[Sbs] Abstract DDVCS

[Alexandre Camsonne]

Hello,

here is the abstract for Double DVCS. I might remove it if I figure out the
acceptance is not large enough to make an interesting measurement.

Alexandre Camsonne for the Hall A SuperBigBite Collaboration

Double Deeply Virtual Compton Scattering in Hall A using SuperBigBite
Spectrometer

The Generalized Parton Distributions (GPDs) are a more general formalism
englobing the concept of elastic form factor (FF) and parton distributions
(PDFs) introducing a third independent variable called skewdness xi in
addition to usual x_bj and t variables which are define for the PDFs. Those
distributions thus contain more informations than the FF and PDFs allowing
to give a more spatial and dynamical description of the nucleon. To measure
GPDs one has to measure exclusive reaction. The simpliest exclusive
reaction is the exclusive production of photons or Deeply Virtual Compton
Scattering. One drawback of this measurement is that the real photon
constrains the measurement of the GPDs to the line xi=x. The Doubly Virtual
Compton Scattering (DDVCS) is the similar process involving a virtual
photon in the final state. This virtual photon decays into a dilepton pair
allowing some values of xi different than x_bj. High luminosity is required
since cross sections are smaller by a factor 100. I will present an idea
using the SuperBigBite Spectrometer to try to measure the DDVCS
particularly in the dimuon channel.

Alexandre
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