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Register a new userFluid Dynamics Study of the $Lambda$ Polarization for Au+Au Collisions at $sqrt{s_{NN}}=200$ GeV
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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).
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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.
Measurements of three-dimensional correlation functions of like-sign low transverse momentum kaon pairs from Au+Au collisions at top RHIC energy $sqrt s_{NN}$=200 GeV are presented. The extracted kaon source function is narrower than the pion one and does not have the long tail along the pair transverse momentum direction. This indicates a much smaller role of long-lived resonance decays and/or of the emission duration on kaon emission. Three-dimensional Gaussian shape of the kaon source function can be adequately reproduced by Therminator simulations with resonance contributions taken into account. Comparison to pion data at the same energy reveals that the kaon Gaussian radii in the outward and sideward directions scale with the transverse mass $m_T$. In the longitudinal direction, unlike at lower SPS energies, the Gaussian radii do not seem to follow the exact $m_T$ scaling between kaons and pions.
We report the STAR measurements of dielectron ($e^+e^-$) production at midrapidity ($|y_{ee}|<$1) in Au+Au collisions at $sqrt{s_{rm NN}}$ = 200,GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 ($rho$-like), 0.76-0.80 ($omega$-like), and 0.98-1.05 ($phi$-like) GeV/$c^{2}$. The spectrum in the $omega$-like and $phi$-like regions can be well described by the hadronic cocktail simulation. In the $rho$-like region, however, the vacuum $rho$ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77$pm$0.11(stat.)$pm$0.24(sys.)$pm$0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the $rho$ meson. The excess yield in the $rho$-like region increases with the number of collision participants faster than the $omega$ and $phi$ yields. Theoretical models with broadened $rho$ contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.
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.
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.
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