole14, H. J. Crawford2,
J. B. Gumming1, R. Debbe1, J. C. Dunlop7, W. Eldredge3, J. Engelage2, S. -Y. Fung3,
E. Garcia11, S. Gushue1, H. Hamagaki12, L. Hansen6, R. S. Hayano15, G. Heintzelman7,
E. Judd2, J. Kang10, E. -J. Kim10, A. Kumagai9, K. Kurita9, J. -H. Lee1, J. Luke6,
Y. Miake9, A. Mignerey11, B. Moskowitz1, M. Moulson14, C. Muentz1 S. Nagamiya4,
M. N. Namboodiri6, C. A. Ogilvie7, J. Olness1, L. P. Remsberg1, H. Sako9,
T. C. Sangster6, R. Seto3, J. Shea11, K. Shigaki1, R. Soltz6, S. G. Steadman7,
G. S. F. Stephans7, M. J. Tannenbaum1, J. H. Thomas1, S. Ueno-Hayashi9, F. Videbaek1,
F. Wang14, Y. Wu14, H. Xiang3, G. H. Xu3, K. Yagi9, H. Yao7, W. A. Zajc14, F. Zhu1
1
Brookhaven National Laboratory, Upton, NY 11973
University of California, Space Sciences Laboratory, Berkeley, CA 94720
3
University of California, Riverside, CA 92507
4
High Energy Accel. Res. Organization (KEK), Oho, Tsukuba, Ibaraki 305, Japan
5
High Energy Accel. Res. Organization (KEK), Tanashi-branch, Midoricho, Tanashi,
Tokyo 188, Japan
6
Lawrence Livermore National Laboratory, Livermore, CA 94550
7
Massachusetts Institute of Technology, Cambridge, MA 02139
8
Department of Physics, University of Tokyo, Tokyo 113, Japan
9
University of Tsukuba, Tsukuba, Ibaraki 305, Japan
10
Yonsei University, Seoul 120-749, Korea
11
University of Maryland,College Park, MD 20742
12
Center for Nuclear Study, School of Science, University of Tokyo, Midoricho, Tanashi,
Tokyo 188, Japan
13
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
14
Columbia University, New York 10027 and Nevis Laboratories, Irvington, New York
10533
15
University of Tokyo, Tokyo 113, Japan
2
Antiproton production in 11.7 A-GeV/c Au+Au collisions over a wide transverse-mass
coverage was studied in the AGS-E866. The inverse slope parameter increases rapidly as a
function of centrality. Antiproton yields in Si+A and Au+Au collisions are consistent with
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the scaling with the 2/3 power of the number of participant nucleons. Transverse-mass
spectra are similar to those of protons from peripheral to central Au+Au collisions.
1. Introduction
Antiproton (p) production in heavy ion collisions reflects subtle interplay between initial
production and absorption by nucleons. Because the ACS energies (10 — 20 A • GeV/c)
are close to the p production threshold, p may be sensitive to cooperative processes such
as QGP [1] and hadronic multi-step processes [2]. On the other hand, p has been proposed
as a probe of baryon density due to large NN annihilation cross sections [3]. Cascade
models [4-6] predict the maximum baryon density reaches about 10 times the normal
nucleus density in central Au+Au collisions, where the strong p absorption is expected.
In this paper, we show systematic studies of p production from p+A to Au+Au collisions.
2. Analysis in AGS-E866 Experiment
The AGS-E866 experiment is aimed at studies of particle production in 10-12 A-GeV/c
Au+Au collisions as a function of centrality. The experimental setup is described elsewhere [7,8]. In this analysis, data taken in 1994 in the Forward Spectrometer are used.
Centrality is defined with the zero-degree calorimeter (ZCAL). The kinematic coverage for
p is 1.0 < y < 2.2 and 0 < mt — mp < 1.2 [GeV/c2], where y, mt, and mp denote rapidity,
transverse mass, and p mass, respectively. About 800 p candidates were extracted out of
about 15 million minimum-bias collisions.
