First measurement of hard exclusive $\ddot{I}{€}^{-}\hat{I}{”}^{++}$ electroproduction beam-spin asymmetries off the proton

Abstract

The polarized cross section ratio $\ddot{I}{ƒ}_{L{T}^{\prime }}/\ddot{I}{ƒ}_0$ from hard exclusive $\ddot{I}{€}^{-}\hat{I}{”}^{++}$ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2 GeV / 10.6 GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from 1.5 GeV${}^2$ up to 7 GeV${}^2$. The reaction provides a novel access to the $d$-quark content of the nucleon and to $p\to \hat{I}{”}^{++}$ transition generalized parton distributions. A comparison to existing results for hard exclusive $\ddot{I}{€}^{+}n$ and $\ddot{I}{€}^{0}p$ electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry.