Speaker
Description
Generalized Parton Distributions (GPDs) describe the correlations between the longitudinal momentum and the transverse position of the partons inside the nucleon. They are nowadays the subject of an intense effort of research, in the perspective of understanding nucleon spin and mechanical properties.
In this talk, we present the first observation of the Timelike Compton Scattering (TCS) process, $\gamma p\to\gamma^* p^\prime\to e^+e^- p^\prime$, 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$ GeV$^2$ and the squared momentum transferred $0.1<-t<0.8$ GeV$^2$ at the average total center mass energy squared of $\bar{s}=14.5$ GeV$^2$. 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 (CFFs, which are complex quantities linked to the GPDs) and provides a way to test the universality of Generalized Parton Distributions. The angular asymmetry of the decay leptons, on the other hand, accesses the real part of the CFF $\mathcal{H}$ which contain the D-term, a quantity directly linked to the mechanical properties of the nucleon.
