Angular and momentum distribution of vector mesons produced in proton-proton and heavy-ion collisions at LHC energies

The spin-orbit coupling causes fine structure in atomic physics and shell structure in nuclear physics, and is a key ingredient in the field of spintronics in materials sciences. In non-central heavy-ion collisions a large initial angular momentum (O(107 ) ~) is expected to be created perpendicular to the reaction plane (subtended by the beam axis and impact parameter direction). In the presence of a large initial angular momentum produced in heavyion collisions, the quarks can be polarized due to the spin-orbital angular momentum interaction of QCD. This leads to the preferential alignment of the intrinsic angular momentum (spin) of vector mesons, formed by hadronization of quarks, along the direction of angular momentum. Spin alignment of vector meson is quantified by measuring the spin density matrix element ρ00 which is the probability of finding a vector meson in spin state 0 out of 3 possible spin states. In the absence of spin alignment all 3 spin states (-1, 0, 1) are equally probable which makes ρ00 = 1/3. The ρ00 will deviate from 1/3 in the presence of spin-orbital angular momentum interactions. Experimentally, the ρ00 can be measured by studying the angular distribution of the decay daughter of vector meson with respect to the direction of angular momentum [1]. We report the first evidence of significant spin alignment effect for vector mesons (K∗0 and φ) in heavy-ion collisions. The measurements are carried out as a function of transverse momentum (pT) and collision centrality with the ALICE detector using the particles produced a mid-rapidity (|y| < 0.5) in Pb–Pb collisions at a center-of-mass energy (√ sNN) of 2.76 TeV. The initial angular momentum due to the extended size of the nuclei and the finite impact parameter in non-central heavy-ion collisions is missing in proton-proton collisions. Determination of ρ00 for vector mesons produced in pp collisions and for spin zero K0 S produced in heavy-ion collisions provide a null test for spin alignment of vector mesons measured in the present work. On the other hand, short lifetimes of hadronic resonances (eg. K∗0 ) compared to other stable hadrons are comparable to the time taken by the dense nuclear matter to evolve to its final state. This can be exploited to investigate the properties of the hadronic phase produced in heavy-ion collisions. K∗0 yields are expected to be modified due to the interaction of their decay daughters within the hadronic medium. In order to find the possible presence of hadronic phase effect in small collisions system, K∗0 production as a function of charged particle multiplicity in pp collisions at √s = 13 TeV are also presented.