# Testing hydrodynamic response to initial-state geometry in Pb+$d^\uparrow$ collisions

## TL;DR

This paper investigates how the polarization of deuterons influences the initial collision geometry and the resulting elliptic flow in Pb + polarized deuteron collisions, offering a new way to test collective behavior in small systems.

## Contribution

It introduces a method to control initial collision geometry via deuteron polarization and predicts measurable effects on elliptic flow, advancing understanding of collective phenomena in small collision systems.

## Key findings

- Hydrodynamic response predicts a sign change in $v_2$ based on deuteron polarization.
- Polarization-dependent initial geometry affects charged hadron elliptic flow.
- Provides a test case for the collective origin in small collision systems.

## Abstract

Deuterons with different polarization states have distinct shapes for their wavefunctions. This offers a unique opportunity to experimentally control the initial-state collision geometry with the polarization of the light-ion targets in relativistic heavy-ion experiments. We study the charged hadron elliptic flow coefficients with respect to the polarization angle of deuterons in Pb + polarized deuteron collisions using a hydrodynamics + hadronic transport model. Hydrodynamic response to initial-state geometry predicts a distinct sign of $v_2$ correlated with the deuteron's polarization states, providing a clean test case for elucidating the collective origin in small collision systems.