The polarization of the Lambda and (Lambda) over bar hyperons along the beam (z) direction, P-z, has been measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P-z comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P-z,P-s2 = < P-z sin(2 phi - 2 Psi(2))>, where phi is the hyperon azimuthal emission angle and Psi(2) is the elliptic flow plane angle. We report the measurement of P-z,P-s2 for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P-z,P-s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at root s(NN) = 200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P-z in Pb-Pb collisions at the LHC. The comparison of the measured P-z,P-s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.
Polarization of Lambda and (Lambda)over-bar Hyperons along the Beam Direction in Pb-Pb Collisions at root s(NN)=5.02 TeV
Alessandro, B.;Alici, A.;Angeletti, M.;Aziz, S.;Banerjee, D.;Baruffaldi, F.;Basu, S.;Bearden, I. G.;Bellini, F.;Biswas, S.;Bruno, G. E.;Colella, D.;Contin, G.;Das, P.;Das, P.;Dash, S.;De Caro, A.;De Falco, A.;Deb, S.;Evans, D.;Faggin, M.;Faivre, J.;Ghosh, P.;Giacalone, M.;Giubilato, P.;Harris, J. W.;Janik, M. A.;Jung, J.;Kim, J. S.;Kundu, S.;Lunardon, M.Membro del Collaboration Group
;Margotti, A.;Martinez M.;Micheletti, L.;Pagano, D.;Rehman, A.;Richter, M.;Riggi, F.;Rotondi, A.;Roy, A.;Rui, R.;Sharma, A.;Sinha, T.;Soramel, F.;Stocco, D.;Swain, S.;Szarka, I;Terrevoli, C.;Thomas, D.;Trogolo, S.;Trombetta, G.;Turrisi, R.;Uras, A.;Vercellin, E.;Wagner, B.;Wang, C.;Wright, J. R.;Wu, W.;Zhou, Y.;Zhu, J.;
2022
Abstract
The polarization of the Lambda and (Lambda) over bar hyperons along the beam (z) direction, P-z, has been measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P-z comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P-z,P-s2 = < P-z sin(2 phi - 2 Psi(2))>, where phi is the hyperon azimuthal emission angle and Psi(2) is the elliptic flow plane angle. We report the measurement of P-z,P-s2 for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P-z,P-s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at root s(NN) = 200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P-z in Pb-Pb collisions at the LHC. The comparison of the measured P-z,P-s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.Pubblicazioni consigliate
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