Rapidity, azimuthal, and multiplicity dependence of mean transverse momentum and transverse momentum correlations inπ+pandK+pcollision

Abstract

Rapidity, azimuthal and multiplicity dependence of mean transverse momentum and transverse momentum correlations of charged particles is studied in p and K p collisions at 250 GeV=c incident beam momentum. For the first time, it is found that the rapidity dependence of the two-particle transverse momentum correlation is different from that of the mean transverse momentum, but both have similar multiplicity dependence. In particular, the transverse momentum correlations are boost invariant. This is similar to the recently found boost invariance of the charge balance function. A strong azimuthal dependence of the transverse momentum correlations originates from the constraint of energy-momentum conservation. The results are compared with those from the PYTHIA Monte Carlo generator. The similarities to and differences with the results from current heavy ion experiments are discussed. The transverse momenta of final state particles are produced after the collision and carry information on system expansion. Their fluctuations are considered to be a good probe for the formation of a quark-gluon plasma (QGP) [1,2] and a trace of local thermal equilibrium [3]. Therefore, these fluctuations have attracted a lot of attention from both theoretical and experimental investigations on relativistic heavy ion collisions [4].All current heavy ion experiments report substantial dynamical fluctuations of transverse momentum p t . Preliminary PHENIX and STAR data on Au Au collisions show that p t fluctuations increase as centrality increases and a similar increase is observed in the mean transverse momentum [5,6]. However, it is not clear how these fluctuations differ from those observed in elementary collisions, where no QGP is expected.In general, it is believed that transverse expansion extends over a wide rapidity range and correlates with longitudinal expansion [7]. However, exactly how the transverse expansion relates to the longitudinal one is unexplored even for the elementary collisions. Detailed investigation on these phenomena in elementary collisions is important for a correct evolution picture of high energy collisions and for the understanding of the related phenomena in current heavy ion experiments. This kind of investigation is possible only in an experiment with full acceptance, such as the hadron-hadron experiment NA22, which makes use of a rapid cycling bubble chamber as an active vertex detector and has excellent momentum resolution in full 4 acceptance.Since the measure of transverse momentum fluctuation in terms of a variable p t was suggested [8], a number of other variables have been recommended [3,6,7,[9][10][11][12]] to extract the genuine dynamical fluctuations. The common and essential part of these variables is the event twoparticle transverse momentum correlation P n ch i1