Beam Energy Dependence of Fifth and Sixth-Order Net-proton Number Fluctuations in Au+Au Collisions at RHIC

The STAR collaboration
Phys.Rev.Lett. 130 (2023) 082301, 2023.

We report the beam energy and collision centrality dependence of fifth and sixth order cumulants ($C_{5}$, $C_{6}$) and factorial cumulants ($\kappa_{5}$, $\kappa_{6}$) of net-proton and proton distributions, from $\sqrt{s_{NN}} = 3 - 200$ GeV Au+Au collisions at RHIC. The net-proton cumulant ratios generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at $\sqrt{s_{NN}}$ = 3 GeV. $C_{6}/C_{2}$ for 0-40% centrality collisions is increasingly negative with decreasing $\sqrt{s_{NN}}$, while it is positive for the lowest $\sqrt{s_{NN}}$ studied. These observed negative signs are consistent with QCD calculations (at baryon chemical potential, $\mu_{B} \leq$ 110 MeV) that include a crossover quark-hadron transition. In addition, for $\sqrt{s_{NN}} \geq$ 11.5 GeV, the measured proton $\kappa_{n}$, within uncertainties, does not support the two-component shape of proton distributions that would be expected from a first-order phase transition. Taken in combination, the hyper-order proton number fluctuations suggest that the structure of QCD matter at high baryon density, $\mu_{B}\sim 750$ MeV ($\sqrt{s_{NN}}$ = 3 GeV) is starkly different from those at vanishing $\mu_{B}\sim 20$MeV ($\sqrt{s_{NN}}$ = 200 GeV and higher).

  • Table 1

    Data from panel (b) of Figure 1.


    Event-by-event proton multiplicity distributions for 0-40$\%$, 0-5$\%$ and 50-60$\%$ Au+Au collisions at $\sqrt{s_{NN}} = 3 GeV. The distributions are not...

  • Table 2

    Data from Figure 2.


    Proton factorial cumulants K4, K5 and K6 in 0-40$\%$ and 50-60$\%$ Au+Au collisions from $\sqrt{s_{NN}}$ = 3 - 200 GeV....

  • Table 3

    Data from Figure 2.


    Proton factorial cumulants K4, K5 and K6 from UrQMD model (0-40$\%$ and 50-60$\%$ centrality) for Au+Au collisions from $\sqrt{s_{NN}}$ =...

  • Table 4

    Data from Figure 3 and Figure 4.


    Net-proton cumulant ratios C4/C2, C5/C1, C6/C2 (0-40$\%$ and 50-60$\%$ centrality) and C3/C1 (0-40$\%$ centrality) in Au+Au collisions from $\sqrt{s_{NN}}$ =...

  • Table 5

    Data from Figure 3 and Figure 4.


    Net-proton cumulant ratios C4/C2, C5/C1, C6/C2 (0-40$\%$ and 50-60$\%$ centrality) and C3/C1 (0-40$\%$ centrality) from UrQMD model in Au+Au collisions...

  • Table 6

    Data from Figure 1 of supplemental note.


    Fifth-to-first (KS5/KS1) and sixth-to-second (KS6/KS2) order K-statistic ratios of net-proton distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 GeV for...

  • Table 7

    Data from Figure 2 of supplemental note.


    Net-proton cumulant ratio C6/C2 and corresponding K-statistic ratio (0-40$\%$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 GeV as a function...

  • Table 8

    Data from Figure 4 of supplemental note.


    Cumulant ratios C4/C2, C5/C1, C6/C2 and C3/C1 of net-proton distributions measured with half rapdity coverage (-0.5 $<$ y $<$ 0)...

Version 2 modifications: In this version, in addition to the data points published in the paper, a new set of data for net-proton C3/C1 measured with half rapidity (see Table 8) are also included.

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