Multiplicity fluctuations and rapidity correlations in ultracentral proton-nucleus collisions

TL;DR

This paper models multiplicity fluctuations and rapidity correlations in ultracentral proton-nucleus collisions, showing that quantum fluctuations dominate and can be described by a correlated gamma distribution, matching existing data and predicting new results.

Contribution

It introduces a simple quantum fluctuation model using a correlated gamma distribution to explain multiplicity and rapidity asymmetries in ultracentral p-A collisions, aligning with experimental data.

Findings

The model reproduces existing multiplicity fluctuation data.

Quantum fluctuations dominate over impact parameter variations in high-multiplicity collisions.

Predictions are made for extremely central collision classes (0-0.1% and 0-0.01%).

Abstract

A collision between a proton and a heavy nucleus at ultrarelativistic energy creates particles whose rapidity distribution is asymmetric, with more particles emitted in the direction of the nucleus than in the direction of the proton. This asymmetry becomes more pronounced as the centrality estimator, defined from the energy deposited in a calorimeter, increases. We argue that for high-multiplicity collisions, the variation of the impact parameter plays a negligible role, and that the fluctuations of the multiplicity and of the centrality estimator are dominated by quantum fluctuations, whose probability distribution can be well approximated by a correlated gamma distribution. We show that this simple model reproduces existing data, and we make quantitative predictions for collisions in the $0-0.1\%$ and $0-0.01\%$ centrality windows. We argue that by repeating the same analysis with a different centrality estimator, one can obtain direct information about the rapidity decorrelation in particle production.