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Register a new userPolarization of $Lambda$ ($bar{Lambda}$) hyperons along the beam direction in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV
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The $Lambda$ ($bar{Lambda}$) hyperon polarization along the beam direction has been measured for the first time in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The polarization dependence on the hyperons emission angle relative to the second-order event plane exhibits a sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude of the signal is about five 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, blast-wave model calculations reproduce the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the signal once the model is required to reproduce the azimuthal dependence of the Gaussian source radii measured via the Hanbury-Brown and Twiss intensity interferometry technique.
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Global polarization of $Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $Lambda$ and $bar{Lambda}$. 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 non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperons 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 $2sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.
Global polarization of $Xi$ and $Omega$ hyperons has been measured for the first time in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The measurements of the $Xi^-$ and $bar{Xi}^+$ hyperon polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay $XirightarrowLambda+pi$, as well as by measuring the polarization of the daughter $Lambda$-hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over $Xi^-$ and $bar{Xi}^+$, is measured to be $langle P_Xi rangle = 0.47pm0.10~({rm stat.})pm0.23~({rm syst.}),%$ for the collision centrality 20%-80%. The $langle P_Xi rangle$ is found to be slightly larger than the inclusive $Lambda$ polarization and in reasonable agreement with a multi-phase transport model (AMPT). The $langle P_Xi rangle$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of $Omega$, $langle P_Omega rangle = 1.11pm0.87~({rm stat.})pm1.97~({rm syst.}),%$ was obtained by measuring the polarization of daughter $Lambda$ in the decay $Omega rightarrow Lambda + K$, assuming the polarization transfer factor $C_{OmegaLambda}=1$.
With a Yang-Mills field, stratified shear flow initial state and a high resolution (3+1)D Particle-in-Cell Relativistic (PICR) hydrodynamic model, we calculate the $Lambda$ polarization for peripheral Au+Au collisions at RHIC energy of $sqrt{S_{NN}}=200$ GeV. The obtained longitudinal polarization in our model agrees with the experimental signature and the quadrupole structure on transverse momentum plane. It is found that the relativistic correction (2nd term), arising from expansion and from the time component of the thermal vorticity, plays a crucial role in our results. This term is changing the signature and exceeds the first term, arising from the classical vorticity. Finally, the global polarization in our model shows no significant dependence on rapidity, which agrees with the experimental data. It is also found that the second term flattens the sharp peak arising from the classical vorticity (1st term).
We present measurements of $e^+e^-$ production at midrapidity in Au$+$Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass ($m_{ee} <$ 5 GeV/$c^2$) and pair transverse momentum ($p_T$ $<$ 5 GeV/$c$), for minimum bias and for five centrality classes. The ee yield is compared to the expectations from known sources. In the low-mass region ($m_{ee}=0.30$--0.76 GeV/$c^2$) there is an enhancement that increases with centrality and is distributed over the entire pair pt range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to $2.3pm0.4({rm stat})pm0.4({rm syst})pm0.2^{rm model}$ or to $1.7pm0.3({rm stat})pm0.3({rm syst})pm0.2^{rm model}$ for minimum bias collisions when the open-heavy-flavor contribution is calculated with {sc pythia} or {sc mc@nlo}, respectively. The inclusive mass and $p_T$ distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the $rho$ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region ($m_{ee}$ = 1.2--2.8 GeV/$c^2$), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
Global hyperon polarization, $overline{P}_mathrm{H}$, in Au+Au collisions over a large range of collision energy, $sqrt{s_mathrm{NN}}$, has recently been measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the Quark-Gluon Plasma (QGP). While na{i}ve extrapolation of data trends suggests a large $overline{P}_mathrm{H}$ as the collision energy is reduced, the behavior of $overline{P}_mathrm{H}$ at small $sqrt{s_mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we have measured the polarization of $Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $sqrt{s_mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91pm0.81(rm stat.)pm0.15(rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.
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