Hadron structure

SPhN is strongly engaged in the study of the partonic structure - in terms of quarks and gluons - of nucleons, protons and neutrons. Nucleons are singular objects as 90% of their mass originates not from their constituent masses, but rather the strong interaction they experience. Nucleons are also a benchmark laboratory to study confinement mechanisms which remain an open key point, although QCD is a well established theroy in the standard model. The study of the nucleon structure in terms of quarks and gluons is thus complementary to the investigations of quark-gluon plasma and its associated phase transition. QCD lattice calculations pave the way to make a link between these approaches, and SPhN is a contributor in this global effort.

 

 

Gluon polarization as a function of x, the momentum fraction of the nucleon carried by the gluon

One of the approach to the problematic of the nucleon structure has been to study global observables, like the decomposition of the nucleon spin in term of its constituents. SPhN was amongst the pionneers in these measurements held at SLAC, CERN and JLAB. These lead to the so called "spin crisis", showing that the three constituent quarks only carry about a third of the nucleon spin, and solving this puzzle is a key priority of this community now. SPhN hadronic physics laboratory is a co-leader of the COMPASS collaboration setup in 1997 with the goal to measure the different contributions to the nucleon spin. The contribution of the gluons and the strange sea quarks could be measured with unprecedent accuracy. The rather small contributions measured so far leave the spin puzzle an open question.

 

DVCS polarized cross sections measured at JLAB

A more modern approach to the study of the nucleon structure consists in measuring Generalized Parton Distributions (GPD). They represent correlated distributions of position and momenta of the nucleon constituants. The first experiments dedicated to the study of these GPDs have been setup at Jefferson Lab (2004-2010), and consist in measuring Deep Virtual Compton Scattering (DVCS). The valence quark domain has been explored initially with the CLAS spectrometer at JLAB with a 6 GeV beam. The upgrade to 12 GeV in 2016 will allow a more systematic study of the GPD in this kinematic region. COMPASS-II GPD program will allow to extend these measurements to the sea quarks and gluons at horizon 2016, what will allow to get a full coverage of the parton position-momentum correlations inside the nucleon. SPhN is co-spokesperson of both of these experiments.

 

First realistic 3D images of the nucleon obtained from a model adjusted to DVCS world data

In addition, a theoretical framework has been setup to perform a global analysis of all experimental data related to GPDs. This framework consists of experimental data data bases, fast algorithms to compute al observables, and graphical display tools. This should allow ultimately to obtain a tomographic 3D view of the nucleon structure.

 

last update : 12-09 12:24:21-2013 (597)

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Structure of the nucleon (SPhN)
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Experiments

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