Feasibility studies for DVCS and first results on exclusive  at COMPASS DVCS studies Physics impact Experimental issues Recoil detector prototype Exclusive  0 production Event Selection Mass and angular distributions Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration 4th Circum-Pan-Pacific Symposium On High Energy Spin Physics August 4, Seattle, WA

Generalized Parton Distributions H(x,0,0) = q(x) measured in DIS F(t) measured in elastic scattering GPDs x+  x-  P P’ hard soft ** ,  Factorisation: Q 2 large, -t<1 GeV 2 Generalized Parton Distributions for quarks : 4 functions H(x, ,t) t

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

Kinematical domain Collider : H1 & ZEUS <x<0.01 Fixed target : JLAB 6-11GeV SSA HERMES 27GeV SSA,BCA COMPASS could provide data on : Cross section (190 GeV) BCA (100 GeV) Wide Q 2 and x bj ranges

Projected errors of a possible DVCS experiment E beam = 100 GeV 6 month data taking 25 % global efficiency 6/18 (x,Q²) data samples Very promising Good constrain for models Model 1 : simplest ansatz Model 2 : from Goeke et al. Beam Charge Asymmetry

Experimental realisation p  ’’  100 GeV/c 2.5 m LH 2 p ~ 80 GeV/c  ~ 1 deg p > 3GeV/c  < 10 deg p : 250 => 750 MeV/c  : 40 => 70 deg COMPASS equipment with additional calorimetry New recoil detector to be designed and built

Compass Set-up polarized target magnets RICH Calorimeters muon filter ~ 200 detection planes Silicon, SciFi, Micromegas, Drift chambers, GEM, Straw chambers, MWPC

Recoil detector Requirements : - Identify and measure protons - Identify neutrals  =>n  +,  0 =>  - Be large and hermetic + COMPASS Set-up

Physical Background DVCS model Mossé, Guichon, Vanderhaegen Source : Pythia 6.1 generated DIS events Apply DVCS-like cuts one  ’, ,p in DVCS range no other charged & neutral in active volumes DVCS is dominant

Proof of Principle  ’’ P Tests performed at COMPASS (summer 2001) Particle Identification Short Plastic scintillators Energy Loss vs TOF Target: 60cm of CH 2 ~ 3x(2.5m of Liquid H 2 ) Nominal beam conditions

Recoil Detector Prototype Goal : full test of feasibility of a 45 degree sector - scintillating material studies (timing at 200 ps over 4 m) - fast triggering and multi-hit ADC/TDC system Candidate to funding by European Community Bonn-Mainz-Saclay-Warsaw are participating Milestones : 2004 – material research, test bench and in-beam studies 2004/5 – development of fast electronics 2005 – construction >2005 – integration and operation downstream of COMPASS

Exclusive  0 production in COMPASS data 2002 Data : E = 160 GeV - 2 x 60 cm polarized 6 LiD target - semi-inclusive trigger - 1/6 of the data analysed Event selection: - ,  ’ - 2 hadrons of opposite charge - no other particles - >30 GeV, E  ’ >20 GeV - -2 <  E < 2.5 GeV - |t| < 0.5 GeV   all Q²   with Q²>1 GeV² Q² Number of events (1/6 of 2002 data): Q²

 0 mass distribution Söding parametrization: No acceptance corrections ! (subtracted)

 0 angular distributions is the fraction of longitudinal (helicity 0)  ’s Definitions : in  *-p CM frame  ’’     p p’    Assuming s-Channel Helicity Conservation in  rest frame with

 0 angular distributions 1/6 of 2002 data No acceptance corrections applied qualitative agreement with expected physics

Conclusions & Perspectives Exciting physics case Theory & Experimental communities very active COMPASS could provide important data DVCS cross section Beam Charge Asymmetry Exclusive meson production High statistics Studies of SCHC Double spin asymmetries Hard Exclusive Meson Production Proposal for DVCS at COMPASS in preparation

Proton detection efficiency Goal : evaluate inefficiency due to  interaction in the target Inefficiency = 30 % occupancy from Möller electrons GEANT Simulation: - real attenuation length - segmentation (24 sectors) - vary target shielding thickness

Existing Data CLAS at JLABHERMES H1 PRL 87 (2001) PRL 87 (2001) Phys.Lett.B517 (2001) 47