Study of Generalized Parton Distributions using the high energy muon beam at COMPASS
Nicole d'Hose (Saclay)
Nucleons are ultimately made of partons, i.e. quarks and gluons. The parton density and longitudinal momentum distributions along the nucleons direction of motion are described by Parton Distribution Functions (PDFs) and have been extensively studied in Deeply Inelastic Scattering. Up to now, PDFs were investigated independently from nucleon form factors which describe the position-space densities. The recently developed theoretical framework of Generalized Parton Distributions (GPDs) embodies both form factors and PDFs and in addition describes correlation between these observables. GPDs provide information on the transverse localisation of a parton at a given fraction it carries of the nucleon’s longitudinal momentum. This description is sometimes referred to as 3-dimensional “nucleon tomography”. GPDS attracted yet more attention after it was shown that they can reveal the orbital angular momentum in the decomposition of the total proton spin, that brings a new insight on the more than 20 years old nucleon spin puzzle. Constrains on GPDs will be provided by a global analysis of exclusive processes such as Deeply Virtual Compton Scattering (DVCS) or Meson Production (DVMP) through a mapping in xBj , Q2 and t the invariant momentum transfer to the target, in a broadest possible kinematic range. A GPD program is part of the Medium and Long Term Plans at COMPASS.The COMPASS apparatus is located in the unique high energy (100-200 GeV) polarised μ± beam of the CERN SPS and uses a high-resolution forward spectrometer in conjunction with a fixed target. By installing a recoil detector around the target to ensure exclusivity of DVCS and DVMP processes, it could be converted into a facility measuring these reactions within a kinematic domain of xBj between 10−2 and 10−1, that cannot be explored by any other existing facility.In the full-scale programme, two years running periods are considered, with the first one using an unpolarised proton target. We propose to measure the t-slope of the exclusive cross sections which is known to reflect the size of the partonic object on which the process takes place. The dependence of the t-slope can be measured as a function of xBj to observe possible shrinkage of the nucleon size for increase values of xBj . Moreover DVCS is identified as the king way for accessing GPDs. It interferes with the Bethe-Heitler (BH) process, the pure electromagnetic effect of radiation of photon by the muons. With polarised muon beam of both charges, COMPASS is the only place to get the real and imaginary parts of the DVCS process amplitude in the interference term, an essential step for GPD determination. In a second running period we consider to use a transversely polarised proton target, in order to collect data that constrain mostly the GPD related to the orbital angular momentum. In this presentation, the proposed experiment will be described as well as the methodology, the new devices to build and the first observation of exclusive single-photon production, a promising but also challenging channel to detect.