First measurement of timelike Compton scattering with CLAS12 at JLAB

While the most attention so far is on studies of GPD using spin (beam/target) observables and cross-sections in DVCS, the other Compton-like process, timelike Compton scattering (TCS) is accessible with high energy electron beams and has much to offer. TCS is the time-reversal symmetric process of DVCS where the incoming photon is real and the outgoing photon has large time-like virtuality.

In this talk, we present the first observation of the TCS process, γp→γ*p'→e⁺e^-p', measured using the CLAS12 detector at Jefferson Lab, with a 10.6 GeV electron beam impinging on a liquid-hydrogen target. The initial photon polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25<Q'²<9 GeV² and the squared momentum transferred0.1<-t<0.8$ GeV² at the average total center mass of energy squared of s=14.5 GeV². The polarization asymmetry, similar to the beam spin asymmetry in Deeply Virtual Compton Scattering (DVCS), projects out the imaginary part of the Compton Form Factors (CFF) and provides a way to test the universality of Generalized Parton Distributions (GPDs). The angular asymmetry of the decay leptons, on the other hand, accesses the real parts of the CFF which contain the D-term, a quantity directly linked to the mechanical properties of the nucleon.