Femtoscopy in p+p vs heavy ions in STAR

Zbigniew Chajecki

The geometric substructure of the particle-emitting source has been characterized via two-particle interferometry by the STAR collaboration for several energies and colliding systems at RHIC. In heavy ion collisions the $m_T$ dependence of femtoscopic radii has been observed for all particle types by several experiments at different collision energies. This dependence has been thought to arise from space-momentum correlations generated by collective flow. On the other hand, there are several reports of a similar mT dependence by experiments measuring elementary particle collisions. Here, quite different physical mechanisms-- including resonances, strings, and uncertainty arguments-- have been proposed to explain the dependence. Determining the differences or similarities in the space-time physics driving the signal in heavy ion versus p+p collisions requires a direct comparison of mT dependence of the raii. Such a comparison has, until now, been sorely lacking. STAR data allow, for the first time, such a direct comparison between A+A, d+A, and p+p collisions, at the same energy, measured in the same detector, and using the same analysis techniques. Surprisingly, our preliminary results indidate an mT-independent scaling of HBT radii with overall system size. Possible physics implications of these observations will be discussed, and the importance of long-range non-femtoscopic correlations for low multiplicity collisions will be emphasized.