# Quark Matter 2018

13-19 May 2018
Venice, Italy
Europe/Zurich timezone
The organisers warmly thank all participants for such a lively QM2018! See you in China in 2019!

## Lambda polarization in heavy ion collisions: from RHIC BES to LHC energies

15 May 2018, 09:40
20m
Sala Mosaici-2, 3rd Floor (Palazzo del Casinò)

### Sala Mosaici-2, 3rd Floor

#### Palazzo del Casinò

Parallel Talk Chirality, vorticity and polarisation effects

### Speaker

Dr Iurii Karpenko (SUBATECH Nantes)

### Description

In hydrodynamic picture of heavy ion collisions, local thermal vorticity of the QGP fluid leads to polarization of nonzero spin hadrons produced out of it [1]. The nonzero polarization of $\Lambda$ hyperons has been recently discovered by STAR in non-central Au-Au collisions in RHIC Beam Energy Scan program [2].

We further study the predictions of the hydrodynamic model for different components of the $\Lambda$ polarization in the framework of 3-dimensional viscous hydrodynamic model with UrQMD initial state, and demonstrate that:

1. at RHIC Beam Energy Scan energies, the global polarization $P$ of produced Lambda is directed along the total angular momentum of the fireball (perpendicular to reaction plane). $P$ decreases with collision energy from 1.7 to 0.2% [3], in agreement with STAR results [2]. The global $\Lambda$ polarization further decreases towards LHC energies,

2. at full RHIC and LHC energies, the dominant component of polarization is the one in the beam direction. This component has a quadrupole structure in transverse momentum plane. Its amplitude $f_2$ shows a mild decrease with collision energy, and it is thus detectable at LHC enegries.

We show that different components of polarization are driven by different properties of the hydrodynamic expansion: whereas the global polarization $P$ is a result of initial shear flow in the reaction plane, $f_2$ is driven by anisotropy of the transverse expansion (which is also responsible for elliptic flow $v_2$). Polarization component in the beam direction is a generic effect present even in a simple 2-dimensional hydrodynamic calculation with longitudinal boost invariance [4].

[1] F. Becattini, V. Chandra, L. Del Zanna, E. Grossi, Ann. Phys. 338 (2013) 32.
[2] STAR collaboration, Nature 548 (2017), 62–65
[3] I. Karpenko, F. Becattini, Eur. Phys. J. C (2017) 77: 213.
[4] F. Becattini, Iu. Karpenko, arXiv:1707.07984, accepted to Phys.Rev.Lett.

Centralised submission by Collaboration Presenter name already specified Theory

### Primary author

Dr Iurii Karpenko (SUBATECH Nantes)

### Co-author

Francesco Becattini (Unversity of Florence)