Influence of the nucleon-nucleon collision geometry on the determination of the nuclear modification factor for nucleon-nucleus and nucleus-nucleus collisions

TL;DR

This paper examines how the geometry of nucleon-nucleon collisions affects the calculation of the nuclear modification factor in nuclear collisions, revealing significant corrections especially at high energies and peripheral collisions.

Contribution

It introduces a correction to the nuclear overlap function and binary collision count based on nucleon substructure, impacting the interpretation of nuclear modification factors.

Findings

Correction to $N_{coll}$ and $T_{AB}$ can be significant in peripheral collisions.

The correction is larger at LHC energies compared to RHIC energies.

Implications for experimental measurements of nuclear modification factors are discussed.

Abstract

The influence of the underlying nucleon-nucleon collision geometry on evaluations of the nuclear overlap function ($T_{AB}$) and number of binary collisions ($N_{coll}$) is studied. A narrowing of the spatial distribution of the hard-partons with large light-cone fraction $x$ in nucleons leads to a downward correction for $N_{coll}$ and $T_{AB}$, which in turn, results in an upward correction for the nuclear modification factor $R_{AB}$. The size of this correction is estimated for several experimentally motivated nucleon-nucleon overlap functions for hard-partons. It is found to be significant in peripheral nucleus-nucleus and nucleon-nucleus collisions, and are much larger at the LHC energy of $\sqrt{s}=5.5$ TeV than for the RHIC energy of $\sqrt{s}$=200 GeV. The implications for experimental measurements are also discussed.