Spin Alignment Measurements of K\(^{*0}\) Vector Mesons with ALICE Detector at the LHC

We present recent results on K\(^{*0}\) polarization from the ALICE experiment at mid-rapidity (\(|y|<\)0.5) in Pb–Pb collisions at \(\sqrt{s_{\mathrm {NN}}} = 2.76\) TeV and 5.02 TeV and in pp collisions at \(\sqrt{s} = 13\) TeV. The polarization of the K\(^{*0}\) vector meson (spin \(=\) 1) is studied with respect to the production plane and second-order event plane by measuring the zeroth element of spin density matrix \(\rho _{00}\). In pp collisions, \(\rho _{00}\) is consistent with 1/3 in all measured \(p_{\mathrm {T}}\) region ranges from 0.0 < \(p_{\mathrm {T}}\) < 10 GeV/c. However, in Pb–Pb collisions the \(\rho _{00}\) values are below 1/3 for \(p_{\mathrm {T}}\) < 2 GeV/c and consistent with 1/3 for \(p_{\mathrm {T}}\) > 2 GeV/c, with respect to both the production and event planes. \(\rho _{00}\) values are also measured for K\(_{\mathrm {S}}^{0}\) (spin = 0) in Pb–Pb collisions at \(\sqrt{s_{\mathrm {NN}}} = 2.76\) TeV in the 20–40% centrality as a null test and the measurements are consistent with 1/3 in 0.0 < \(p_{\mathrm {T}}\) < 5 GeV/c. We have also observed a centrality dependence of measured \(\rho _{00}\) values for K\(^{*0}\), with the maximum deviation from 1/3 occurring in mid-central collisions.