Hot spots and gluon field fluctuations as causes of eccentricity in small systems

TL;DR

This paper investigates the origins of eccentricity in small systems like proton-nucleus collisions, finding that geometric hot spot fluctuations are the primary cause, with color charge fluctuations playing a minor role.

Contribution

It provides an analytical calculation of eccentricities considering both hot spot and color charge fluctuations, highlighting the dominance of geometric fluctuations in small systems.

Findings

Geometric hot spot fluctuations are the main source of eccentricity.

Color charge fluctuations have a negligible effect on eccentricity.

Hot spot size and number critically influence eccentricity values.

Abstract

We calculate eccentricities in high energy proton-nucleus collisions, by calculating correlation functions of the energy density field of the Glasma immediately after the collision event at proper time tau = 0. We separately consider the effects of color charge and geometrical hot spot fluctuations, analytically performing the averages over both in a dilute-dense limit. We show that geometric fluctuations of hot spots inside the proton are the dominant source of eccentricity whereas color charge fluctuations only give a negligible correction. The size and number of hot spots are the most important parameters characterizing the eccentricities.