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Register a new userStudy of Lambda polarization in relativistic nuclear collisions at sqrt{s}_NN = 7.7-200 GeV
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Iu. Karpenko
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We present a calculation of the global polarization of Lambda hyperons in relativistic Au-Au collisions at RHIC Beam Energy Scan range sqrt{s}_NN = 7.7 - 200 GeV with a 3+1 dimensional cascade + viscous hydro + cascade model, UrQMD+vHLLE. Within this model, the mean polarization of Lambda in the out-of-plane direction is predicted to decrease rapidly with collision energy from a top value of about 2% at the lowest energy examined. We explore the connection between the polarization signal and thermal vorticity and estimate the feed-down contribution to Lambda polarization due to the decay of higher mass hyperons.
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A comprehensive and detailed analysis of hadronic abundances measured in Au-Au collisions at RHIC at sqrt(s)_NN = 130 and 200 GeV is presented. The rapidity densities measured in the central rapidity region have been fitted to the statistical hadronization model and the chemical freeze-out parameters determined as a function of centrality, using data from experiments BRAHMS, PHENIX and STAR. The chemical freeze-out temperature turns out to be independent of centrality to a few percent accuracy, whereas the strangeness under-saturation parameter gamma_S decreases from almost unity in central collisions to a significantly lower value in peripheral collisions. Our results are in essential agreement with previous analyses, with the exception that fit quality at sqrt(s)_NN = 200 GeV is not as good as previously found. From the comparison of the two different energies, we conclude that the difference in fit quality, as described by chi2 values, is owing to the improved resolution of measurements which has probably exceeded the intrinsic accuracy of the simplified theoretical formula used in the fits.
Predictions for the global polarization of $Lambda$ hyperons in Au+Au collisions at moderately relativistic collision energies, 2.4 $leqsqrt{s_{NN}}leq$ 11 GeV, are made. These are based on the thermodynamic approach to the global polarization incorporated into the model of the three-fluid dynamics. Centrality dependence of the polarization is studied. It is predicted that the polarization reaches a maximum or a plateau (depending on the equation of state and centrality) at $sqrt{s_{NN}}approx$ 3 GeV. It is found that the global polarization increases with increasing width of the rapidity window around the midrapidity.
We analyze single particle momentum spectra and interferometry radii in central Au+Au collisions at RHIC within a hydro-inspired parametrization accounting for continuous hadron emission through the whole lifetime of hydrodynamically expanding fireball. We found that a satisfactory description of the data is achieved for a physically reasonable set of parameters when the emission from non space-like sectors of the enclosed freeze-out hypersurface is fairly long: $ 9$ fm/c. This protracted surface emission is compensated in outward interferometry radii by positive $r_{out} - t$ correlations that are the result of an intensive transverse expansion. The main features of the experimental data are reproduced: in particular, the obtained ratio of the outward to sideward interferometry radii is less than unity and decreases with increasing transverse momenta of pion pairs. The extracted value of the temperature of emission from the surface of hydrodynamic tube approximately coincides with one found at chemical freeze-out in RHIC Au+Au collisions. A significant contribution of the surface emission to the spectra and to the correlation functions at relatively large transverse momenta should be taken into account in advanced hydrodynamic models of ultrarelativistic nucleus-nucleus collisions.
The polarization of $Lambda$ hyperons from relativistic flow vorticity is studied in peripheral heavy ion reactions at FAIR and NICA energies, just above the threshold of the transition to the Quark-Gluon Plasma. Previous calculations at higher energies with larger initial angular momentum, predicted significant $Lambda$ polarization based on the classical vorticity term in the polarization, while relativistic modifications decreased the polarization and changed its structure in the momentum space. At the lower energies studied here, we see the same effect namely that the relativistic modifications decrease the polarization arising from the initial shear flow vorticity.
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.
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