3. Results
Fig. 1 shows mt spectra in minimum-bias events. Kinematic reflections of the spectra in
each rapidity are consistent within statistical uncertainties. E886 [9] and E878 [10] results
at pt ~ 0 agree with our data. Fig. 2 shows mt spectra in 1.0 < y < 2.2 in centrality
windows of 0 - 8 %, 8 - 23 %, 23 - 38 %, and 38 - 77 % (zero corresponds to most
central). Inverse slope parameters increase rapidly as a function of centrality from 0.18 to
0.28 GeV/c2. E864 [11] and E878 [10] data at pt ~ 0 agree with our data except for in the
most centrality window, where the E864 point is 4 times larger than the E878 point, and
the exponential extrapolation of our data comes between them. It is an open question
whether this is due to acceptance difference of the p decaying from A. The acceptance in
our spectrometer is estimated to be 42 % including the branching ratio of 64 %.
Fig. 3 shows comparison of dN/dy among p+A [12], Si+Al and Si+Au data [13] at
14.6 A-GeV/c in J/MV — 0.6 < y < y^N and Au+Au data at 11.7 A-GeV/c in \y — y^N\ <
0.6 as a function of the number of participants (Npart}. The Npart was calculated with
FRITIOF 1.7 [14]. A beam energy correction factor of 0.47 is applied to p+A and Si+A
data. Si+A and Au+Au data are consistent with the Np£t scaling.
These data are compared with RQMD (solid line) and the first collision model (dashed
line). RQMD calculations are from Ref. [15] for p+A and were done with version 2.3
for Si+A and 2.1 for_Au+Au. In RQMD, initial p production is enhanced by multi-step
processes and free NN annihilation cross sections are used. The first collision model gives
p yields as dN/dy = dN/dyp+p • Nf, where dN/dyp+p is dN/dy in p+p collisions, and Nf
DISCLAIMER
This repor was prepared ar an account of work sponsored by an agency of the
United States Government Neither the United States Government nor any agency
thereof, nor any of their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents
that its use would not infnnge privately owned rights Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof
The views and opinions of authors expressed herein do not necessarily state or
reflect those of the United States Government or any agency thereof
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centrality windows. See text for details.
is the number of binary collisions between unstruck nucleons. No absorption is assumed.
Both models reproduce p+A data, and the scaling of N^t from Si+A to Au+Au data.
In Fig. 4, mt spectra are compared with those of protons. For all centrality windows,
their shapes appear similar, but more data are needed for a quantitative evaluation.
4. Conclusions and Outlook
E866 measured p production in Au+Au collisions at 11.7 A-GeV/c in wide transverse
mass coverage. The dN/dy from Si+A to Au+Au collisions scales with Np^. Both
RQMD and the first collision model reproduce the global system dependence of p yields.
However, by construction, the latter cannot reproduce the rapid development of the inverse slope parameter with centrality in Au+Au collisions. This observation implies that
it is important to investigate rat spectra to explore p production mechanisms. The mt
spectra of p are similar to those of the proton from peripheral to central Au+Au events,
and this will be investigated in more detail with a larger data sample in 1995, as well as
the data in E866's large angle spectrometer, Henry Higgins.
This work is supported by the U.S. Department of Energy under contracts with BNL
(DE-AC02-98CH10886), Columbia University (DE-FG02-86-ER40281), LLNL (W-7045ENG-48), MIT (DE-AC02-76ER03069), UC Riverside (DE-FG03-86ER40271), by NASA
(NGR-05-003-513), under contract with University of California, by Ministry of Education
and KOSEF (951-0202-032-2) in Korea, and by the Ministry of Education, Science, Sports,
and Culture of Japan.
Antiproton dN/dy vs Nport
Preliminary Antiproton and Proton
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Figure 3. The dN/dy in p+A, Si+A and Figure 4. Comparison of mt spectra beAu+Au collisions as a function of Npart. See tween p (scaled by 4000) and the proton
text for details.
in Au+Au collisions. See text for details.
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M98004992
Report Number (14)
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Publ. Date (11)
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Sponsor Code (18) lx>£/£fi\ rfA*A\ K*M& /To 5UC Category (19) ^g^/f;^c-QO^^.^oo; U<L~OOQ\
DOE
Hiroyuki Sako