Analyses of Quark Helicity Distributions in the Polarized Nucleon Measured at HERMES

Abstract

Quark helicity distributions were measured by experiment at DESY-HERA HERMES. The HERMES experiment measures the spin structure of the nucleon with Deep Inelastic Scattering (DIS) of 27.6 GeV longitudinally polarized positron beam at HERA and longitudinally polarized gas targets (H, D). The experiment was motivated to solve the “nucleon spin puzzle” found by the EMC experiment in 1988. HERMES uses the Ring Imaging CHerenkov (RICH) detector which has dual radiators: aerogel and C4F10 gas. The RICH allows us to identify pions, kaons and protons in the momentum region of 2 15 GeV/c. Hadrons are detected in coincidence with the scattered positron. HERMES is the first DIS experiment with a full hadron identification capability. Using informations from the RICH, cross section asymmetries between the parallel and anti-parallel configuration of the beam and target spins were measured for charged pions and kaons separately for the first time. From the measured double spin asymmetries, helicity distributions of individual quark flavors were extracted for u, ū, d, d and s. Main focuses of this thesis are 1) evaluation of efficiency of the hadron identification with RICH, and 2) evaluation of systematic uncertainties in the quark helicity distributions due to uncertainties on the unpolarized parton distribution functions (PDFs) used in the analyses. The uncertainty in the hadron identification with RICH is mainly due to imperfection of the RICH description in the HERMES Monte Carlo simulation. The estimation of the hadron identification efficiency was performed by hadron tagging using ’decaying particle method’ which uses decaying particles such as ρ, φ, Λ, K s in the experimental data. The evaluated RICH uncertainty was propagated to the systematic error on the double spin asymmetries. The effect of the uncertainties in the unpolarized PDFs was evaluated using the 40 eigenvector PDF sets of the CTEQ6.1M parameterization. The quark helicity distributions are obtained by HERMES with high precision in a self-consistent way with less assumptions in the analysis than any of previous experiments, making use of its hadron identification capabilities.