Fluctuations of conserved quantities such as baryon number, charge, and strangeness are sensitive to the correlation length of the hot and dense matter created in relativistic heavy-ion collisions and can be used to search for the QCD critical point. We report the first measurements of the moments of net-kaon multiplicity distributions in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. The collision centrality and energy dependence of the mean ($M$), variance ($\sigma^2$), skewness ($S$), and kurtosis ($\kappa$) for net-kaon multiplicity distributions as well as the ratio $\sigma^2/M$ and the products $S\sigma$ and $\kappa\sigma^2$ are presented. Comparisons are made with Poisson and negative binomial baseline calculations as well as with UrQMD, a transport model (UrQMD) that does not include effects from the QCD critical point. Within current uncertainties, the net-kaon cumulant ratios appear to be monotonic as a function of collision energy.
Collisions energy dependence of $M/\sigma^2$ for $\Delta N_k$ multiplicity distributions from 0–5% most central and 70–80% peripheral collisions in Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV. The error bars are statistical uncertainties and the caps represent systematic uncertainties.
Collisions energy dependence of $S\sigma$ for $\Delta N_k$ multiplicity distributions from 0–5% most central and 70–80% peripheral collisions in Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV. The error bars are statistical uncertainties and the caps represent systematic uncertainties.
Collisions energy dependence of $\kappa\sigma^2$ for $\Delta N_k$ multiplicity distributions from 0–5% most central and 70–80% peripheral collisions in Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV. The error bars are statistical uncertainties and the caps represent systematic uncertainties.
We report a measurement of cumulants and correlation functions of event-by-event proton multiplicity distributions from fixed-target Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 3 GeV measured by the STAR experiment. Protons are identified within the rapidity ($y$) and transverse momentum ($p_{\rm T}$) region $-0.9 < y<0$ and $0.4 < p_{\rm T} <2.0 $ GeV/$c$ in the center-of-mass frame. A systematic analysis of the proton cumulants and correlation functions up to sixth-order as well as the corresponding ratios as a function of the collision centrality, $p_{\rm T}$, and $y$ are presented. The effect of pileup and initial volume fluctuations on these observables and the respective corrections are discussed in detail. The results are compared to calculations from the hadronic transport UrQMD model as well as a hydrodynamic model. In the most central 5% collisions, the value of proton cumulant ratio $C_4/C_2$ is negative, drastically different from the values observed in Au+Au collisions at higher energies. Compared to model calculations including Lattice QCD, a hadronic transport model, and a hydrodynamic model, the strong suppression in the ratio of $C_4/C_2$ at 3 GeV Au+Au collisions indicates an energy regime dominated by hadronic interactions.
Collision energy dependence of the cumulant ratios: $C_2/C_1=\sigma/M$, $C_3/C_2=S\sigma$, and $C_4/C_2=\kappa\sigma^2$, for protons (open squares) and net protons (red circles) from top 0–5% (top panels) and 50–60% (bottom panels) Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for protons from $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions is shown as a filled square. UrQMD results with $|y|<0.5$ for protons are shown as gold bands while those for net protons are shown as green dashed lines or green bands. At 3GeV, the model results for protons (−0.5) are shown as blue crosses. UrQMD results of proton and net-proton $C_4/C_2$, see right panels, are almost totally overlapped. The open cross is the result of the model with a fixed impact parameter $b < 3$ fm. The hydrodynamic calculations, for 5% central Au+Au collisions, for protons from $|y|<0.5$ are shown as dashed red linea and the result of the 3 GeV protons from $−0.5<y<0$ is shown as an open red star.
Collision energy dependence of the cumulant ratios: $C_2/C_1=\sigma/M$, $C_3/C_2=S\sigma$, and $C_4/C_2=\kappa\sigma^2$, for protons (open squares) and net protons (red circles) from top 0–5% (top panels) and 50–60% (bottom panels) Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for protons from $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions is shown as a filled square. UrQMD results with $|y|<0.5$ for protons are shown as gold bands while those for net protons are shown as green dashed lines or green bands. At 3GeV, the model results for protons (−0.5) are shown as blue crosses. UrQMD results of proton and net-proton $C_4/C_2$, see right panels, are almost totally overlapped. The open cross is the result of the model with a fixed impact parameter $b < 3$ fm. The hydrodynamic calculations, for 5% central Au+Au collisions, for protons from $|y|<0.5$ are shown as dashed red linea and the result of the 3 GeV protons from $−0.5<y<0$ is shown as an open red star.
Collision energy dependence of the cumulant ratios: $C_2/C_1=\sigma/M$, $C_3/C_2=S\sigma$, and $C_4/C_2=\kappa\sigma^2$, for protons (open squares) and net protons (red circles) from top 0–5% (top panels) and 50–60% (bottom panels) Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for protons from $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collisions is shown as a filled square. UrQMD results with $|y|<0.5$ for protons are shown as gold bands while those for net protons are shown as green dashed lines or green bands. At 3GeV, the model results for protons (−0.5) are shown as blue crosses. UrQMD results of proton and net-proton $C_4/C_2$, see right panels, are almost totally overlapped. The open cross is the result of the model with a fixed impact parameter $b < 3$ fm. The hydrodynamic calculations, for 5% central Au+Au collisions, for protons from $|y|<0.5$ are shown as dashed red linea and the result of the 3 GeV protons from $−0.5<y<0$ is shown as an open red star.
The cross section for $e^+ e^- \to \pi^+ \pi^- J/\psi$ between 3.8 GeV and 5.5 GeV is measured with a 967 fb$^{-1}$ data sample collected by the Belle detector at or near the $\Upsilon(nS)$ ($n = 1,\ 2,\ ...,\ 5$) resonances. The Y(4260) state is observed, and its resonance parameters are determined. In addition, an excess of $\pi^+ \pi^- J/\psi$ production around 4 GeV is observed. This feature can be described by a Breit-Wigner parameterization with properties that are consistent with the Y(4008) state that was previously reported by Belle. In a study of $Y(4260) \to \pi^+ \pi^- J/\psi$ decays, a structure is observed in the $M(\pi^\pm\jpsi)$ mass spectrum with $5.2\sigma$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5) {\rm MeV}/c^2$ and width $\Gamma=(63\pm 24\pm 26)$ MeV/$c^{2}$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmonium-like state.
Measured cross section with statistical errors only.
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