Spin alignment measurements of vector mesons with ALICE at the LHC

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 ( ρ 00 ) of the 3×3 spin density matrix is measured at mid-rapidity (| y | < 0.5) as a function of transverse momentum ( p T ) and collision centrality. ρ 00 values are found to be less than 1/3 (1/3 corresponds to no spin alignment) at low p T (< 2 GeV/ c ) for both K *0 and ϕ vector mesons, whereas at high p T , extracted ρ 00 values are consistent with 1/3. Deviation of ρ 00 from 1/3 is maximum in mid-central collisions and in central and peripheral collisions the ρ 00 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 K 0 S in mid-central Pb–Pb collisions are consistent with ρ 00 = 1/3.