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The main goal of the Beam Energy Scan (BES) program at RHIC is to map the phase
diagram of QCD. The second order critical point or the Critical End Point (CP) at which the
first-order transition between hadron matter and quark-gluon plasma (QGP) ends is one of
the distinctive features of the phase diagram. Its location experimentally would provide one
of the most stringent tests for Lattice calculations. To map out the phase diagram, in BES,
√
the center of mass energy s N N was varied from 200 to 7.7 GeV, which in turn changed the
temperature and chemical potential of the produced matter. The strongly coupled QGP then
expands and cools, following a path on the phase diagram characterized by approximately
constant entropy density to baryon number density ratio until freeze-out. One can control that
the CP lies as close to the freeze-out by varying the center of mass energy. At the CP, the critical
mode becomes massless and develops large long wavelength correlations (ξ), resulting in large
fluctuations of the various moments of the distributions of conserved quantities, such as net
baryon, net charge and net strangeness[1]. The variance (σ 2 = (∆N ) 2 ; ∆N = N − M ;where
M is the mean) of these distributions are related to ξ as σ 2 ξ 2 [2]. It was recently shown that
higher moments of distributions, measuring the deviations from a Gaussian, have a sensitivity
to CP fluctuations that is better than that of σ 2 , due to a stronger dependence on ξ[2, 3, 4].
The numerators in skewness (S = (∆N ) 3 /σ 3 ) go as ξ 4.5 and kurtosis (κ = [ (∆N ) 4 /σ 4 ] − 3)
go as ξ 7 [5].
In this talk the measurements of the various moments (mean (M ), standard deviation (σ),
skewness (S) and kurtosis (κ)) and their volume independent products (such as κσ 2 and Sσ) of
+
−
net kaon (∆N K = N K
− N K
) multiplicity measured by the STAR detector at mid-rapidity for
√
Au+Au collisions at s N N =7.7 to 200 GeV center of mass energies. The energy and centrality
dependence of higher moments and their products (Sσ and κσ 2 ) of net-kaons along with net-
charge and net-proton as a proxy of net-strangeness, net-charge and net-baryon will be reported.
All the results are corrected for particle reconstruction efficiency and will be compared to
baseline distributions constructed following Poisson and Negative Binomial/Binomial statistics.
They will also be compared to calculations from HRG model, which do not include a CP.
Deviations from some of the baseline measures and model calculations are observed for net-
kaon and net-proton distributions. The implications of the observation to CP physics will be
presented.
References
[1] V. Koch A. Majumder, and J. Randrup, Phys. Rev. Lett. 95, 182301 (2005); M. Asakawa
U. Heinz, and B. Muller, Phys. Rev. Lett. 85, 2072 (2000).
[2] M. A. Stephanov, Phys. Rev. Lett. 102, 032301 (2009); Phys. Rev. Lett. 107, 052301 (2011);
C. Athanasiou, et al., Phys. Rev. D 82, 074008 (2010).
[3] M. M. Aggarwal, et al., (STAR Collaboration), Phys. Rev. Lett. 105, 22302(2010).
[4] F. Karsch and K. Redlich, Phys. Lett. B 695, 136 (2011).
[5] J. Cleymans et al., Phys. Rev. C 73, 034905 (2006). |