Global polarization of Λ and (Λ)over-bar hyperons in Au plus Au collisions at √sNN=19.6 and 27GeV

Abdulhamid, M. I.; Aboona, B. E.; Adam, J.; Adamczyk, L.; Adams, J. R.; Aggarwal, I.; Aggarwal, M. M.; Ahammed, Z.; Anderson, D. M.; Aschenauer, E. C.; Aslam, S.; Atchison, J.; Bairathi, V.; Baker, W.; Cap, J. G. Ball; Barish, K.; Bellwied, R.; Bhagat, P.; Bhasin, A.; Bhatta, S.; Bielcik, J.; Bielcikova, J.; Brandenburg, J. D.; Butterworth, J.; Cai, X. Z.; Caines, H.; Sanchez, M. Calderon de la Barca; Cebra, D.; Ceska, J.; Chakaberia, I.; Chaloupka, P.; Chan, B. K.; Chang, Z.; Chatterjee, A.

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摘要

In relativistic heavy-ion collisions, a global spin polarization, P-H, of Lambda and (Lambda) over bar hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing P-H with decreasing root sNN. A splitting between Lambda and (Lambda) over bar polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of root s(NN) = 19.6 and 27 GeV in the BNL Relativistic Heavy Ion Collisions Beam Energy Scan Phase II using the STAR detector, with an upper limit of P ((Lambda)) over bar - P-Lambda < 0.24% and P<((Lambda))over bar> - P-Lambda < 0.35%, respectively, at a 95% confidence level. We derive an upper limit on the naive extraction of the late-stage magnetic field of B < 9.4 x 10(12) T and B < 1.4 x 10(13) T at root s(NN) = 19.6 and 27 GeV, respectively, although more thorough derivations are needed. Differential measurements of P-H were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of |y| < 1 and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.

关键词

ELLIPTIC FLOW VORTICITY