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Register a new userGlobal and local polarization of $Lambda$ hyperons in Au+Au collisions at 200 GeV from STAR
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Takafumi Niida
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New results on global polarization of $Lambda$ hyperons in Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV reveal non-zero signal on the order of a few tenths of a percent. Compared with lower energy results, the current measurement shows that the polarization decreases at higher collision energy, the trend being well reproduced by theoretical calculations. The polarization is found to be larger in more peripheral collisions, as well as in the in-plane than in the out-of-plane direction. The signal seems to depend on the asymmetry between positive and negative charge in each event, which may indicate an influence of the axial current induced by the magnetic field. In addition, the first measurement of a local polarization along the beam direction was performed. The results show a quadrupole modulation relative to the second-order event plane, as expected from the elliptic flow.
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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 $C_{OmegaLambda}=1$.
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.
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.
The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.
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