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E05-015: Measurement of the Target Single-Spin Asymmetry in Quasi-Elastic 3He(e,e')
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  Two HRS spectrometers on each side of the beam will be used to independently detect the scattered electrons at the same scattering angle for the Q2 = 1.0 GeV2 measurement.  
This experiment will measure the target single-spin asymmetry, Ay , for the neutron using the inclusive quasi-elastic 3He(e,e') reaction in Hall A with a vertically polarized 3He target at Q2 = 1.0 and 2.3 GeV2. In the one-photon exchange approximation, Ay is identically zero due to time-reversal invariance. However, it is also sensitive to the two-photon exchange amplitude which can be non-zero and enters Ay through the interference between the one- and two-photon amplitudes. For large enough Q2, where the scattering predominantly occurs from asymptotically-free partons, there are contributions from both the elastic and inelastic nucleon response during two-photon exchange. Calculation of the elastic response is straight-forward and yields an asymmetry of Any,elas ≅ -0.005 for the neutron at the kinematics of this experiment. The inelastic response was recently calculated using models of Generalized Parton Distributions (GPD's) as input and is on the order of Any,inel ≅ -0.01, which gives a total expected asymmetry of Any ≅ -0.015. Two different moments, each containing two of the GPD's, are needed to fully describe the inelastic response. However for the neutron, Ay is dominated by just one of these moments that contains the GPD's, Hq and Eq, that are also related to the nucleon form factors and the total angular momentum contribution to the nucleon spin from the quarks. This experiment will be performed using the standard Hall A spectrometers and a vertically polarized 3He target and will measure Any with an absolute statistical uncertainty of δAy ≅ 0.0023 at each Q2 (15% relative to the prediction above). This experiment will be the first to firmly establish a non-vanishing Ay, a T-odd quantity that is identically zero in the Born approximation, providing new constraints on GPD models and new information on the dynamics of the two-photon exchange process.
This experiment is scheduled to run in Jefferson Lab's Experiment Hall A from April 20 through May 8, 2009.