Scaling Properties of Hyperon Production in Au + Au Collisions at sqrt sNN = 200 GeV Page: 3 of 7
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PACS numbers: 25.75.Dw
Lattice Quantum ChromoDynamics calculations pre-
dict that a new state of matter, the Quark Gluon Plasma
(QGP), can be formed at zero baryon density in nu-
clear collisions when the temperature exceeds 160 - 170
MeV [1]. Strange quarks, whose mass is comparable to
the critical temperature, are expected to be abundantly
produced by thermal parton interactions in the high tem-
perature QGP phase. Due to the corresponding increase
in the strange quark density, hyperon production is ex-
pected to be enhanced in high energy nuclear collisions,
the enhancement increasing with the number of strange
valence quarks in the hyperon [2]. Such an effect has al-
ready been observed in various fixed-target experiments
at lower energy by comparing the number of hyperons
produced per participating nucleon in nucleus-nucleus
and proton-nucleus collisions [3, 4, 5]. In this letter, we
study the centrality dependence of hyperon production
in Au + Au collisions at a collision energy of sNN
200 GeV, which is an order of magnitude higher than
that previously achieved. We also study the transverse
momentum dependence of hyperon production in central
and peripheral collisions in an attempt to shed light upon
the possible production mechanisms.
Previous studies have shown that ratios of hadron
yields in high energy nucleus-nucleus collisions are gen-
erally well described by statistical models in the grand
canonical limit, where baryo-chemical potential and tem-
perature are parameters [6, 7, 8]. A strangeness phase-
space occupancy factor, >3, is sometimes introduced to
describe the extent to which strangeness reaches its equi-
librium abundance. In this framework, the amount of
strangeness produced per participating nucleon (Npart)
is directly related to the value of -y. The centrality de-
pendence of , therefore provides a quantitative measure
of strangeness equilibration as a function of system size
in nucleus-nucleus collisions [9], under the assumption
that the grand canonical approximation remains valid in
non-central collisions.
By contrast, at high transverse momentum, hadrons
are thought to be produced via incoherent hard scatter-
ings, which, in the absence of any nuclear medium effects,
should scale with the number of binary nucleon-nucleon
collisions (Nbinary) [10, 11]. Measurements of hadron
production in Au + Au collisions at RHIC have shown
that not only is there a deviation from binary scaling in
central collisions [10, 12], but also a distinct difference
in the scaling behaviour of baryons and mesons in the
intermediate transverse momentum range, 2 < PT < 5
GeV/c [13, 14]. A strong particle-type dependence is not
predicted by conventional Monte Carlo (MC) event sim-
ulators such as HIJING, where hadron formation in this
region is dominated by independent parton fragmenta-
tion [15]. On the other hand, quark recombination (co-alescence) models have been successful in explaining the
observed deviation from binary scaling for baryons and
mesons in central collisions [16, 17, 18, 19], as well as pro-
viding an explanation for the particle-type dependence
of measured azimuthal anisotropies at intermediate PT
in non-central collisions [14]. By extending these previ-
ous studies to include multi-strange baryons we provide
a more stringent test of recombination models. Further-
more, it may allow us to probe the differences between
strange and light (up and down) quark distributions pro-
duced in nucleus-nucleus collisions.
The STAR Time Projection Chamber (TPC) mea-
sures the trajectories and momenta of charged particles
produced in each collision in the pseudo-rapidity range
lyj < 1.8 [20]. The detector operates within a solenoidal
magnetic field of 0.5 Tesla whose axis is aligned with
the beam. A central trigger barrel, covering the pseudo-
rapidity region Iyj < 1, and two zero-degree calorime-
ters are used as trigger detectors. A total of 1.6 x 106
minimum-bias trigger collisions and 1.5 x 106 central trig-
ger collisions were used for this analysis. A detailed
description of the analysis including particle reconstruc-
tion, track quality, decay vertex topology cuts and cal-
culation of the detection efficiency can be found else-
where [21, 22, 23]. In this study A(A), B-(E) and
Q- ((+) have been measured in rapidity intervals of
yJ < 1, 0.75 and 0.75, respectively. In order to increase
statistics, the results for Q- and + have been com-
bined. Within the chosen rapidity intervals the particle
reconstruction efficiency is a function of transverse mo-
mentum and lifetime. The efficiency calculations were
based on the probability of finding Monte Carlo gen-
erated particles after processing them through a TPC
detector response simulation, embedding them into real
events and then reconstructing them as real data. The
collision centrality was defined by the charged particle
multiplicity measured in the TPC in the pseudo-rapidity
range Iyr < 0.5. Five centrality bins were selected cor-
responding to the following ranges in the total hadronic
cross section (0 - 5%, 10 - 20%, 20 - 40%, 40 - 60%,
60 - 80%). The 0 - 5% bin represents the most cen-
tral collisions and was obtained from the central trig-
ger sample. The remaining bins were obtained from the
minimum-bias sample. Due to relatively poor statistics,
the 5-10% bin and the Q 10-20% and 60-80% bins were
omitted from this analysis.
Figure 1 shows the transverse momentum distributions
of A(A), --(+) and Q- +U+ measured at mid-rapidity
and as function of centrality. The errors shown on the
data points are statistical only. The A spectra were cor-
rected for feed-down from multi-strange baryon weak de-
cays, based upon the measured 8 and Q spectra. The
feed-down correction depends sensitively on both exper-
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Adams, J. Scaling Properties of Hyperon Production in Au + Au Collisions at sqrt sNN = 200 GeV, article, June 8, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc895898/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.