Physics - Nucleon Structure
Generalized Parton Distributions
The theoretical framework of Generalized Parton Distributions (GPDs) has recently been developed and caused excitement in the field of understanding the structure of the nucleon. It has recently been shown that exclusive pp annihilation into two photons at large s and t can be described in terms of GPDs. Estimates of the expected count rates based on a simple model predict a few thousand γγ events per month for a luminosity of 2 · 1032 cm-2s-1 at an energy of √s = 3.2GeV/c2.Other estimates, based on cross section measurements of the inverse process γγ → pp, predict count rates up to a factor 50 above the later estimate of but still are consistent with the handbag ansatz.
It is proposed to measure the crossed-channel Compton scattering and the related exclusive annihilation processes with a scalar meson, a vector meson, or a lepton pair in the final state. The comparison of the differential cross sections for the various processes and the comparison with GPD based models will allow new insights into the annihilation process in terms of quark models and QCD.
Time-like Form Factor of the Proton
The electromagnetic form factor of the proton in the time-like region can be extracted from the cross sections for the process pp → e+ e-. The proton time-like form factors have been measured by several experiments in the low Q2 region down to threshold. At high Q2 the only measurements have been achieved by E760 and E835 at Fermilab up to Q2 of 15GeV2/c2. However, due to limited statistics |GM| and |GE| have not been measured separately and could only be extracted using the assumption |GE| = |GM|. In PANDA, it will be possible to determine the form factors over the widest Q2 range with a single experiment, from threshold to 20 GeV2/c4 and above, depending on the beam time availability. Due to much improved statistics and angular coverage it will be possible to significantly improve the results of E760 and E835 and to measure |GM| and |GE| separately.