Speaker
Description
Timelike Compton Scattering (TCS), accessed through exclusive dilepton photoproduction $\gamma p \to p' \ell^+\ell^-$, is a key complementary channel to Deeply Virtual Compton Scattering (DVCS) for extracting generalized parton distributions (GPDs) from experiment. While DVCS primarily probes the imaginary part of Compton form factors, TCS provides independent access to the real part, and the combination of both channels is essential for a reliable and complete GPD extraction. However, the conventional theoretical framework for analyzing TCS relies on approximate treatments of the Bethe-Heitler (BH) mechanism, which shares the same final state but carries no GPD information. The standard approach generates an infinite series of azimuthal harmonics from the BH propagators, entangling kinematic artifacts with the GPD signal and introducing systematic bias into the extraction.
In this work, we recast TCS within the Single Diffractive Hard Exclusive Process (SDHEP) framework. Rather than treating BH as background to be subtracted, SDHEP organizes BH and TCS as different channels of the same process---a leading-power $\gamma^*$ channel and a next-to-leading-power $[q\bar{q}]$ channel---within a systematic expansion in $\sqrt{-t}/q_T$. The dedicated SDHEP frame, with its $z$-axis along the incoming photon, yields a cross section with finite azimuthal harmonics at each order in the power expansion, with no approximation of the BH propagators required. We derive the complete helicity amplitudes for both the BH and TCS subprocesses in this frame and construct the differential cross section in terms of polarization asymmetry parameters that depend linearly on GPD moments, providing an over-constrained system for GPD extraction. We show that the two frameworks are related by a simple rotation, which explains analytically how a single SDHEP harmonic maps to an infinite series in the conventional frame. Our results demonstrate that the SDHEP formalism provides a more systematic and precise pathway for extracting GPDs from current and future TCS measurements at Jefferson Lab and the Electron-Ion Collider.