Spin alignment measurements using vector mesons with ALICE detector at the LHC

TL;DR

This paper reports measurements of spin alignment of K*0 vector mesons in Pb-Pb collisions at LHC energies, showing slight deviations from random spin orientation at low transverse momentum and centrality dependence, with no alignment observed in pp collisions.

Contribution

First measurements of K*0 spin alignment in Pb-Pb collisions at LHC energies, revealing centrality and momentum dependence with no alignment in pp collisions.

Findings

$ ho_{00}$ slightly below 1/3 at low $p_T$

$ ho_{00}$ independent of collision energy $\

Abstract

We present new measurements related to spin alignment of $\mathrm{K^{*0}}$ vector mesons at mid-rapidity for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 and 5.02 TeV. The spin alignment measurements are carried out with respect to production plane and second order event plane. At low $p_{\mathrm{T}}$ the spin density matrix element $\rho_{00}$ for $\mathrm{K^{*0}}$ is found to have values slightly below 1/3, while it is consistent with 1/3, i.e. no spin alignment, at high $p_{\mathrm{T}}$. Similar values of $\rho_{00}$ are observed w.r.t. both production plane and event plane. Within statistical and systematic uncertainties, $\rho_{00}$ values are also found to be independent of $\sqrt{s_{\mathrm{NN}}}$. $\rho_{00}$ also shows centrality dependence with maximum deviation from 1/3 for mid-central collisions w.r.t. both the kinematic planes. The measurements for $\mathrm{K^{*0}}$ in pp collisions at $\sqrt{s}$ = 13 TeV and for $\mathrm{K^{0}_{S}}$ (a spin 0 hadron) in 20-40% central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV are consistent with no spin alignment.