Investigating the proton structure through polarization transfers in real Compton scattering processes at JLAB

Abstract

Abstract. In the present work, preliminary results of the analysis of polarization transfers in real Compton scattering (RCS) performed in Hall-C at JLAB are presented, for data collected at large scattering angle ( cm =70 o ) in the experiment E07-002. A previous experiment, E99-114, has already demonstrated a complete inconsistence of its results with a pQCD mechanism at JLAB energy regime. This kind of experiment, therefore, is of crucial importance for understanding the nature of the reaction mechanism for such a simple process, involving a real photon. Furthermore, RCS experiments allow the extraction of Compton form factors and Generalized Parton Distributions (GPDs), the latter connected to the total angular momentum of the nucleon. Physics motivation and preliminary resultsReal Compton scattering (RCS) in the hard scattering limit is a powerful probe of the structure of the nucleon. With a real photon at wide angle regime [1], RCS provides access to the high-t transverse structure of the hadron, whereas deep virtual Compton scattering (DVCS) provides access mostly to the high-Q 2 low-t structure. For RCS on proton, the hard scale is reached when the Mandelstam variables s, -t, and -u are larger than the proton mass. Under such conditions the transition amplitude is expected to factorize as T if (s,t) = f ⊗ K(s,t) ⊗ i , where K(s,t) is the perturbative hard scattering amplitude, involving the coupling of the external photons to the active quarks, and the 's are the soft initial and final wave functions, describing the coupling of the active quarks to the proton. Two different approaches are usually taken as a reference in the interpretation of the results: the "handbag" mechanism [2,3], which involves only one active constituent ( Fig. 1 (left)), with the soft physics for the coupling of this active quark to the proton described in the Generalized Parton Distributions (GPD's) [4,5], and the "perturbative QCD" (pQCD) mechanism [6], in which all three valence quarks are active participants in the hard subprocess, the latter effected by the exchange of two hard gluons ( Fig. 1 (right)), with the soft physics contained in the valence quark distribution. In principle, both mechanisms contribute to a