Measurements of spin alignment of vector mesons and global polarization of hyperons with ALICE at the LHC

TL;DR

This study measures the global polarization of Lambda hyperons and spin alignment of K*0 vector mesons in Pb-Pb collisions at the LHC, finding no significant polarization or alignment signals across various conditions.

Contribution

It provides the first measurements of hyperon polarization and vector meson spin alignment at LHC energies, expanding understanding of spin phenomena in high-energy heavy-ion collisions.

Findings

No global polarization detected for Lambda hyperons.

K*0 mesons show slight spin alignment at low transverse momentum.

No spin alignment observed in proton-proton collisions or for K0_S in Pb-Pb collisions.

Abstract

We present the measurements related to global polarization of $\Lambda$ hyperons and spin alignment of $\mathrm{K^{*0}}~$ vector mesons at mid-rapidity for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}~$= 2.76 TeV using the ALICE detector at the LHC. The global polarization measurements are carried out with respect to the first order event plane while the spin alignment measurements are carried out with respect to the production plane. No global polarization signal for $\Lambda$ is observed for 5-15\% and 15-50\% central Pb-Pb collisions. The spin density matrix element $\rho_{00}$ is found to have values slightly below 1/3 at low transverse momentum ($p_{\mathrm{T}}$) for $\mathrm{K^{*0}}~$ mesons, while it is consistent with 1/3 (no spin alignment) at higher $p_{\mathrm{T}}$. No spin alignment is observed for $\mathrm{K^{*0}}~$ in pp collisions at $\sqrt{s}$ = 13 TeV and for the spin zero hadron $\mathrm{K^{0}_{S}}~$ in 20-40\% Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}~$ = 2.76 TeV.