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

Abstract

We perform a systematic analysis of several HBT parameters in heavy ion collisions from $E_{\mathrm{beam}}=2$ AGeV to $\sqrt{s_{\mathrm{NN}}}=200$ 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.

arXiv comment11 pages, 2 figs

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