Heavy quark polarization anisotropy as a novel probe of fireball geometry

TL;DR

This paper introduces a novel method using heavy quark polarization harmonics to probe the initial geometric anisotropies of the quark-gluon plasma in heavy-ion collisions, linking polarization patterns to early collision geometry.

Contribution

It proposes polarization harmonics of heavy hadrons as a new observable sensitive to fireball shape, connecting polarization anisotropies to initial spatial eccentricities.

Findings

Polarization harmonics relate to initial spatial eccentricities.

Quantitative estimates match observed $D^{*+}$ spin alignment.

Medium-induced depolarization encodes geometric information.

Abstract

We propose a new approach to probe the initial fireball geometry in relativistic heavy-ion collisions using spin polarization. Specifically, we introduce polarization harmonics of open heavy hadrons as a novel observable sensitive to geometric anisotropies. Heavy quarks are produced in early hard scatterings and can acquire spin polarization from the strong, transient electromagnetic fields present at early times. As they propagate through the anisotropic quark-gluon plasma, medium-induced interactions lead to path-length dependent depolarization, imprinting an azimuthally anisotropic polarization pattern. Within the framework of rotational Brownian motion, we show that the resulting polarization harmonics are directly related to the initial spatial eccentricities, thereby establishing heavy-flavor polarization anisotropies as a sensitive and complementary probe of the early-time collision geometry. We present quantitative estimates of the second polarization harmonic associated with the recently observed $D^{*+}$ spin alignment reported by the ALICE Collaboration.