CLAS/DVCS

Schematic diagram of DVCS. A virtual photon interacts with a quark within the nucleon, and that quark re-emits a real photon (DVCS) or a meson. The GPDs depend on the kinelatical variables given here.

Objectives

Study of deeply virtual Compton scattering (DVCS) at Jefferson Lab, at 6 GeV, as part of the general program to determine generalized parton distributions (GPD) in the nucleon.

Recent theoretical advances resulted in the definition of these new distributions, which yield a much more complete information about the way quarks and gluons are distributed within nucleons (and within all hadrons): 3D holography, quark orbital angular momentum... New theorems, called factorization theorems, link these GPDs to the electroproduction of photons (DVCS) and of mesons. This first generation experiment aims at determining the range of validity of these theorems and at bringing the first constraints on the shape of these distributions.

Topic/program:

Ultimate constituants of matter/Structure of hadrons

Construction of a superconducting solenoidal magnet and of an electromagnetic calorimeter, which are addition s to the standard CLAS setup (Cebaf Large Acceptance Spectrometer dans le Hall B au JLab) in order to allow the detection at very forward angles with respect to the beam direction.

Context

First results obtained in 2001 through non-dedicated experiments by the CLAS collaboration at JLab, as well as by HERMES and H1 at DESY, were strong incentives in launching dedicated DVCS programs.

The first two dedicated experiments were performed at JLab (Hall A in 2004 and Hall B/CLAS in 2005, object of the present description). HERMES followed in 2006-2007 and further data will be taken at JLab in 2008-2010. Other programs are being designed at CERN/COMPASS (as of 2010 with Saclay participation) and at JLab at 12 GeV (as of 2013 with Saclay participation).

Localization

Experiment performed in Hall B of Thomas Jefferson Laboratory (JLab) - Newport News - Virginia - USA (2005). Second data taking period in 2008.

View of equipment delivered at JLab. Calorimeter in blue and magnet in yellow.

Dapnia contributions

The contributions

Design and construction of a superconducting solenoid, operating at 4.5 Teslas and with large aperture for recoil proton detection; includes an original cryogenic system and the associated instrumentation (2002-2004).

Design and construction of an electromagnetic calorimeter in collaboration with ITEP/Moscow, JLab and Orsay (2003-2004): Dapnia was responsible for the laser-light monitoring system; gave expert advice on the mechanical design and on the temperature stabilization; our physicists initiated the simulation and analysis methods of the electromagnetic showers. This lead-tungstate calorimeter, read-out with avalanche photodiodes, benefitted from R&D for LHC (CMS and ALICE), but is the first of its type to operate in a real physics experiment.

Responsabilities

Co-spokesperon of the experiment(2001-2008) and chairman of the CLAS collaboration (2005-2007): Michel Garçon.

Run coordination : Michel Garçon and Franck Sabatié.

First thesis (2006) and publication (2007): François-Xavier Girod (CFR).

Size of experiment

This has been a mid-size project for Dapnia.

Brief summary of scientific impact

We include here results obtained by the two dedicated experiments at JLab:

- Hall A, DVCS on proton: provides first experimental evidence for scaling in DVCS (i.e. Compton scattering is really occuring at the quark level); PhD thesis of a Dapnia student; published in Phys. Rev. Lett.

- Hall A, DVCS on neutron; provides (model-dependent) constraints on the quark angular momentum in the nucleon; accepted for publication in Phys. Rev. Lett.

- Hall B/CLAS, DVCS on proton; provides the first large kinematical scan of DVCS observables, which will be later translated into the kinematical dependences of GPDs; PhD thesis of a Dapnia student; to besubmitted to Phys. Rev. Lett. before the end of 2007.