We report the first measurement of the target single-spin asymmetry, Ay, in quasielastic scattering from the inclusive reaction 3He↑(e,e′ ) on a 3He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A nonzero Ay can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the substructure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at Q2=0.13, 0.46, and 0.97 GeV2. These measurements demonstrate, for the first time, that the 3He asymmetry is clearly nonzero and negative at the 4σ–9σ level. Using measured proton-to-3He cross-section ratios and the effective polarization approximation, neutron asymmetries of −(1–3)% were obtained. The neutron asymmetry at high Q2 is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at Q2=0.97 GeV2 agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.
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This work was supported in part by the U.S. National Science Foundation, the U.S. Department of Energy, and DOE Contract No. DE-AC05-06OR23177, under which Jefferson Science Associates, LLC operates the Thomas Jefferson National Accelerator Facility, the National Science Foundation of China, and UK STFC Grants No. 57071/1 and No. 50727/1.
Dutta, Chiranjib; Korsch, Wolfgang; Zhang, Y.-W.; Long, E.; Mihovilovič, M.; Jin, G.; Allada, K.; Anderson, B.; Annand, J. R. M.; Averett, T.; Ayerbe-Gayoso, C.; and Boeglin, W., "Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the Reaction 3He↑(e,e′ )" (2015). Physics and Astronomy Faculty Publications. 379.