Measurement of the Target-Normal Single-Spin Asymmetry in Quasi-Elastic Scattering from the Reaction ${}^3$He${}^{\uparrow }(e,e^{\prime })$

Abstract

We report the first measurement of the target single-spin asymmetry, $A_y$, in quasi-elastic scattering from the inclusive reaction ${}^3$He${}^{\uparrow }(e,e^{\prime })$ on a ${}^3$He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A non-zero $A_y$ can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the sub-structure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at $Q^2=$ 0.13, 0.46 and 0.97 GeV${}^2$. These measurements demonstrate, for the first time, that the ${}^3$He asymmetry is clearly non-zero and negative with a statistical significance of (8-10)$\sigma$. Using measured proton-to-${}^3$He cross-section ratios and the effective polarization approximation, neutron asymmetries of $-$(1-3)% were obtained. The neutron asymmetry at high $Q^2$ is related to moments of the Generalized Parton Distributions (GPDs). Our measured neutron asymmetry at $Q^2=0.97$ GeV${}^2$ agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.

References in corpus (4)

• Spin Structure of the Nucleon - Status and Recent Results
• Two-Photon Physics in Hadronic Processes
• Transverse target spin asymmetry in inclusive DIS with two-photon exchange
• Neutron single spin asymmetries from semi-inclusive deep inelastic scattering off transversely polarized ${}^3$He

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