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Deeply Virtual ω Production

e -’. ω. e -.  *. p. p’. Deeply Virtual ω Production. Michel Gar çon & Ludyvine Morand (SPhN-Saclay) for the CLAS collaboration. MENU 2004, Beijing. Which picture prevails at high Q 2 ?. Meson and Pomeron (or two-gluon) exchange …. (Photoproduction). ω. π , f 2 , P.

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Deeply Virtual ω Production

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  1. e-’ ω e- * p p’ Deeply Virtual ω Production Michel Garçon & Ludyvine Morand (SPhN-Saclay) for the CLAS collaboration MENU 2004, Beijing

  2. Which picture prevails at high Q2 ? Meson and Pomeron (or two-gluon) exchange … (Photoproduction) ω π, f2, P …or scattering at the quark level ? Flavor sensitivity of DVMP on the proton: ωL γ*L (SCHC)

  3. e- p e- pω π+π- π0 CLAS: high luminosity run at 5.75 GeV • First JLab experiment with GPDs in mind • (october 2001 – january 2002) • polarized electrons, E = 5.75 GeV • Q2 up to 5.5 GeV2, • Integrated luminosity: 30 fb-1 • W up to 2.8 GeV W = 2.8 2.4 2 1.8 GeV

  4. e- p e- pω π+ π- π0 - 0 n+…   0 m2 (2m)2 Identification of channel through missing mass Separation of :   + - 0(M = 783 MeV,  = 8 MeV) 0 + -(M = 770 MeV,  = 151 MeV)

  5. 28 millions events 20 days Q2 (GeV2) xB Determination of CLAS efficiency in 4D through simulation 4 kinematical variables => 4D efficiency table Q2, xB, t,  Ngen Nacc Event Generator Analysis Code CLAS GEANT simulation For each 4D bin :eff = Nacc/Ngen eff ~ 5%

  6. Background subtraction Q2 from 1.6 to 5.6 GeV2 xB from 0.16 to 0.70 + binning in  or t

  7. 0.16 < xB < 0.22 0.22 < xB < 0.28 0.28 < xB < 0.34 0.34 < xB < 0.40 *pp (b) 0.40 < xB < 0.46 0.46 < xB < 0.52 0.52 < xB < 0.58 0.58 < xB < 0.64 Q2 (GeV2) Q2 (GeV2) Q2 (GeV2) Q2(GeV2) Extraction of σγ*p->pω • Bin to bin syst. errors included • = 15-18% • Main sources : • background subtraction = 8 or 12% • efficiency calculation = 8%

  8. Comparison with previous data • Compatibility with DESY data (1977) • Disagreement with Cornell data (1981) Q2 range much extended with our data

  9. d/d(b/rd) 0.16 < xB < 0.22 0.22 < xB < 0.28 0.28 < xB< 0.34 0.34 < xB < 0.40 (deg) 0.40 < xB < 0.46 0.46 < xB < 0.52 0.52 < xB < 0.58 0.58 < xB < 0.64 TT et TL (b) Q2 (GeV2) Q2 (GeV2) Q2 (GeV2) Q2 (GeV2) Differential cross sections in  => First indication of helicitynon conservation in s-channel

  10. Differential cross sections in t Large t range, up to 2.7 GeV2. Small t : diffractive regime (d/dt  ebt) Large t : cross sections larger than anticipated

  11. THIS WORK Small |t| Large |t| * ω π0 p p’ Comparison with JML model (J.-M. Laget, to be published in Phys. Rev. D) Model includes exchange of π0, f2, P * ω p p’

  12. 0  6D efficiency table Q2, xB, t, , θN, φN eff ~ 0,5% Second part of data analysis: study of ω decay • Angular distribution of ω decay products • (study of e-pπ+π-X final state) • Determination of ω polarization • Test of s-channel helicity conservation (SCHC) • ω channel identification • Determination of CLAS efficiency in 6D • Study of ω decay

