Description
Fluctuations of and correlations among conserved charges of strong
interactions have long been considered sensitive observables
for the exploration of the structure of the phase diagram of QCD. Cumulants
of conserved charge fluctuations are the most promising experimental observables
in the search for a critical point in the phase diagram of QCD performed in the
beam energy scan (BES) at RHIC. The published data on cumulants of net-proton number
fluctuations and, in particular, the still preliminary data set on net-proton
fluctuations which covers a larger transverse momentum range show obvious
deviations from the thermodynamics of a hadron resonance gas (HRG). This
naturally raises the question whether ratios of cumulants of net-proton number
fluctuations, and the relation between them and net electric charge cumulants
measured by STAR and PHENIX can be understood in terms of equilibrium
thermodynamics obtained from lattice QCD calculations.
We present results for cumulants of net-baryon number and net electric
charge fluctuations calculated
in a next-to-leading order (NLO) Taylor series in the baryon chemical potential
($\mu_B$). We discuss the resulting pattern of ratios of cumulants,
e.g. $\kappa_B\sigma_B^2\equiv \chi_4^B/\chi_2^B$ and
$S_B\sigma_B^3/M_B\equiv \chi_3^B/\chi_1^B$. We note that both quantities are
identical at $\mu_B=0$ and that the curvature of the
latter is about three times larger than the former. Comparing this generic
structure with STAR results on net-proton number fluctuations we conclude that
current BES results at energies $\sqrt{s} \ge 19.6$~GeV
are compatible with QCD thermodynamics and can be understood in a NLO Taylor
expansion. We also discuss changes of freeze-out parameters that arise from the new,
preliminary STAR data on net-proton fluctuations compared to the published data and the
recent PHENIX data on electric charge fluctuations.
| On behalf of collaboration: | [Other] |
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