# Polarization of $\Lambda$ ($\bar{\Lambda}$) hyperons along the beam direction in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

## TL;DR

This study reports the first measurement of $\Lambda$ hyperon polarization along the beam direction in Au+Au collisions at 200 GeV, revealing a sine modulation pattern linked to vorticity, with results challenging existing hydrodynamic models.

## Contribution

It provides the first experimental measurement of hyperon polarization along the beam axis in high-energy heavy-ion collisions, highlighting discrepancies with theoretical predictions.

## Key findings

- Polarization exhibits sine modulation with emission angle.
- Polarization increases with collision centrality.
- Measured polarization is significantly smaller than model predictions.

## Abstract

The $\Lambda$ ($\bar{\Lambda}$) hyperon polarization along the beam direction has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The polarization dependence on the hyperons' emission angle relative to the second-order event plane exhibits a sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude of the signal is about five times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, blast-wave model calculations reproduce the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the signal once the model is required to reproduce the azimuthal dependence of the Gaussian source radii measured via the Hanbury-Brown and Twiss intensity interferometry technique.