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Title:Measuring the contribution of low Bjorken-x gluons to the proton spin with polarized proton-proton collisions
Author(s):Wolin, Scott
Director of Research:Grosse Perdekamp, Matthias
Doctoral Committee Chair(s):Peng, Jen-Chieh
Doctoral Committee Member(s):Grosse Perdekamp, Matthias; Eckstein, James N.; Stack, John D.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
nuclear physics
gluon contribution to the proton spin
Abstract:The PHENIX experiment is one of two detectors located at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, NY. Understanding the spin structure of the proton is a central goal at RHIC, the only polarized proton-on-proton collider in existence. The PHENIX spin program has two primary objectives. The first is to improve the constraints on the polarized parton distributions of the anti-u and anti-d quarks within the proton. The second objective is to improve the constraint on the gluon spin contribution to the proton spin, ∆G. The focus of this thesis is the second objective. The motivation to study ∆G originates with polarized Deep Inelastic Scattering (DIS) experiments, in which a polarized lepton is scattered off of a polarized proton. Polarized DIS scattering experiments have found that the quark polarization is significantly less than expected and too small to account for the proton spin of 1/2h. This result was published first in 1989 by the European Muon Collaboration. After it was further confirmed through DIS experiments at SLAC, CERN and DESY, it was natural to try measuring ∆G, as a non-zero value would imply that the missing quark spin was due to gluon spin. Polarized DIS experiments are able to indirectly access gluons, however, the kinematic reach has so far been insufficient to put a strong constraint on ∆G. Even extreme scenarios for the gluon polarization could not be excluded by these experiments. At a polarized proton-on-proton collider, one gains direct access to gluons and more powerful constraints to the gluon polarization become possible with relatively small data samples. The PHENIX experiment has been successful at providing the first meaningful constraints on ∆G, along with STAR, the other detector located at RHIC. These constraints have, in fact, eliminated the extreme scenarios for gluon polarization through measurements of the double spin asymmetry, ALL, between the cross section of like and unlike sign helicity pp interactions. ALL measurements can be performed with a variety of final states at PHENIX. Until 2009, these final states were only measured for pseudo-rapidities of |η| < 0.35. This range of η is referred to as mid-rapidity. These mid-rapidity measurements, like the polarized DIS measurements, suffer from a limited kinematic reach. Final states containing a measured particle with pT > 1 GeV/c are considered to have occurred in the hard scattering domain where the pp interaction is well approximated as an interaction of a quark or gluon in one proton and a quark or gluon in the second proton. Each of these interacting particles has a momentum fraction, x, of its parent proton’s momentum. The gluon polarization is dependent on the momentum fraction and the net gluon polarization can be written as the integral of the momentum fraction dependent polarization: ∆G = \int_{0}^{1} ∆g(x)dx. The momentum fractions of the two interacting particles give information about the final state jets. Likewise, one can work backwards. By measuring the kinematics of final state hadrons or jets, information about quark and gluon momentum fractions can be learned. It turns out that mid-rapidity measurements of ALL are primarily sensitive to pp collisions in which the gluon momentum fraction was in the range 0.05 < x < 0.2. Therefore, mid-rapidity measurements are capable of constraining ∆g(x) only within this range and the polarization of gluons having a momentum fraction outside this range do not play a significant role in the observed ALL. This leaves a large gap in understanding as the gluon number density at low-x, x < 0.05, grows rapidly. It is, therefore, precisely the region not constrained by mid-rapidity ALL measurements that is the most interesting place to look for a potentially large gluon polarization. This provides the motivation to build a new calorimeter for PHENIX that is able to measure final states of pp interactions in which a low-x gluon was a participant. Like a fast moving car crashing into a slow moving car and the debris ending up mostly along the line of motion of the fast moving car, the debris of a high-x quark interacting with a low-x gluon will result in debris at forward rapidity at small angles to the initial quark momentum. The Muon Piston Calorimeter (MPC) was installed in 2006 and 2007 at forward rapidity, 3.1 < |η| < 3.9, with the intention of giving PHENIX the ability to constrain ∆g(x) for x < 0.05. In this thesis, the first two measurements of ALL using the MPC to measure a single hadron in the final state will be presented. Following this, an electronics upgrade to the MPC will be described which enables the selection of events with two hadrons detected in the MPC. This requirement favors gluons at even lower x than the single hadron event selection. The di-hadron measurement that this upgrade makes possible will allow PHENIX to produce an ALL measurement that constrains ∆g(x) in the range of 5 × 10−4 < x < 0.01. Finally, we discuss the most important systematic uncertainty common to all ALL measurements which arises from the determination of the relative luminosity. A precision ALL measurement requires measuring the final state yield from the portions of the proton beams that collide like and unlike sign helicity protons separately. It also requires understanding the ratio of the collision rates of these two portions of the beam exquisitely well. This is a long standing problem and, until recently, had threatened to severely restrict the ability of PHENIX to utilize the large data sets that have been acquired in the last two years to improve the constraints on ∆G. We will conclude this thesis with a comprehensive overview of the relative luminosity systematic uncertainty and present a new framework within which this uncertainty can be determined. It will be demonstrated that not only were very large effects previously overlooked, but that by accounting for these effects the systematic uncertainty is reduced by an order of magnitude, from O(10−3) to O(10−4). This improvement has consequences for all high statistics measurements at PHENIX which were previously limited by their systematic uncertainty. The measurement of the gluon contribution to the proton spin at the PHENIX experiment is a multi- faceted problem which requires a multi-faceted solution. This thesis describes several aspects of the solution as the single- and di-hadron measurements from MPC data are likely to provide the best constraints to ∆G at low-x for the next decade. Eventually, an Electron-Ion Collider (EIC) will be designed and commissioned that will further extend the kinematic reach of the polarized DIS experiments that motivated the spin program at RHIC. In the meantime, the goal of PHENIX in general, and the MPC in particular, is to glean as much information about the gluon polarization as possible before the EIC era arrives.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Scott Wolin
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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