Thermal blurring of event-by-event fluctuations provoked by rapidity conversion

TL;DR

This paper investigates how thermal motion causes a blurring effect in the measurement of conserved charge fluctuations in heavy ion collisions, affecting the comparison between experimental data and theoretical predictions.

Contribution

It introduces the concept of thermal blurring as a significant factor altering the relation between momentum-space and coordinate-space fluctuations in relativistic heavy ion collisions.

Findings

Thermal motion significantly modifies fluctuation measurements.

Thermal blurring can be modeled as part of a diffusion process.

Rapidity window dependence reveals the impact of thermal blurring.

Abstract

We study the effect of thermal blurring caused by the use of (momentum-space) rapidity as a proxy of coordinate-space rapidity in experimental measurements of conserved charge fluctuations in relativistic heavy ion collisions. In theoretical studies assuming statistical mechanics, calculated fluctuations are those in a spatial volume. Experiments, on the other hand, can measure fluctuations only in a momentum-space in the final state. In a standard argument to compare experimental results for a momentum space with theoretical studies for a coordinate space, rapidities of particles are implicitly regarded as equivalent to their coordinate-space rapidity. We show that the relation of two fluctuations is significantly altered by the existence of the thermal motion, i.e. thermal blurring. We discuss that the thermal blurring can be regarded as a part of the diffusion process, and the effect can be understood by studying the rapidity window dependences of fluctuations. Centrality dependence of the thermal blurring effect is also discussed.