Probing Hadron Structure in Proton-Nucleus Collisions

TL;DR

This paper investigates high-energy proton-nucleus collisions to test gluon distribution models, analyzing azimuthal correlations and inclusive hadron production, and explores methods to improve theoretical predictions at high transverse momentum.

Contribution

It provides a detailed analysis of gluon distributions using azimuthal correlations and introduces methods to address negative cross sections in high-order calculations.

Findings

Azimuthal correlations are sensitive probes of gluon distributions.

Next-to-leading order calculations yield negative cross sections at high transverse momentum.

Resummation and exact kinematic methods can produce more realistic predictions.

Abstract

Understanding the behavior of large atomic nuclei (heavy ions) in high-energy collisions has been the focus of a concerted research effort over the past 10-15 years, with a recent focus on transverse momentum-dependent (or "unintegrated") parton distributions and their high-energy behavior. With the advent of high-energy proton-nucleus collisions at RHIC and the LHC, we are able to experimentally test this behavior for the first time. In this dissertation, I examine two sample predictions of this high-energy behavior. First, I analyze the azimuthal angular correlation for Drell-Yan pair and associated hadron production. I show that the correlation is a sensitive probe of the underlying gluon distribution, and a proper prediction of the correlation at all angles requires a gluon distribution with physically realistic behavior at all momenta. I'll then describe a numerical calculation of the cross section for inclusive hadron production, incorporating all corrections up to next-to-leading order in the strong coupling. The results of the calculation are negative at high transverse momentum, which is surprising but may be mathematically reasonable, since the perturbative approximation to the cross section may break down under those kinematic conditions. However, it may be possible to make meaningful predictions for the nuclear modification ratio $R_{pA}$ despite the negative cross section. Moving beyond next-to-leading order, I examine two proposed methods of curing the negativity: first, a straightforward resummation of selected higher-order terms corresponding to gluon loop diagrams, and the use of exact kinematic formulas, incorporating terms which disappear in the infinite-energy limit. Using these methods, the calculation can be adapted to produce reasonable results at high transverse momentum.