Resonance contributions to Hanbury-Brown–Twiss correlation radii

We study the effect of resonance decays on intensity interferometry for heavy ion collisions. Collective expansion of the source leads to a dependence of the two-particle correlation function on the pair momentum $\mathbf{K}$. This opens the possibility to reconstruct the dynamics of the source from the $\mathbf{K}$ dependence of the measured Hanbury-Brown–Twiss (HBT) radii. Here we address the question to what extent resonance decays can fake such a flow signal. Within a simple parametrization for the emission function we present a comprehensive analysis of the interplay of flow and resonance decays on the one- and two-particle spectra. We discuss in detail the non-Gaussian features of the correlation function introduced by long-lived resonances and the resulting problems in extracting meaningful HBT radii. We propose to define them in terms of the second-order $q$ moments of the correlator $C(\mathbf{q},\mathbf{K})$. We show that this yields a more reliable characterisation of the correlator in terms of its width and the correlation strength $\lambda$ than other commonly used fit procedures. The normalized fourth-order $q$ moments (kurtosis) provide a quantitative measure for the non-Gaussian features of the correlator. At least for the class of models studied here, the kurtosis helps separating effects from expansion flow and resonance decays, and provides the cleanest signal to distinguish between scenarios with and without transverse flow.