Origin of the high-multiplicity excess in IP-Glasma

Identify the origin of IP-Glasma’s overprediction of the most central, below-one-percent multiplicity events in proton-proton, proton-nucleus, and nucleus-nucleus collisions, including whether excessive fluctuations in proton substructure are responsible.

Background

IP-Glasma reproduces the self-normalized multiplicity distributions over most of the studied centrality range but overpredicts the rarest, highest-multiplicity events. The discrepancy is concentrated below approximately one-percent centrality and is therefore not expected to strongly affect the principal observables analyzed in the paper.

The authors identify excessive proton-substructure fluctuations, such as the heavy tail of the log-normal saturation-scale fluctuations, as one possible explanation, but do not establish the cause.

References

The origin of this excess is not clear; the excess may reflect excessive fluctuations of the proton substructure, for example, in the heavy tail of the log-normal $Q_s2$ fluctuations from \cref{eqn:saturation_scale_fluctuations}.

— Centrality-dependent nuclear modification from hard-soft correlations in the glasma  (2609.29886 - Faraday et al., 24 Sep 2026) in Section 3.1, Multiplicity distributions