1. 1 Exclusive electroproduction of the    on the proton at CLAS  Outline: Physics motivations: GPDs CLAS experiment: e1-dvcs Data analysis:   cross section n Ahmed FRADI, IPN Orsay Bosen Workshop 2007

2. 2  A ’’hard’’ part exactly calculable in pQCD, which describes the interaction between the virtual photon and a quark of the nucleon and the exchange of a gluon.  A “soft’’ part which represents the non-perturbative structure of the nucleon and describes this structure in terms of 4 GPDs.  A second ’’soft’’ part which describes the structure of the meson with the distribution amplitude  z). For the electroproduction of mesons, the reaction amplitude can be factorized in 3 parts: Large Q 2, small t Mesons :  L -1<x<1  t=   the dominant process is the handbag diagram Physics motivations: GPDs (Generalized Parton Distributions) (Ji, Radyushkin, Collins, Strikman, Frankfurt,…) ~~ p n(=p+  ) H,E,H,E(x, ,t) x-  t  x+    z) Meson ~~

3. 3 H, H, E, E (x,ξ,t) ~~ “Ordinary” parton distributions H(x,0,0) = q(x), H(x,0,0) = Δq(x) ~ x Elastic form factors  H(x,ξ,t)dx = F 1 (t) (Dirac FF) (  ξ) x Ji’s sum rule 2J q =  x(H+E)(x,ξ,0)dx (nucleon spin) x+ξx-ξ t γ, π, ρ, ω… GPDs are not completely unknown -2ξ  E(x,ξ,t)dx = F 2 (t) (Pauli FF) (  ξ) X.Ji,Phys.Rev.Lett.78,610(1997); Phys.Rev.D55,7114(1997) Elastic scattering Exclusive scattering Deep inelastic scattering

4. 4 Physics motivations : interest of  + Separate flavors H q,E q :  Vector meson H,E    H u +1/3H d    H u - H d CLAS analysis S. Morrow/M. Guidal (almost finalized) The GPDs H(x, ,t) and E(x, ,t) must satisfy a polynomiality rule: the n th x moment of GPDs must be a polynomial in  of order n+1.In GPDs models based on Double Distributions,for n odd the n+1 order is missing. The so-called D-term has been introduced 1 to take into account this missing power  n+1. D-term: must be an odd function of x. D-term can be interpreted as an isoscalar scalar(0 + ) meson contribution (e p  e p  0 ) D-term ** 00 p p ++ p n ** NO D-term (1) M.Polyakov and C.Weiss,Phys.Rev. D60,114017(1999)

5. 5 Prediction for the cross section:GPDs (VGG) M.Vanderhaeghen,P.A.M. Guichon and M.Guidal,Phys.rev.D 60 094017 (1999)    ≈   0 /5

6. 6 Electromagnetic Calorimeter Electron ID, detection of neutral particles Time-of-Flight Counters Measure speed → mass (particle identification) Gas Cherenkov Counters Separation e/  Hydrogen target Drift Chambers: to determine the trajectories and momenta of charged particles Torus Coil : to bend the trajectory of charged particles beam Hall B / JLab (VA,USA) CLAS : CEBAF Large Acceptance Spectrometer Inner Calorimeter: detection of photons in the forward direction

7. 7 e1 - dvcs Beam energy = 5.75 GeV.1 < xB <.8 Q 2 up to 5 GeV 2 Integrated Luminosity ≈ 40fb -1 e p  e n    e’ n  +  0  n e    (February-June 2005) Detected in CLAS Missing mass

8. 8 Cross section  (  * p  n  +  0 )  V (Q 2, x B ) : the virtual photon flux. L int : integrated luminosity ≈ 40fb -1. 1 L int  Q 2  x B N   +  0 (Q 2, x B )  (Q 2, x B )   p  n       V (Q 2, x B ) Acc(Q 2, x B ) Acc(Q 2, x B ): Acceptance of the CLAS detector.  Q 2  x B :bin width.

9. 9 Channel selection e p  e n    e’ n  +  0  n e    Neutron missing mass   invariant mass N  +  0

10. 10 Counts/20 MeV 100% e1-dvcs statistics  +  0 invariant mass =√( p   p    N  +  0

11. 11 1 L int  Q 2  x B N   +  0 (Q 2, x B )  (Q 2, x B )   p  n       V (Q 2, x B ) Acc(Q 2, x B ) Cross section  (  * p  n  +  0 )

12. 12 MC Acceptance calculation in 7D 120 million events generated with a realistic generator and simulated with a GEANT CLAS simulator. Acc Acc(Q 2,x B,t,…) = rec(Q 2,x B,t,…) / gen(Q 2,x B,t,…)

13. 13 Q2Q2 xb -tW  +  0 invariant mass  Cos  +    Kinematical variables( Data+ Simulation:    phase space) Acc

14. 14 Cross section  (  * p  n  +   ) 1 L int  Q 2  x B N   +  0 (Q 2, x B )  (Q 2, x B )   p  n       V (Q 2, x B ) Acc(Q 2, x B )

15. 15 Reduced cross section :  p  n    Preliminary Arbitrary units Statistical errors only

16. 16 Cross section  (  * p  n  +   ) Cross section  (  * p  n  + ) Background subtraction

17. 17 Background subtraction: for each (Q 2,x B ) bin Fit :5 parameters Skewed BreitWigner : 4 parameters  + normalisation  + mass  + width  + skew parameter Phase space:simulation 1 parameter (background) IM [  +  0 ] (GeV) total fit result   p → n  (Q 2,x B ) d  /d IM [  +  0 ]

18. 18 Fit to      invariant mass for each (Q 2,x B,t) bin Good fits unexpected peaks ! d  dt(Q 2,x B,t)

19. 19 To-do list  Improve the fits to the  +  0 invariant mass.  Extract   * p →n  + and d  / dt  Separate the longitudinal(   L ) from the tranverse(  T ) cross section by fitting the cos  HS  + and relying on SCHC(« s-channel helicity conservation » ).  Comparison with theory: GPDs(VGG),Regge approach(JML),…

20. 20 e-e- 10 LR 6 LR OUT IN Pion rejection from EC Particle identification: electron 25 0.06 e- e-  -

21. 21 Particle identification:  + Information from DC + SC DC  SC      e+e+ ++ p

22. 22 To define completely the reaction e p  e n  +  e’ n    0 7 independent variables are needed: Q 2 = -q 2 = -(e-e’) 2, x B = Q 2 /2pq, t=(p-p’) 2,    invariant mass,  (angle between leptonic and hadronic planes),   decay angles in    CM  One can also define W 2 =(p+q) 2 (CM energy squared of the virtual photon-proton system). Simulations and CLAS acceptance studies

23. 23 I calculate the acceptance for each of the elementary 7-dim bins. Acc(Q 2,x B,t,…) = rec(Q 2,x B,t,…) / gen(Q 2,x B,t,…) Event generator:GENEV GSIMReconstruction In the limit of the bin size being small enough the acceptance calculation is entirely model independent MC Acceptance calculation in 7D

24. 24 Correction Factors : F corr = F rad.... F rad : radiative correction: ≈ 20 % Cross section  (  * p  n  +  0 )  V (Q 2, x B ) : the virtual photon flux L int : integrated luminosity 1 L int  Q 2  x B N   +  0 (Q 2, x B )  (Q 2, x B )   p  n       V (Q 2, x B ) Acc(Q2, xB) F corr