Measurement of the Longitudinal Single-Spin Asymmetry for W± Boson Production in Polarized Proton-Proton Collisions at √S = 510 GeV at RHIC

Abstract

Understanding the spin structure of the nucleon can be considered as one of the fundamental goals in nuclear physics. Following the introduction of the quark model in 1964, the spin of the proton was naively explained by the alignment of spins of the valence quarks. However, in our current understanding, the valence quarks, sea quarks, gluons, and their possible orbital angular momentum are all expected to contribute to the overall spin of the proton. Despite this significant progress, our understanding of the individual spin contributions of quarks and antiquarks to the proton is not yet complete. Measurements of W± single spin asymmetries in longitudinally polarized proton-proton collisions at RHIC provides unique and clean access to the individual helicity distributions of light quarks and antiquarks of the proton. W± boson are produced through the annihilation of up + anti-down (anti-up + down) quarks and can be detected through their leptonic decays to electrons and anti-electron neutrinos (positrons and electron neutrinos). Due to maximal violation of parity during the production, W bosons couple to left-handed quarks and right-handed anti-quarks and hence offer direct probes of their respective helicity distributions in the nucleon. The STAR experiment at RHIC is well equipped to measure W decay electrons and positrons in longitudinally polarized p+p collisions, where only the charged lepton is observed in the final state with a large missing transverse energy opposite in azimuth due to the undetected neutrino. In this dissertation, the details of the analysis and the results of the longitudinal single spin asymmetry, AL, for W boson production at RHIC are presented. The total integrated luminosity of the data analyzed is 246 pb-1 with an average beam polarization of ~54%. The data are collected during 2013 in longitudinally polarized proton-proton collisions at √S =510 GeV by the STAR experiment at RHIC. The analysis includes the procedure, the results and the evaluation of the systematic uncertainty of the calibration of the STAR Barrel Electromagnetic Calorimeter which was performed coincident with the primary W AL analysis. The W AL analysis is discussed in terms of data QA, the reconstruction of W bosons via decayed electrons and positrons, and the estimation of the electroweak and QCD type background contributions. The reconstruction of W decay events includes the use of the Time Projection Chamber for the tracking purposes and the Barrel Electromagnetic Calorimeter for the identification and isolation of electron and poistron candidates by measuring their transverse energies in the calorimeter towers. Finally the results of AL for W+ (W-) are reported as a function of decay positron (electron) pseudo-rapidity, η, between -1 and +1. The theoretical predictions for the spin asymmetries calculated using recent polarized and unpolarized parton distribution functions, are compared with the measured values.