Mean pt scaling with m/nq at the LHC: Absence of (hydro) flow in small systems?

TL;DR

This paper investigates how the average transverse momentum scales with hadron mass and quark content in different collision systems at the LHC, revealing deviations from hydrodynamic predictions in small systems.

Contribution

It demonstrates that m/nq scaling holds in low-multiplicity events but breaks in high-multiplicity collisions, challenging hydrodynamic models and supporting a quark content scaling.

Findings

m/nq scaling observed in low-multiplicity events

Scaling breaks in high-multiplicity p-Pb and Pb-Pb collisions

Hydro models do not reproduce the observed scaling behavior

Abstract

In this work, a study of the average transverse momentum (pt) as a function of the mid-rapidity charged hadron multiplicity (Nch) and hadron mass (m) in p-Pb and Pb-Pb collisions at LHC energies is presented. For the events producing low Nch, the average pt is found to scale with the reduced hadron mass, i.e., mass divided by the number of quark constituents (m/nq), this scaling also holds for inelastic pp collisions at RHIC and LHC energies. The scaling is broken in high multiplicity p-Pb and Pb-Pb collisions, where, for <Nch/deta> < 60 the average pt is higher for baryons than that for mesons, though they increase linearly with m/nq. This behavior is qualitatively well reproduced by Pythia 8, but not by hydro calculations, where an universal scaling with the hadron mass (and not with m/nq) is predicted for all the multiplicity event classes. Only the central (0-60%) Pb-Pb collisions behave as expected from hydro.