Pion correlations in relativistic heavy ion collisions for three symmetric systems.

Abstract

The method of two-pion interferometry was used to obtain source-size and lifetime parameters for the pions produced in heavy ion collisions. Two acceptances (centered at approximately 0\ifmmode^\circ\else\textdegree\fi{} and approximately 90\ifmmode^\circ\else\textdegree\fi{}, in the center of mass) were used for each of three systems, 1.70 GeV/nucleon $^{56}\mathrm{Fe}$+Fe, 1.82 GeV/nucleon $^{40}\mathrm{Ar}$+KCl, and 1.54 GeV/nucleon $^{93}\mathrm{Nb}$+Nb, allowing a search for dependences on nuclear mass and viewing angle. The correlation functions were calculated by comparing data samples to event-mixed reference samples. The effect of the particle correlations on the reference samples was corrected by weighting the events appropriately to remove the residual correlation effect. The source parameters, in the nucleus-nucleus center-of-mass frame, show an oblate source (i.e., ${\mathit{R}}_{\mathrm{\ensuremath{\perp}}}$g${\mathit{R}}_{\mathrm{\ensuremath{\parallel}}}$) for the lighter systems and an approximately spherical source for the heaviest system. The dependence on nuclear mass shows that ${\mathit{R}}_{\mathrm{\ensuremath{\perp}}}$ is essentially constant (under both viewing angles), whereas ${\mathit{R}}_{\mathrm{\ensuremath{\parallel}}}$ for the 90\ifmmode^\circ\else\textdegree\fi{} (c.m.) data increases with the nuclear mass. No evidence was found for a dependence of the source size on the pion momentum.