We present the first measurement of the timelike Compton scattering process, $\gamma p\to p^{\prime }{\gamma }^{*}({\gamma }^{*}\to e^{+}{e}^{-})$, obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities $2.25<Q^{\prime 2}<9{\mathrm{GeV}}^2$, squared momentum transferred $0.1<-t<0.8{\mathrm{GeV}}^2$, and average total center-of-mass energy squared $s=14.5{\mathrm{GeV}}^2$. The photon beam polarization asymmetry, similar to the beam-spin asymmetry in deep virtual Compton scattering, is sensitive to the imaginary part of the Compton form factors and provides a way to test the universality of the generalized parton distributions. The angular asymmetry of the decay leptons accesses the real part of the Compton form factors and thus the $D$-term in the parametrization of the generalized parton distributions.

• Received 31 August 2021
• Revised 21 October 2021
• Accepted 11 November 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.262501

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Published by the American Physical Society