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

PHENIX;proton spin;gluon polarization;double spin asymmetry;Delta G

McKinney, Cameron Palmer

关键词: PHENIX;    proton spin;    gluon polarization;    double spin asymmetry;    Delta G;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/88073/MCKINNEY-DISSERTATION-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In the 1980s, polarized deep inelastic lepton-nucleon scattering experimentsrevealed 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 thepuzzle of how to account for the remaining spin. As gluons carry roughly50\% of the proton's momentum, it seemed most logical to look to thegluon spin as another significant contributor. However, lepton-nucleonscattering experiments only access the gluon helicity distribution,$\Delta g$, through effects on the quark distributions via scalingviolations. Constraining $\Delta g$ through scaling violations requiresexperiments that together cover a large range of $Q^{2}$. Such experimentshad been carried out with unpolarized beams, leaving $g(x)$ (theunpolarized gluon distribution) relatively well-known, but the polarizedexperiments 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 firstpolarized $pp$ running in 2002, the gluon distributions could beaccessed 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 includedin a QCD global analysis performed by Daniel de Florian, Rodolfo Sassot,Marco Stratmann, and Werner Vogelsang (DSSV), yielding the first directconstraints on the gluon helicity. The DSSV group found that the contributionof the gluon spin to the proton spin was consistent with zero, butthe data provided by PHENIX and STAR was all at mid-rapidity, meaning$\Delta g$ was constrained by data only a range in $x$ from 0.05to 0.2, leaving out helicity contributions from the huge number oflow-$x$ gluons. A more recent analysis by DSSV from 2014 includingRHIC data through 2009 for the first time points to significant gluonpolarization at intermediate momentum fractions, meaning gluon polarizationmeasurements may be more interesting than anticipated, especiallyat 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 sensitivityto $\Delta g$ to lower $x$. Monte Carlo simulations indicate thatmeasurements of hadrons in the MPC's pseudorapidity of range $3.1