Charged pion production in $\mathbf{Au+Au}$ collisions at $\mathbf{\sqrt{s_{NN}}}$ = 2.4$\mathbf{GeV}$

TL;DR

This study provides detailed measurements of charged pion production in gold-gold collisions at 2.4 GeV, revealing non-isotropic angular distributions and discrepancies with existing transport models, thus offering new insights into sub-threshold particle production.

Contribution

It offers high-statistics data on pion emission across various centralities and compares these results with multiple transport models, highlighting significant differences and undershoot compared to previous experiments.

Findings

Pion multiplicity per participant decreases with centrality.

Angular distributions are non-isotropic even in central collisions.

Data undershoot FOPI results by 2.5 sigma at similar energies.

Abstract

We present high-statistic data on charged pion emission from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 2.4 GeV (corresponding to $E_{beam}$ = 1.23 A GeV) in four centrality classes in the range 0 - 40$\%$ of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the world data, but undershoot by $2.5 \sigma$ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.