Measurement of global polarization of {\Lambda} hyperons in few-GeV heavy-ion collisions

TL;DR

This paper reports the first measurement of global polarization of { extLambda} hyperons in heavy-ion collisions below the strangeness production threshold, revealing a strong centrality dependence and larger-than-expected polarization values.

Contribution

It provides the first measurement of hyperon polarization at these energies and compares results with theoretical models, challenging existing predictions.

Findings

Polarization increases towards peripheral collisions.

Measured polarization magnitudes are the largest observed so far.

Results align with 3D fluid dynamics and UrQMD models, but exceed AMPT predictions.

Abstract

The global polarization of {\Lambda} hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au collisions at \sqrt{s_{NN}} = 2.4 GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon transverse momentum (p_T) and rapidity (y_{CM}) for the range of centrality 0--40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20--40%) collisions the polarization magnitudes are <P_{\Lambda}>(%) = 6.8 \pm 1.3 (stat.) \pm 2.1 (syst.) for Au+Au and <P_{\Lambda}>(%) = 6.2 \pm 0.4 (stat.) \pm 0.6 (syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3 GeV. The observed polarization is of a similar magnitude as predicted by 3D fluid dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model.