Abstract
We present the measurements related to global polarization of Λ hyperons and spin alignment of K*0 vector mesons at mid-rapidity for Pb-Pb collisions at [see formula in PDF] = 2.76 TeV using the ALICE detector at the LHC. The global polarization measurements are carried out with respect to the first order event plane while the spin alignment measurements are carried out with respect to the production plane. No global polarization signal for Λ is observed for 5-15% and 15-50% central Pb-Pb collisions. The spin density matrix element ρ00 is found to have values slightly below ⅓ at low transverse momentum (pT) for K*0 mesons, while it is consistent with ⅓ (no spin alignment) at higher pT. No spin alignment is observed for K*0 in pp collisions at √s = 13 TeV and for the spin zero hadron K0S in 20-40% Pb-Pb collisions at [see formula in PDF] = 2.76 TeV.
Highlights
Large magnetic field [1] and large angular momentum [2] are expected to be present in the initial stages of high energy heavy-ion collisions
The angular distributions are measured with respect to a quantization axis, which can either be perpendicular to the production plane of the hadron, or normal to the reaction plane of the system
The production plane is defined by the momentum of the hadron under study and the beam direction, whereas the reaction plane is defined by the impact parameter and beam direction
Summary
Large magnetic field [1] and large angular momentum [2] are expected to be present in the initial stages of high energy heavy-ion collisions. One of the physics interests of the heavy-ion program using the ALICE detector at the LHC is to look for signatures of these effects. This can be achieved by studying the angular distributions of the decay daughters of hyperons and vector mesons [3,4,5]. In this work we present the first results at LHC energies related to the spin alignment of K∗0 vector mesons through the measurement of ρ00 in pp and Pb-Pb collisions with respect to the production plane. Pb-Pb collisions with respect to the event plane using the ALICE detector [7]
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