Ensemble meanptversus charged-hadron multiplicities in high energy nuclear collisions

Abstract

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