Local spin alignment of vector mesons in relativistic heavy-ion collisions

TL;DR

This paper explores how local vorticity in heavy-ion collisions can cause spin alignment of vector mesons, offering new insights into the observed spin polarization phenomena beyond global effects.

Contribution

It demonstrates that local spin polarization, not just global polarization, can explain vector meson spin alignment, and proposes experimental methods to distinguish these effects.

Findings

Local spin polarization can cause vector meson spin alignment.

Proposed observables differentiate local from global polarization.

Measurements can reveal vorticity patterns in heavy-ion collisions.

Abstract

We investigate the spin alignment of vector mesons arising from locally polarized quarks and anti-quarks (local spin alignment) in heavy-ion collisions. We find that $\rho_{00}\neq 1/3$ does not necessarily signal the global polarization of quarks and anti-quarks along the orbital angular momentum of the system, but may also originate from their local spin polarization. Such local spin polarization could be induced by local vorticity arising from the anisotropic expansion of the fireball. We further propose experimental observables that can distinguish between the local and the global spin alignments. Measurements of these observables in heavy-ion collisions can probe the vorticity pattern and shed light on the puzzles of $\phi$ and $K^{*0}$ spin alignments.