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Title:Gluon polarization in the proton: constraints at low x from the measurement of the double longitudinal spin asymmetry for forward-rapidity hadrons with the PHENIX detector at RHIC
Author(s):McKinney, Cameron Palmer
Director of Research:Grosse Perdekamp, Matthias
Doctoral Committee Chair(s):Makins, Naomi
Doctoral Committee Member(s):Stack, John; Willenbrock, Scott S.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
proton spin
gluon polarization
double spin asymmetry
Delta G
Abstract:In the 1980s, polarized deep inelastic lepton-nucleon scattering experiments revealed that only about a third of the proton's spin of $\frac{1}{2}\hbar$ is carried by the quarks and antiquarks, leaving physicists with the puzzle of how to account for the remaining spin. As gluons carry roughly 50\% of the proton's momentum, it seemed most logical to look to the gluon spin as another significant contributor. However, lepton-nucleon scattering experiments only access the gluon helicity distribution, $\Delta g$, through effects on the quark distributions via scaling violations. Constraining $\Delta g$ through scaling violations requires experiments that together cover a large range of $Q^{2}$. Such experiments had been carried out with unpolarized beams, leaving $g(x)$ (the unpolarized gluon distribution) relatively well-known, but the polarized experiments have only thus far provided weak constraints on $\Delta g$ in a limited momentum fraction range. With the commissioning in 2000 of the Relativistic Heavy Ion Collider, the first polarized proton-proton ($pp$) collider, and the first polarized $pp$ running in 2002, the gluon distributions could be accessed directly by studying quark-gluon and gluon-gluon interactions. In 2009, data from measurements of double longitudinal spin asymmetries, $A_{LL},$ at the STAR and PHENIX experiments through 2006 were included in a QCD global analysis performed by Daniel de Florian, Rodolfo Sassot, Marco Stratmann, and Werner Vogelsang (DSSV), yielding the first direct constraints on the gluon helicity. The DSSV group found that the contribution of the gluon spin to the proton spin was consistent with zero, but the data provided by PHENIX and STAR was all at mid-rapidity, meaning $\Delta g$ was constrained by data only a range in $x$ from 0.05 to 0.2, leaving out helicity contributions from the huge number of low-$x$ gluons. A more recent analysis by DSSV from 2014 including RHIC data through 2009 for the first time points to significant gluon polarization at intermediate momentum fractions, meaning gluon polarization measurements may be more interesting than anticipated, especially at momentum fractions where no constraints exist as of yet. A forward detector upgrade in PHENIX, the Muon Piston Calorimeter (MPC), was designed with the purpose of extending the sensitivity to $\Delta g$ to lower $x$. Monte Carlo simulations indicate that measurements of hadrons in the MPC's pseudorapidity of range $3.1<\eta<3.9$ probe asymmetric collisions between high-$x$ quarks and low-$x$ gluons, with the $x$ of the gluons reaching below 0.01 at a collision energy $\sqrt{s}=500\,GeV$. We access $\Delta g$ through measurements of $A_{LL}$ for electromagnetic clusters in the MPC; this thesis details the measurement from the Run 11 (2011) data set at $\sqrt{s}=500\,GeV$. We find $A_{LL}\approx0$, but the statistical uncertainties from this measurement mean we likely cannot resolve the small expected asymmetries. However, improved techniques for determining the relative luminosity between bunch crossings with different helicity configurations will allow data from a much larger data set in Run 13 to be most impactful in constraining $\Delta g,$ whereas previous measurements of $A_{LL}$ have had difficulties limiting the systematic uncertainty from relative luminosity. In this thesis, we begin by presenting an overview of the physics motivation for this experiment. Then, we discuss the experimental apparatus at RHIC and PHENIX, with a focus on those systems integral to our analysis. The analysis sections of the thesis cover calibration of the Muon Piston Calorimeter, a careful examination of the relative luminosity systematic uncertainty, and the process of obtaining a final physics result.
Issue Date:2015-07-17
Rights Information:Copyright 2015 by Cameron Palmer McKinney
Date Available in IDEALS:2015-09-29
Date Deposited:August 201

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