Prospects for Future measurement of GPDs using COMPASS at CERN E. Burtin, CEA/Saclay - Dapnia/SPhN for the COMPASS Collaboration Expression of Interest SPSC-EOI-2005 Physics motivations & context Experimental realisation INPC 2007 Tokyo, June 5, 2007

Generalized Parton Distributions GPDs x+ x- P P’ hard soft g* g,p,r t Factorisation: Q2 large, -t<1 GeV2 contains pdf H(x,0,0) = q(x) measured in DIS Generalized Parton Distributions for quarks : 4 functions H,E,H,E(x,,t) ~~ contains form factors F(t) measured in elastic scattering contains information on the nucleon spin : Ji’s sum rule : and 1/2 = 1/2  + Lz + G = Jz + G

Towards a 3-D nucleon picture (Px,ry,z) Lattice calculation (unquenched QCD): Negele et al., NP B128 (2004) 170 Göckeler et al., NP B140 (2005) 399  small valence quark core  widely spread sea q and gluons Hard Exclusive Scattering Deeply Virtual Compton Scattering ep ep GPDs * Q² x+ x- p  t mp=0.87 GeV p z x P r y x boost measure H(x,0,t) as function of t Chiral dynamics: Strikman et al., PRD69 (2004) 054012 gluon density generated by the  cloud increase of the N transverse size for xBj < mπ/mp=0.14 GPDs : H( x,,t ) Fourier access to correlations : ( Px, ry,z ) Burkardt,Belitsky,Müller,Ralston,Pire

DVCS observables Deep VCS Bethe-Heitler High energy beam Lower energy => use interference - holography Cross section Single Spin Difference Polarised beam Beam Charge Difference +/- charged beam

What makes Compass a special place ? CERN High energy muon beam 100 / 190 GeV 80% Polarisation +q and -q available (change once/day) Same intensity & scrapper settings Opposite polarization Kinematical domain : Hera Ix2 E=190, 100GeV Targets liquid Hydrogen & Deuterium Polarised target (no recoil) Calendar Now : COMPASS with polarised target 2010-2015 : Extensive program with Recoil detection & Calo 2014+ : Jlab 12 and GSI/FAIR startup Valence + sea + gluons

Projected errors of a possible DVCS experiment Beam Charge Asymmetry Ebeam = 100 GeV 180 days, 25 % efficiency L = 1.3 1032 cm-2 s-1 6/18 (x,Q²) data samples Double Distribution : VGG PRD60(1999) and Prog.Part. NP47(2001) Factorized t-dependence H(x,ξ,t) ~ q(x) F(t) Reggeized t-dependence <b2>=’ln1/x (’=0.8,1.1) H(x,0,t) = q(x) et<b2>= q(x)/xα’t Dual parametrization Mellin moments, QCD evolution V. Guzey PRD74(2006)054027 Good constrains for models

Contribution of Ju and Jd to the nucleon spin Through the modeling of GPD E in Double Distribution formalism 1-Transversaly polarised target and unpolarized beam In Meson production : Under study with COMPASS 2002-03-04-06 6LiD deuteron Data and with 2007 NH3 proton Data In DVCS : But… no recoil detection around the polarized target 2-Neutron target - liquid deuterium for the complete program after 2010

Experimental realisation all COMPASS trackers: SciFi, Si, μΩ, Gem, DC, Straw, MWPC 2.5 m Liquid H2 & D target to be designed and built μ’ ECAL 1 or 2   10°  COMPASS equipment with additional calorimetry at large angle (p0 bkg) μ p’ Recoil detector to insure exclusivity to be designed and built TOF~300ps

Recoil Detector Prototype of scint. counters: B: 4m x 5cm A: 2.8m x 0.4cm TOF = 315 ps achieved (MIP) Using 1GHz sampler and waveform Analysis (mandatory for the expt)

GEANT Simulation of PMT signals with beam Red is DVCS proton Blue is background upstream PMT downstream 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 INNER OUTER Proton efficiency Effective signal Beam intensity (muons/5s) recording the waveform of all signals and segmentation are mandatory

Calorimeter acceptance xbj Q2 Existing Calorimeters Xbj-bins + 3m x 3m ECAL0

Physical Background to DVCS Source : Pythia 6.1 generated DIS events Apply DVCS-like cuts ’,,p in DVCS range no other charged & neutral in active volumes detector requirements: 24° coverage for neutral 40° for charged particles in this case DVCS is dominant

Proton signal with the prototype deuterons protons Measured β E in B (MeV) Target : 10 cm CH Nominal beam intensity 8 hours of data

Conclusion & prospects Possible physics ouput Sensitivity to spatial distribution of partons Sensitivity to total spin of partons : Ju & Jd Working on a variety of models (VGG, Müller, Guzey and FFS-Sch) to quantify the Physics potential of DVCS at COMPASS Experimental realisation DVCS proton is accompanied by a high background Recoil Detection is feasible (waveforms DAQ) Extension of the calorimetry is desirable Roadmap “exclusive” meson production on transverse target results soon… Building a 1m long Recoil Detector for the COMPASS hadron program Submission of a proposal as soon as possible Get ready to take data with recoil detection and extended calorimetry from 2010 to 2015 before JLab12 & GSI/Fair