1. GPDs studies at COMPASS DVCS studies for a GPD experiment in 2010 Physics impact Experimental issues Recoil detector prototype Exclusive  0 production in present COMPASS data Principle Preliminary 2002 results Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration SIR workshop 2005 May 19, 2005 – Newport News, VA

2. 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

3. 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 COMPASS muon beam can do all !

4. Requirements for DVCS: -same energy -maximum intensity -opposite polarisation to a few % -change of beam charge frequent Solution proposed by Lau Gatignon: 1)To select Pπ=110GeV and Pμ=100GeV to maximise the muon flux 2) To keep constant the collimator settings which define the π and μ momentum spreads  Pol μ+ = -0.8 and Pol μ- = +0.8 I(μ+) ~ 2.  I(μ-) switch in ~10 mins (once per shift?) Muon section 400m Hadron decay section 600m Compass target scrapers Be absorbers Protons 400 GeV T6 primary Be target Collimators 1 2 3 4 H V 1.3 10 13 protons/spill 2.10 8 muons/spill Polarized μ+ and μ- beams

5. Extraction of GPDs in the case of dσ (μp  μp  ) = dσ BH + dσ DVCS unpol + P μ dσ DVCS pol + e μ a BH Re A DVCS + e μ P μ a BH Im A DVCS  cos nφ  sin nφ φ θ μ’μ’ μ **  p P μ+ =-0.8 P μ- =+0.8 t, ξ~x Bj/2 fixed Extraction of GPDs in the case of μ+ / μ-

6. Kinematical domain Collider : H1 & ZEUS 0.0001<x<0.01 Fixed target : JLAB 6-11GeV SSA,BCA? HERMES 27 GeV SSA,BCA COMPASS could provide data on : Cross section (190 GeV) BCA (100 GeV) Wide Q 2 and x bj ranges Limitation due to luminosity E=190, 100GeV Nx2 new LINAC 4 (SPS injection) in 2010 could COMPASS benefit from this ?

7. BCA  model 1 model 2 COMPASS Model 2: from Goeke, Polyakov and Vanderhaeghen sensitivity to the different spatial distribution of partons  when x Bj  Model 1: H(x,ξ,t) ~ q(x) F(t) H(x,0,t) = q(x) e t = q(x) / x α ’ t Sensitivity of BCA to models Good sensitivity to models in COMPASS x Bj range

8. Projected errors of a possible DVCS experiment E beam = 100 GeV 6 month data taking 25 % global efficiency 6/18 (x,Q²) data samples Good constrain for models Model 1 : H(x,ξ,t) ~ q(x) F(t) Model 2 : Beam Charge Asymmetry L = 1.3 10 32 cm -2 s -1 H(x,0,t) = q(x) / x α ’t 3 bins in x Bj = 0.05, 0.1, 0.2 6 bins in Q 2 from 2 to 7 GeV 2

9.  μ’μ’ p’ μ Additionnal equipment to the COMPASS setup COMPASS equipment with additional calorimetry at large angle Recoil detector to insure exclusivity to be designed and built 2.5 m Liquid H2 target to be designed and built ECAL 1 or 2   12° all COMPASS trackers: SciFi, Si, μ Ω, Gem, DC, Straw, MWPC

10. Physical Background to DVCS detector requirements: 24° coverage for neutral 50 MeV calorimeter threshold 40° for charged particles Source : Pythia 6.1 generated DIS events Apply DVCS-like cuts one  ’, ,p in DVCS range no other charged & neutral in active volumes in this case DVCS is dominant

11. 12° 24° 4m ECAL0 Possible solution for the recoil detector proposed in the Workshop on the Future Physics at COMPASS 26 Sept2002 Received funding by EU FP6 (Bonn-Mainz-Saclay-Warsaw ) Goal : full test of a prototype of a 45 degree sector - scintillating material studies (timing at 200 ps over 4 m) - fast triggering and multi-hit ADC/TDC system Also studying a scintillating fibers solution (Monte-Carlo) with good timing provided by incoming or scattered muons

12. Vector meson production (ρ,ω,  …)  H & E Pseudo-scalar production (π,η… )  H & E ~ ~ Hρ 0 = 1/  2 ( 2/3 H u + 1/3 H d + 3/8 H g ) Hω = 1/  2 ( 2/3 H u – 1/3 H d + 1/8 H g ) H  = - 1/3 H s - 1/8 H g Single spin asymmetry ~ E/H for a transverse polarized target Cross section: Hard Exclusive Meson Production Can be investigated with present COMPASS data It comes for free with the recoil detector and existing COMPASS trackers

13. Compass Set-up 2002-2003 polarized target magnets RICH Calorimeters muon filter ~ 200 detection planes Silicon, SciFi, Micromegas, Drift chambers, GEM, Straw chambers, MWPC at CERN 250 physicists 26 institutes 160 GeV pol.  beam

14. Incoherent exclusive  0 sample selection Exclusivity of the reaction E miss =(M² X -M² N )/2M N -2.5 < E miss < 2.5 GeV Incoherent production: 0.15 < p t ²< 0.5 GeV² scattering off a quasi-free nucleon Assuming both hadrons are  0.5 < M  < 1 GeV M  E miss pt²pt² Event topology: Kinematics : > 30 GeV E  ’ > 20 GeV Q² > 0.01 GeV²  ’’   N N’ ** 00

15.  0 angular distributions Definitions : in  *-p CM frame  ’’     p p’    in  rest frame The angular distribution W(cos  depends on the  0 spin density matrix elements  23 (15) measurable with polarised (unpol.) beam This analysis : one-dimension distributions  test of s-Channel Helicity Conservation SCHC Distributions are corrected for acceptance. Background not subtracted. Statistical error limited by MC sample (except last Q² bin).

16. Measurement of r 04 00 Distribution : Spin density matrix elements: T  are helicity amplitudes meson photon

17. Measurement of r and Im r if SCHC holds: 04 1-1 3 1-1 weak violation beam polarisation R can be extracted

18. 2002 Determination of R  ° =  L /  T 2003+ as SCHC holds …: only T 00 ≠0 T 11 ≠0 Then : High statistics from  -production to hard regime better coverage at high Q² more data to come … Impact on GPD :  L is dominant at high Q² (factorisation only valid for  L )

19. Conclusions Exclusive vector meson production in COMPASS data - preliminary results from 2002 data - allow test of SCHC - shows dominance of  L at Q²>2GeV² - more data on tape with better coverage at high Q² to come - insight on GPDs : transverse asymmetry and cross section measurements Towards a GPD experiment using COMPASS… - COMPASS is complementary to other experiments - has good sensitivity to GPD models through BCA - has good Q² range for 0.03<x bj <0.2 - will be able to measure Hard Exclusive Meson Production - seeking collaborators… This initiative is now an “Express of Interest” : SPSC-EOI-005 http://doc.cern.ch//archive/electronic/cern/preprints/spsc/public/spsc-2005-007.pdf COMPASS should also be a good place to study GPD in 2010 !

20. Proton luminosity upgrade at CERN Gatignon compass

21. DVCS+ Bethe Heitler The high energy muon beam at COMPASS allows to play with the relative contributions DVCS-BH which depend on 1/y = 2 m p E ℓ x Bj /Q 2 Higher energy: DVCS>>BH  DVCS Cross section Smaller energy: DVCS~BH  Interference term will provide the DVCS amplitude φ θ μ’μ’ μ **  p μ p μ p BH calculable