Evidence for x-dependent proton color fluctuations in pA collisions at the LHC

TL;DR

This paper presents experimental evidence and a model for x-dependent proton color fluctuations in pA collisions at the LHC, linking parton size and interaction strength to observed deviations from Glauber predictions.

Contribution

It introduces a quantitative model incorporating interaction strength fluctuations to explain centrality-dependent jet production, highlighting x-dependent proton size variations.

Findings

Partons with large x have smaller interaction cross sections.

The model accurately describes the x=0.6 data.

Results suggest a connection to the EMC effect.

Abstract

We argue that the pattern of the deviation from the Glauber approximation prediction for the centrality dependence of the rate of forward jet production observed in pA collisions at the LHC provides the first experimental evidence that parton configurations in the projectile proton containing a parton with large $x$ interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the $x$-dependent interaction strength $\sigma(x)$. We find that \sigma(x)\sim 0.6 ~\sigma_{tot}(pp) gives a good description of the x=0.6 data and may shed a light on the origin of the EMC effect.