Non - identical particle correlation analysis as a probe of transverse flow

Non-identical two particle correlation functions probe asymmetries between the average space-time emission points of different particle species. The system collective expansion would produce such asymmetry because massive particles, such as protons, are on average more pushed towards the edge of the system, than lighter ones, i.e. pions. Measuring pion-kaon, pion-proton and kaon-proton correlation functions, using the STAR detector, we show that the data are consistent with the effect of transverse flow in Au-Au collisions at √ s N N = 130 GeV. 1. Transverse expansion in ultra-relativistic heavy ion collisions High energy densities are reached in ultra-relativistic heavy ion collisions [ 1], which may lead to a transition from hadronic matter to partonic matter. In order to assess the formation of a partonic state of matter, which requires forming a large volume of interacting matter, it is mandatory to show evidences that the particles emerging from the system exhibit collective behavior. While it is not a required condition, showing that the system reaches thermal equilibrium would provide such clue. Models that assume chemical equilibrium at hadronization describe very well the relative abundance of most particle species [ 2]. However, such models are also successful at describing yields in p − p and e + −e − collisions where thermalization is not achieved. Another clue, may be obtained from studying collective expansion of the system which must arise if particles interact with one another. Indeed, a pressure gradient would then build up due to the large initial energy density gradient found between the core and the edge of the system. Conclusive evidence of the system's collective expansion have been gathered from several independent analysis: elliptic flow, transverse mass spectra and two-particle correlations [ 3]. These measurements in Au–Au collisions at √ s N N = 130 GeV are consistently interpreted in the blast wave framework relying on transverse flow, i.e. collective expansion in the transverse plane. The blast wave parameterization is described in Ref [ 4]. Dealing with central events, cylindrical symmetry is assumed, and the calculations parameters are: temperature (T = 110 MeV), transverse flow rapidity (ρ(r) = 3/2ρ(r/R) with ρ = 0.6), radius of the system (R = 13 fm), and emission duration (∆τ = 1.5 fm/c). T and ρ are extracted from pion, kaon and proton transverse mass spectra. R and ∆τ are extracted from the pion source radii, R out and R side. The interpretation of data …