First Measurement of the Beam Normal Single Spin Asymmetry in $Δ$ Resonance Production by $Q_{\rm weak}$

Abstract: The beam normal single spin asymmetry ($B_{\rm n}$) is generated in the scattering of transversely polarized electrons from unpolarized nuclei. The asymmetry arises from the interference of the imaginary part of the two-photon exchange with the one-photon exchange amplitude. The $Q_{\rm weak}$ experiment has made the first measurement of $B_{\rm n}$ in the production of the $\Delta$(1232) resonance, using the $Q_{\rm weak}$ apparatus in Hall-C at the Thomas Jefferson National Accelerator Facility. The final transverse asymmetry, corrected for backgrounds and beam polarization, is $B_{\rm n}$ = 43 $\pm$ 16 ppm at beam energy 1.16 GeV at an average scattering angle of about 8.3 degrees, and invariant mass of 1.2 GeV. The measured preliminary $B_{\rm n}$ agrees with a preliminary theoretical calculation. $B_{\rm n}$ for the $\Delta$ is the only known observable that is sensitive to the $\Delta$ elastic form-factors ($\gamma$*$\Delta\Delta$) in addition to the generally studied transition form-factors ($\gamma$*N$\Delta$), but extracting this information will require significant theoretical input.