Pion freeze-out as seen through HBT correlations in heavy ion collisions from FAIR/AGS to RHIC energies

TL;DR

This study analyzes pion emission characteristics in heavy ion collisions across a wide energy range using UrQMD simulations, revealing discrepancies in source lifetime and volume compared to experimental data, and supporting a universal freeze-out criterion.

Contribution

It provides a systematic UrQMD-based analysis of HBT parameters over a broad energy spectrum, highlighting differences from experimental observations and proposing a universal freeze-out condition.

Findings

Calculated source lifetime exceeds experimental values at all energies.

Pion source volume increases with beam energy, contrary to some experimental trends.

Mean free path at freeze-out supports a universal kinetic decoupling criterion.

Abstract

We perform a systematic analysis of several HBT parameters in heavy ion collisions from $E_{\rm beam}=2$ AGeV to $\sqrt {s_{\rm NN}}=200$ GeV within the UrQMD transport approach and compare the results to experimental data where available. We find that the 'lifetime' of the emission source as calculated from $\tau \sim \sqrt{R_O^{2}-R_S^{2}}$, is larger than the experimentally observed values at all investigated energies. The calculated volume of the pion source ($V_f$) is found to increase monotonously with increasing beam energy and the experimentally observed decrease of the measured $V_f$ at AGS is not seen. Finally, we calculate the mean free path $\lambda_f=0.5-1$ fm of pions at freeze-out and find a good description of the experimental data above the AGS energy region, supporting the suggestion of a universal kinetic decoupling criterion up to the highest RHIC energies.