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.

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.

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.

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, $v_2\{2\}$ and $v_2\{4\}$, for charged hadrons from $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{\mathrm{NN}}}=193\mathrm{GeV}$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in zero degree calorimeters (ZDCs). Within this sample, the observed dependence of $v_2\{2\}$ on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in $\mathrm{U}+\mathrm{U}$ collisions. We also show that $v_2$ vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions.

We report the first measurement of the elliptic anisotropy ($v_2$) of the charm meson $D^0$ at midrapidity ($|y|<1$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured $D^0$ $v_2$ in 0%–80% centrality $\mathrm{Au}+\mathrm{Au}$ collisions can be described by a viscous hydrodynamic calculation for a transverse momentum ($p_T$) of less than $4\mathrm{GeV}/c$. The $D^0$ $v_2$ as a function of transverse kinetic energy ($m_T-m_0$, where $m_T=\sqrt{p_T^2+m_0^2}$) is consistent with that of light mesons in 10%–40% centrality $\mathrm{Au}+\mathrm{Au}$ collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature-dependent, dimensionless charm spatial diffusion coefficient ($2\pi T{D}_s$) in the range of $\sim 2–12$, are able to simultaneously reproduce our $D^0$ $v_2$ result and our previously published results for the $D^0$ nuclear modification factor.