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 2013 (597)

A team of physicists, engineers and technicians from IRFU are developing a new generation of MicroMegas trackers. The planned Compass II experiment at CERN, together with the Clas12 experiment at the Jefferson Lab, will impose new operational constraints preventing the current generation of trackers from working with nominal performance. Tests on a new generation of detectors have been carried out using particle beams generated at CERN. These tests have achieved both of their objectives; a reduction of the ... More »
   The pion, predicted by Yukawa in 1935 and discovered in 1947, was the first of a family of particles called mesons: a family that has continued to grow ever since. Ordinary mesons consist of a quark and an antiquark. The theory of strong interaction also predicts the existence of more complex mesons, called ‘exotic' mesons. The existence of exotic mesons has not yet been formally proven, but scientists have been searching for them for over more than a decade. The Compass experiment at CERN, an ... More »
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 ... More »


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