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

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 τ=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.