Spin structure of the proton and deuteron with CLAS at Jefferson Lab

P. E. Bosted
2005 AIP Conference Proceedings  
The EG1 experiment at Jefferson Lab used the CLAS detector to measure inclusive, semi-inclusive, and exclusive reactions with longitudinally polarized electrons scattering from longitudinally polarized protons and deuterons. Preliminary results for g p 1 and g d 1 are presented for a subset of the inclusive data. The data span a large kinematic range, 0 02 ¡ Q 2 ¡ 5 GeV 2 and 0 05 ¡ x ¡ 1, covering the region where nucleon resonances dominate as well as the traditional DIS region. The
more » ... gion. The spin-dependent structure function g 1 ¢ x£Q 2 ¤ for inelastic lepton-nucleon scattering is of fundamental importance in understanding the quark and gluon spin structure of the proton and neutron in the deep inelastic region of high Q 2 , as well as the resonant structure at low Q 2 and excitation energy. The goal of the CLAS EG1 experiment was to make precise measurements over the entire ¢ x£Q 2 ¤ range currently accessible at Jefferson Lab in a single experiment, using both polarized proton and deuteron targets. Results from an initial data run in 1998 (EG1a) have recently been published [1, 2]. We report here on some of the data from the much higher statistics run in 2000. Beam currents ranged from 0.3 nA to 6 nA. The helicity was frequently reversed to minimize instrumental asymmetries. The 1-cm-long polarized target cells contained pre-irradiated granules of 15 NH 3 or 15 ND 3 immersed in liquid He at 1 K in a uniform magnetic field of 5 T. Microwaves near 140 GHz were used to drive the hyperfine transition which aligns (or anti-aligns) the nucleon spins with the magnetic field, producing proton polarizations of typically 70% (30%) for NH 3 (ND 3 ). The sub-millimeter diameter electron beam spot was rastered over the 1.8 cm 2 front surface of the target to uniformly distribute beam heating and radiation damage. The target polarization direction was periodically reversed. The product of target polarization P t and beam polarization P B was determined using the well-known double-spin asymmetry in ep elastic (quasielastic) scattering for NH3 (ND3). Results from single arm (e only) and double arm (both e and p detected) measurements were found to be consistent within statistical errors, and the average for the experiment was P b P t ¥ 0¦5 for protons and P b P t ¥ 0¦2 for deuterons. Scattered electrons were tracked in the CLAS [3] toroidal field using drift chambers, and separated from hadrons by requiring a substantial signal in the gas Cherenkov counters and most of the particle energy to be deposited in the electromagnetic calorimeter. Data were taken with approximately equal running times with positive and negative torus polarity (corresponding to electrons bending inward or outward) with beam energies near 1.7, 2.4, 4.2, and 5.7 GeV. Approximately 2¦3 §
doi:10.1063/1.1871644 fatcat:lncsl34dijbs5gfzoempe4llja