Structure of the nucleon (SPhN)

The PbF2 electromagnetic calorimeter for the DVCS experiment at JLab/Hall A.

The object of these studies is to understand how nucleons are constituted from interacting quarks and gluons, and to predict their properties (charge distribution, magnetic moment, spin, resonances …). The service has been involved for many years in the study of the spin of the nucleon. Several experiments have shown that the quarks only contribute of the order of 30% of the spin of the nucleon. The other contributions, in particular that of the gluons, remain to be determined. This is the objective of the COMPASS experiment using a beam of polarised muons at CERN. SPhN is heavily involved in this experiment, both in the construction of the instrumentation localisation chambers, polarised target) and in the analysis of the data. The electromagnetic probe is the most important tool for investigating the properties of the nucleon. Although our knowledge of the nucleon remains very incomplete, important results have been obtained at JLab (United States) in the measurement of the form-factors of the neutron and proton.

Exclusive measurements, based among others on deep inelastic virtual Compton scattering (DVCS), will enable the determination of new aspects of the structure of nucleons, such as the contributions of the angular momentum of the quarks to the spin of the nucleon and correlations between quarks. These new structure functions are described in a recently developed theoretical formalism, to which SPhN has contributed, that of generalised parton distributions (GPD). Our teams are taking part in the first DVCS measurements on protons and neutrons at JLab. These studies will be continued at higher energy with polarised beams at JLab (12 GeV) and CERN.

#576 - Last update : 02/10 2005

Nuclear Physics Division

Institute of Research into the Fundamental Laws of the Universe