Fitted HBT radii versus space-time variances in flow-dominated models

Abstract

The inability of otherwise successful dynamical models to reproduce the "HBT radii" extracted from twoparticle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the "RHIC HBT Puzzle". Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source which can be directly computed from the emission function, without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models some of which exhibit significant deviations from simple Gaussian behaviour. By Fourier transforming the emission function we compute the 2-particle correlation function and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and measured HBT radii remain, we show that a more "apples-to-apples" comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data.

Photon Hanbury-Brown–Twiss interferometry for noncentral heavy-ion collisions

“…To this end we generalize the Gaussian fitting algorithm developed in Refs. [38,60] to include the R os as an additional fit parameter. (Note that R os cannot be determined from one-dimensional Gaussian fits to slices of the correlation function along any one of the osl axes; it requires at least a two-dimensional Gaussian fit in the os plane.)…”

“…We here use a generalization to noncentral collisions of the Gaussian fit technique developed in Refs. [38,60]. Since photons, unlike charged hadrons, do not suffer from final-state Coulomb interactions, this technique is even more appropriate here than it is for hadron correlations.…”

Photon HBT interferometry for non-central heavy-ion collisions

“…The HBT radii are determined by fitting the computed correlation function with the functional form (14). To this end we generalize the Gaussian fitting algorithm developed in [38,60] to include the R os as an additional fit parameter. (Note that R os cannot be determined from 1-dimensional Gaussian fits to slices of the correlation function along any one of the osl axes; it requires at least a 2-dimensional Gaussian fit in the os plane.)…”

“…For two-photon correlations it is therefore even more important than for hadrons that the HBT radius parameters are computed through a procedure that matches their experimental extraction. We here use a generalization to non-central collisions of the Gaussian fit technique developed in [38,60]. Since photons, unlike charged hadrons, do not suffer from finalstate Coulomb interactions, this technique is even more appropriate here than it is for hadron correlations.…”