Impact of globally spin-aligned vector mesons on the search for the chiral magnetic effect in heavy-ion collisions

TL;DR

This paper investigates how the spin alignment of vector mesons, especially rho mesons, can influence the observables used to detect the chiral magnetic effect in heavy-ion collisions, highlighting the importance of rho meson measurements.

Contribution

It demonstrates that globally spin-aligned rho mesons can significantly affect CME-sensitive observables, emphasizing the need to measure rho meson spin alignment for accurate CME interpretation.

Findings

CME observables depend on rho_{00} of rho mesons.

Positive or negative contributions to CME signals depend on rho_{00} values.

Rho meson spin alignment measurements are crucial for CME studies.

Abstract

In high-energy heavy-ion collisions, the chiral magnetic effect (CME) is predicted to arise from the interplay between the chirality imbalance of quarks in the nuclear medium and the intense magnetic field, and will cause a charge separation along the magnetic field direction. While the search for the CME is still ongoing in experiments at Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), the CME-sensitive observables need to be scrutinized to exclude the non-CME contributions. In this work, we examine the influence of globally spin-aligned $\rho$ mesons on the $\gamma_{112}$ correlator, the $R_{\Psi_2}(\Delta S)$ correlator, and the signed balance functions, via a toy model and a multiphase transport model (AMPT). The global spin alignment of vector mesons could originate from non-CME mechanisms in heavy-ion collisions, and is characterized by the 00-component of the spin density matrix, $\rho_{00}$. We find that the CME observables show similar dependence on $\rho_{00}$, and could receive a positive (negative) contribution from $\rho$-decay pions, if the $\rho_{00}$ of $\rho$ mesons is larger (smaller) than 1/3. Since pions are the most abundant particles in such collisions, the $\rho_{00}$ measurements for $\rho$ mesons become crucial to the interpretation of the CME data.