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Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the Reaction He 3 ↑ (e,e′)
Zhang, YW Long, E Mihovilovič, M Jin, G Allada, K Anderson, B Annand, JRM Averett, T Ayerbe-Gayoso, C Boeglin, W
Zhang, YW
Long, E
Mihovilovič, M
Jin, G
Allada, K
Anderson, B
Annand, JRM
Averett, T
Ayerbe-Gayoso, C
Boeglin, W
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Journal Article
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2015-10-22
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10.1103/PhysRevLett.115.172502
Abstract
© 2015 American Physical Society. We report the first measurement of the target single-spin asymmetry, Ay, in quasielastic scattering from the inclusive reaction He3↑(e,e′) on a He3 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 0.97GeV2. These measurements demonstrate, for the first time, that the He3 asymmetry is clearly nonzero and negative at the 4σ-9σ level. Using measured proton-to-He3 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=0.97GeV2 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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Physical Review Letters
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