Collectivity in pPb Collisions with Femtoscopy

TL;DR

This paper proposes using two-particle femtoscopic correlations of non-identical hadrons, specifically proton-pion pairs, to detect and analyze collective, vortex-like fluid behavior in small pPb collision systems at the LHC.

Contribution

It introduces a novel femtoscopic observable based on emission asymmetry between protons and pions to measure initial vorticity in small collision systems.

Findings

Emission asymmetry correlates with initial vorticity.

Proton-pion femtoscopy can detect fluid rotation.

Method provides a new way to study collective flow in small systems.

Abstract

Collisions of protons with lead nuclei (pPb), such as those measured by the LHCb experiment, provide a unique environment to study the surprising emergence of collective, fluid-like phenomena in small systems. A key signature of this hydrodynamic behavior is the predicted formation of a toroidal vorticity structure. In this work, I use two-particle femtoscopic correlations of non-identical hadrons, specifically proton-pion ($p\pi^+$) pairs, as a novel probe for this phenomenon. Previous works indicate that the collective flow of the system is consistent with the formation of a vortex ring created by the passage of the proton through the lead nucleus, which modifies the collective flow profile. I establish that the resulting emission asymmetry between protons and pions, driven by their mass difference and differential response to the vortical flow, is directly linked to the initial vorticity and can be measured using femtoscopy. This method therefore presents a new, sensitive observable for characterizing the rotational dynamics of the matter created in small collision systems.