Speaker
Francois-Xavier Girod
(JLab)
Description
The exclusive electroproduction of a hard photon off a nucleon $eN\rightarrow eN\gamma$
provides three-dimensional information on the nucleon structure.
This reaction proceeds via the Bethe-Heitler (BH) process (with the photon emitted by the electron),
and the Deeply Virtual Compton Scattering (DVCS) process (with the photon emitted by the proton).
BH and DVCS are indistinguishable, and interfere at the amplitude level.
In the Bjorken regime of large $Q^2$ at fixed $x_B$, and for $-t/Q^2<1$, the DVCS amplitude factorizes with the non-perturbative part described in terms of
Generalized Parton Distributions (GPDs).
The imaginary part of the amplitude parameterized by GPDs provide three-dimensional imaging through the spatial distributions of partons in the transverse plane at fixed longitudinal momentum fraction $x_B$.
The real part of the amplitude is sensitive to the Form Factors parameterizing the Energy Momentum Tensors of partons in the nucleon, in particular the elusive D-term.
Existing strategies to access the D-term include dispersion relations between the imaginary parts of the amplitude accessed through spin asymetries,
and the real parts of the amplitude accessed through cross-sections and double spin asymetries.
These dispersion relation strategies are difficult to implement, and include several forms of systematical uncertainties.
The availability of a positron beam gives a more direct access to the real part of the amplitude through beam charge asymetry measurements
and would be an invaluable tool to fully realize the potential of the exclusive reaction program at Jefferson Lab.
Primary author
Francois-Xavier Girod
(JLab)
