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Extraction of Beam-Spin Asymmetries from the Hard Exclusive π+ Channel off Protons in a Wide Range of Kinematics
We have measured beam-spin asymmetries to extract the sinϕ moment AsinϕLU from the hard exclusive ep→e′nπ+ reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center of mass. The A^sinϕ_{LU} moment has been measured up to 6.6 GeV^2 in −t, covering the kinematic regimes of generalized parton distributions (GPD) and baryon-to-meson transition distribution amplitudes (TDA) at the same time. The experimental results in very forward kinematics demonstrate the sensitivity to chiral-odd and chiral-even GPDs. In very backward kinematics where the TDA framework is applicable, we found A^sinϕ_LU to be negative, while a sign change was observed near 90° in the center of mass. The unique results presented in this Letter will provide critical constraints to establish reaction mechanisms that can help to further develop the GPD and TDA frameworks.