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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Published in Physical Review Letters, v. 115, no. 17, article 172502, p. 1-6.

© 2015 American Physical Society

The copyright holders have granted the permission for posting the article here.

This work is a creation of the Jefferson Lab Hall A Collaboration. Due to the large number of authors involved, only the first 10 and the ones affiliated with the University of Kentucky are listed in the author section above. For the complete list of authors, please download this article or visit the following link: http://dx.doi.org/10.1103/PhysRevLett.115.172502.

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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.

Ay_QE_Supplement_PRL_TDA.pdf (21 kB)
The file contains a detailed description of our systematic uncertainties.