Spin alignment of vector mesons in heavy ion and proton - proton collisions

TL;DR

This paper measures the spin alignment of vector mesons in heavy ion and proton-proton collisions, revealing different polarization behaviors depending on the reference plane, explained by a model involving Thomas precession during hadronization.

Contribution

It provides the first measurements of rho_{00} for K^{*0} and phi(1020) mesons in these collisions and introduces a simple model to interpret the observed spin alignment patterns.

Findings

No polarization with respect to the reaction plane in mid-central Au+Au collisions.

Non-zero, p_perp-dependent rho_{00} in production plane and p+p collisions.

The observed behavior is explained by a model involving Thomas precession during hadronization.

Abstract

The spin alignment matrix element rho_{00} for the vector mesons K^{*0} and phi(1020) has been measured in RHIC at central rapidities. These measurements are consistent with the absence of polarization with respect to the reaction plane in mid-central Au + Au collisions whereas, when measured with respect to the production plane in the same reactions and in p + p collisions, a non-vanishing and p_perp-dependent rho_{00} is found. We show that this behavior can be understood in a simple model of vector meson production where the spin of their constituent quarks is oriented during hadronization as the result of Thomas precession.