Ensemble mean $\bf p_t$ vs charged-hadron multiplicities in high energy nuclear collisions

Abstract

Measurements of event-ensemble mean transverse momentum $\langle p_t \rangle$ vs charged-hadron multiplicity $n_{ch}$ for $p_t$ spectra from 5 TeV p-Pb and 2.76 TeV Pb-Pb collisions and from p-p collisions for several energies have been reported recently. While in all cases $\langle p_t \rangle$ increases monotonically with $n_{ch}$ the rate of increase is very different from system to system. Comparisons with several theory Monte Carlos reveal substantial disagreements and lead to considerable uncertainty on how to interpret the $\langle p_t \rangle$ data. In the present study we develop a two-component (soft+hard) model (TCM) of $p_t$ production in high energy nuclear collisions and apply it to the $\langle p_t \rangle$ data. The soft component is assumed to be a universal feature of high energy collisions independent of A-B system or energy. The hard-component model is based on the observation that dijet production in p-p collisions does not satisfy the eikonal approximation but does so in A-A collisions. Hard-component properties are determined from previous measurements of hadron spectrum hard components, jet spectra and fragmentation functions. The TCM describes the p-p and Pb-Pb $\langle p_t \rangle$ data accurately, within data uncertainties, and the p-Pb data appear to transition smoothly from p-p to A-A $n_{ch}$ trends.