Multiplicity fluctuations and correlations in 5.02 TeV p+Pb collisions at zero impact parameter

TL;DR

This paper introduces a Bayesian method to reconstruct event-by-event multiplicity fluctuations and correlations in p+Pb collisions at 5.02 TeV, revealing insights into proton structure and fluctuation behavior across rapidity.

Contribution

It develops a model-independent Bayesian approach to analyze fluctuations and correlations in p+Pb collisions, tested on simulations and applied to experimental data.

Findings

Angantyr overestimates fluctuations

Fluctuations increase towards the Pb-going side

Gluon branching explains fluctuation behavior

Abstract

We present a Bayesian method to reconstruct event-by-event multiplicity fluctuations and rapidity correlations in p+Pb collisions at zero impact parameter from minimum-bias data, without assuming any model of the collision dynamics. We test it on Monte Carlo simulations with the Angantyr model, then apply it to ATLAS data on the distribution of charged multiplicity and transverse energy in p+Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Fluctuations in $b=0$ collisions are quantum fluctuations which originate mostly from the proton wave function, and therefore have the potential to constrain the subnucleonic structure of the proton. The Angantyr model is found to overestimate fluctuations. In addition, we find that as the rapidity increases (towards the Pb-going side), not only the multiplicity density increases, but also its relative event-by-event fluctuation. This counter-intuitive phenomenon is also observed in simulations with Angantyr, and with the QCD dipole model, where its origin can be traced back to the branching process through which gluons are produced.