Hanbury Brown–Twiss correlation functions from event generators: A reliable approach to determine the size of the emitting source in ultrarelativistic heavy ion collisions?

Abstract

Employing NEXUS, one of the most recent simulation programs for heavy ion collisions, we investigate in detail the Hanbury Brown--Twiss correlation function of charged pions and kaons for central reactions 158-A GeV $\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}.$ For this study we supplement the standard simulation program by electromagnetic interactions. We find a wide distribution of freeze-out times which corresponds to a broad distribution of source sizes. Strong space-momentum space correlations as well as the electromagnetic interaction between the correlated pairs and the environment, which is of the same size as that between the hadrons of the correlated pair, strongly modify the correlation function. It can still be well parametrized by $C(\mathbf{k})=1+\ensuremath{\lambda}{e}^{\ensuremath{-}4{\mathbf{k}}^{2}{R}^{2}},$ where k is the relative momentum. R, however, is not a simple function of the source radius; the complicated space-time structure of the source as well as the space-momentum space correlations and the final-state Coulomb interactions are encoded. The true source radius, as given by the rms radius of the emission points, and R differ by up to 30%.