Mechanism behind p–Pb normalized multiplicity following INEL pp energy dependence

Ascertain the mechanism responsible for the observation that the normalized mean charged-particle multiplicity per participating nucleon pair, (2/⟨N_part⟩)⟨N_ch⟩, in non-single diffractive proton–lead collisions at 5.02 TeV aligns with the energy dependence observed in inelastic proton–proton collisions rather than central nucleus–nucleus collisions.

Background

By fitting power-law dependencies across systems, the authors find that central AA collisions exhibit a faster energy growth of normalized mean multiplicity than pp. Surprisingly, the p–Pb value at 5.02 TeV falls on the INEL pp curve and is half the Pb–Pb magnitude.

This unexpected similarity between NSD p–Pb and INEL pp scaling raises a fundamental question about the underlying particle production mechanism in p–Pb, which remains unresolved in current theoretical descriptions.