Spin alignment measurements of vector mesons with ALICE at the LHC

Abstract

We report the first measurements of spin alignment of K*0 and ϕ vector mesons (spin = 1) in heavy-ion collisions at LHC energies. Measurements are carried out with the ALICE detector in Pb–Pb collisions at sNN=2.76TeV. The second diagonal element (ρ00) of the 3×3 spin density matrix is measured at mid-rapidity (|y| < 0.5) as a function of transverse momentum (pT) and collision centrality. ρ00 values are found to be less than 1/3 (1/3 corresponds to no spin alignment) at low pT (< 2 GeV/c) for both K*0 and ϕ vector mesons, whereas at high pT, extracted ρ00 values are consistent with 1/3. Deviation of ρ00 from 1/3 is maximum in mid-central collisions and in central and peripheral collisions the ρ00 values are consistent with 1/3. The observed spin alignment is qualitatively predicted by the recombination model of quarks which are polarized in the presence of initial angular momentum via the spin-orbital angular momentum interaction of QCD. Control measurements such as the spin alignment of K*0 and ϕ in pp collisions at s=13TeV and the spin alignment of the spin zero hadron K0S in mid-central Pb–Pb collisions are consistent with ρ00 = 1/3.