Two-pion correlations and multiplicity effects in La on La collisions

Bose-Einstein correlations of negative pions in heavy ion collisions have been investigated for the reaction ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ ${+}^{\mathrm{nat}}$La→2${\mathrm{\pi }}^{\mathrm{-}}$X at 12.6 GeV/nucleon at two acceptances, centered at laboratory observation angles of approximately 0° and 45° with respect to the beam axis. A scintillation counter array downstream of the target was used to sample the charged particle multiplicity of each event and hence give some information on the impact parameter of the collision. Including results from previous experiments, space-time dimensions of the pion source are now available for mass-symmetric collisions in the mass range of A=40 to 139. The sources are oblate for nearly all systems, except for La+La central collisions viewed near 45° in the laboratory (90° in the center of mass) where the source is spherical. The perpendicular radius ${\mathit{R}}_{\mathrm{\perp }}$ is never less than 4 fm, regardless of the centrality of the La+La collision. Furthermore, ${\mathit{R}}_{\mathrm{\perp }}$ seems independent of the mass of the collision system.