Spin alignment measurements of vector mesons with ALICE at LHC

TL;DR

This study measures the spin alignment of vector mesons in heavy-ion collisions at the LHC, revealing deviations from random spin orientation that suggest quark polarization effects during hadronization.

Contribution

First measurement of vector meson spin alignment at LHC energies, providing insights into quark polarization and hadronization mechanisms in heavy-ion collisions.

Findings

Deviations from 1/3 in $ ho_{00}$ at low $p_T$ for Pb--Pb collisions.

No significant spin alignment observed in pp collisions.

Results support models of quark polarization due to initial angular momentum.

Abstract

We present the measurements related to the spin alignment of $\mathrm{K^{*0}}$ and $\mathrm{\phi}$ 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 second diagonal spin density matrix element ($\rho_{00}$) is measured from the angular distribution of the decay daughters of the vector meson in the decay rest frame, with respect to the event plane and the production plane. The $\rho_{00}$ values are found to be less than 1/3 at low $p_{\mathrm{T}}$ ($<$ 2 GeV/$c$) for both vector mesons. The observed deviations from 1/3 are maximal for mid-central collisions at a level of 3$\sigma$ for $\mathrm{K^{*0}}$ and 2$\sigma$ for $\mathrm{\phi}$ mesons. As a control measurement the \rh~values were found to be consistent with 1/3, i.e., no significant spin alignment, for pp collisions with respect to the production plane. The results are qualitatively consistent with the expectation from a model that attributes the spin alignment to a polarization of quarks in the presence of large initial angular momentum and a subsequent hadronization by the process of recombination.