Measurements of the Double-Spin Asymmetry A1 on Helium-3: Toward a Precise Measurement of the Neutron A1.

摘要

The spin structure of protons and neutrons has been an open question for nearly twenty-five years, after surprising experimental results disproved the simple model in which valence quarks were responsible for nearly 100% of the nucleon spin. Diverse theoretical approaches have been brought to bear on the problem, but a shortage of precise data---especially on neutron spin structure---has prevented a thorough understanding.;Experiment E06-014, conducted in Hall A of Jefferson Laboratory in 2009, presented an opportunity to add to the world data set for the neutron in the poorly covered valence-quark region. Jefferson Laboratory's highly polarized electron beam, combined with Hall A's facilities for a high-density, highly polarized ³He target, allowed a high-luminosity double-polarized experiment, while the large acceptance of the BigBite spectrometer gave coverage over a wide kinematic range: 0.15 x 0.95. In this work, we present the analysis of a portion of the E06-014 data, measured with an incident beam energy of 4.74 GeV and spanning 1.5 Q² 5.5 (GeV/c)². From these data, we extract the longitudinal asymmetry in virtual photon-nucleon scattering, A1, on the ³He nucleus. Combined with the remaining E06-014 data, this will form the basis of a measurement of the neutron asymmetry An1 that will extend the kinematic range of the data available to test models of spin-dependent parton distributions in the nucleon.