Evidence of spin-orbital angular momentum interactions in relativistic heavy-ion collisions

TL;DR

This paper reports the first evidence of spin alignment of vector mesons in heavy-ion collisions at the LHC, indicating spin-orbital angular momentum interactions and quark polarization effects.

Contribution

It provides the first experimental measurement of spin alignment of vector mesons in heavy-ion collisions, supporting models of quark polarization due to angular momentum.

Findings

Spin alignment observed at low transverse momentum for $K^{*0}$ and $\,phi$ mesons.

No significant spin alignment for $K^0_S$ mesons or in proton-proton collisions.

Results are consistent with quark polarization models in heavy-ion collisions.

Abstract

The first evidence of spin alignment of vector mesons ($K^{*0}$ and $\phi$) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element $\rho_{00}$ is measured at midrapidity ($|y| <$ 0.5) in Pb-Pb collisions at a center-of-mass energy ($\sqrt{s_{\rm NN}}$) of 2.76 TeV with the ALICE detector. $\rho_{00}$ values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum ($p_{\rm T} <$ 2 GeV/$c$) for $K^{*0}$ and $\phi$ at a level of 3$\sigma$ and 2$\sigma$, respectively. No significant spin alignment is observed for the $K^0_S$ meson (spin = 0) in Pb-Pb collisions and for the vector mesons in $pp$ collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.