We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=7.7,11.5,19.6,27$, and 39 GeV using identified hadrons (${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\overline{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($\left|y\right|<0.1$) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T\rangle$, and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

We report the beam energy ($\sqrt{s_{NN}}=7.7–200\mathrm{GeV}$) and collision centrality dependence of the mean ($M$), standard deviation ($\sigma$), skewness ($S$), and kurtosis ($\kappa$) of the net-proton multiplicity distributions in $\mathrm{Au}+\mathrm{Au}$ collisions. The measurements are carried out by the STAR experiment at midrapidity ($|y|<0.5$) and within the transverse momentum range $0.4<p_T<0.8\mathrm{GeV}/c$ in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the quantum chromodynamic phase diagram. The products of the moments, $S\sigma$ and $\kappa {\sigma }^2$, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and antiproton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation and also to a hadron resonance gas model.

Global polarization of $\mathrm{\Lambda }$ hyperons has been measured to be of the order of a few tenths of a percentage in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV, with no significant difference between $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{{}_{NN}}}$ = 7.7–62.4 GeV and indicate noticeable vorticity of the medium created in noncentral heavy-ion collisions at the highest Relativistic Heavy Ion Collider collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from a multi-phase transport model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity ${\eta }^H$ within $|{\eta }^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

We report the first measurements of the moments—mean ($M$), variance (${\sigma }^2$), skewness ($S$), and kurtosis ($\kappa$)—of the net-charge multiplicity distributions at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at seven energies, ranging from $\sqrt{s_{\mathit{\text{NN}}}}=7.7$ to 200 GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, ${\sigma }^2/M$, $S\sigma$, and $\kappa {\sigma }^2$, with the expectations from Poisson and negative binomial distributions (NBDs). The $S\sigma$ values deviate from the Poisson baseline and are close to the NBD baseline, while the $\kappa {\sigma }^2$ values tend to lie between the two. Within the present uncertainties, our data do not show nonmonotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models.

We report the first measurements of the kurtosis ($\kappa$), skewness ($S$), and variance (${\sigma }^2$) of net-proton multiplicity ($N_p-N_{\overline{p}}$) distributions at midrapidity for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=19.6$, 62.4, and 200 GeV corresponding to baryon chemical potentials (${\mu }_B$) between 200 and 20 MeV. Our measurements of the products $\kappa {\sigma }^2$ and $S\sigma$, which can be related to theoretical calculations sensitive to baryon number susceptibilities and long-range correlations, are constant as functions of collision centrality. We compare these products with results from lattice QCD and various models without a critical point and study the $\sqrt{s_{NN}}$ dependence of $\kappa {\sigma }^2$. From the measurements at the three beam energies, we find no evidence for a critical point in the QCD phase diagram for ${\mu }_B$ below 200 MeV.

The $\mathrm{\Lambda }$ ($\overline{\mathrm{\Lambda }}$) hyperon polarization along the beam direction has been measured in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, for the first time in heavy-ion collisions. The polarization dependence on the hyperons’ emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T$ greater than $1\mathrm{GeV}/c$. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown–Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.