Measurements of two particle Bose-Einstein correlations can be interpreted in terms of the space-time structure of the emitting source (HBT interferometry) and may provide a signature of QGP formation [, ]. Theoretical studies show that kaon interferometry offers significant advantages over pions with respect to final state interactions and resonance formation, which obscure the interpretation of the data [, ]. Also, studies of interferometry based on hydrodynamic models [] reveal that in order to best determine the structure of the emitting system the two-particle spectrum should include a wide range of the total two particle momenta including very low momenta. We discuss two applications of HBT interferometry for STAR in which the SVT makes a vital contribution: (1) by extending the acceptance to low and (2) by enabling neutral kaon () interferometry.
Increasing the acceptance improves the two pion statistics and might enable STAR to perform HBT on an event-by-event basis. Also, the study of two-particle correlations over an extended range of two-particle mean will provide better constraints on collision dynamics.
Neutral kaon interferometry via SVT is unique to STAR. Central Au+Au collisions produce on the order of 200 short lived neutral kaons per event and of these approximately 10% are expected to be reconstructed using SVT tracking of the decay pions. The K correlation function offers an obvious advantage due to the absence of Coulomb effects. A comparison between the results from charged and neutral kaon HBT will quantitatively determine the Coulomb effect. K HBT is also not affected by two-track resolution, which is a limiting factor in charged particle correlations. K are identified by their decay into charged pions. These pions are emitted back-to-back along a line oriented randomly in the K rest frame. Even for two K at exactly zero relative momentum the decay pions are well separated in configuration and momentum space and can be measured with the same efficiency as if the relative momentum of the kaons was large. Moreover, because the K is not a strangeness eigenstate, the K-K correlation includes a unique interference term that provides additional space-time information [] as well as insight into possible strangeness distillation effects [] in cases where the source baryon density is appreciable.
We simulated the K HBT performance of STAR with the SVT for central Au+Au HIJING events including an imposed Bose-Einstein correlation enhancement as expected from a 10 fm radius spherical source. Considering that the kaons can be expected to freeze out earlier than the pions, this radius is plausible. The event reconstruction was simplified by assuming perfect matching between SVT and TPC tracks. Tracking inefficiencies and detector resolution were taken into account however. Details of the procedure can be found in []. Fig. 4 shows the correlation functions C(Q) and C(Q) [, ]. Q, which refers to the relative transverse momentum along the direction of the average pair momentum is predicted to offer the best discrimination between plasma and hadron gas scenarios [, ]. Our simulation suggests that with the SVT a measurement of Q from K HBT is achievable. The SVT is essential for K interferometry, because of its tracking capability which enables efficient secondary vertex reconstruction.
Figure: Measurements of correlation functions C(Q) and C(Q) from K pair reconstruction with SVT and TPC