  13. Study of ω decay (2)

  14. Determination of => other indication of non conservation of helicity in the s-channel ω decay study : φN dependence

  15. <W> = 2.1 GeV <W> = 2.8 GeV Contribution of Lfrom VGG (GPDs) and JML (Regge) models VGG model: M. Vanderhaeghen, P. Guichon, M. Guidal T + L L

  16. Conclusions and perspectives • Precise measurements of the e-p e-p reaction at high Q2 • and over a wide range in t • Exclusive reactions are measurable at high Q2 • (even with the current « modest » CEBAF energy) • First significant analysis of  decay in electroproduction What do we learn ? • Cross sections larger than anticipated at high t • SCHC does not hold for this channel • Evidence for unnatural parity exchange •  0 exchange very probable even at high Q2

  17. Comparison to models • JML model (Regge) : • Good agreement when introducing a t dependence in the  form factor • suggests coupling to a « point » object at high t (hep-ph/0406153) • VGG model (GPDs) : • Direct comparison not possible since Lnot measured • Nevertheless • handbag diagram contributes only about 1/5 of measured cross sections • no incompatibility → ω most challenging/difficult channel to access GPD Perspectives • Explore physical significance of high t behaviour • Systematic study of other processes (e-p e-p0, , ) • in view of an interpretation in terms of GPDs • Measurements feasible up to Q2=8-9 GeV2with CEBAF@12 GeV

  18. 0 linear saturation  *  0 exchange dominance at W ~ few GeV in ω photoproduction parameters = coupling constants gij Extension to electroproduction case  add a parameter = electromagnetic form factor Regge theory applied to vector meson production (Jean-Marc Laget et al., see also talk of Marco Battaglieri) Regge trajectories exchange in t channel α(t) t

  19. Amplitude: Cross section: t=2 Scaling law GPD formalism for vector mesons e- = 0, ,  (see talk by Michel Guidal) M e- L t small q q- * M z L x+ In the Bjorken regime : x- FACTORIZATION H, E p p+ Collins et al. p p

  20. Elastic Q2 (GeV2) Deep inelastic Resonances W (GeV) 10 1 W=M xB = 0,8 0,1 W=1,8 xB = 0,1 ν (GeV) 0,1 1 10 100 M M 1,8 1,8 Kinematical domain explored valence quarks Elastic pQCD gluons sea quarks Inelastic Resonances Regge

  21. 10 RL OUT 6 RL IN e- - e- Electron identification Information from DC + CC + EC Electron selection Pion rejection

  22. SC DC + p Hadron identification Information from DC + SC

  23. 0  0 e- p e- pω   m2 π+π- π0 Identification of channel through missing mass

  24. Background subtraction  Event weighting : w=1/eff(Q2,xB,t,)  Fit of MX[e-pX] with : y(m) = Skewed Gaussian + Pol2 (peak + radiative tail) (background)  Subtraction of y(m).  Integration of event number between 720 and 850 MeV.

  25. 118 millions events 63 days Determination of CLAS efficiency in 6D through simulation 6 kinematical variables => 6D efficiency table Q2, xB, t, , θN, φN Ngen Nacc Event Generator Analysis Code GSIM GPP For each 6D bin :eff = Nacc/Ngen eff ~ 0,5%

  26. Phase space (1)

  27. Phase (2)

  28. Experimental Q2, xB, t, , … distributions e-p+X e-p+-X

  29. CLAS system efficiency 4D 6D

  30.  virtual photon polarization i, j   meson helicity • ω polarization via • SCHC test : (necessary condition) • If SCHC, then separation σL, σT • Test of hypothesis of natural parity exchange in the t-channel (NPE) : • (necessary condition) * ω 0-, 1+UNPE 0+,1- NPE p p’ Formalism for ω decay Decay angular distribution : elements of ωspin density matrix ε parameter of virtual photon polarization R = σL/σT (Rp :  = T + L) Deduce :

  31. Determination of Study ofω decay (2)

  32. Method of moments : 2 Determination of 15 … parameters which should vanish if SCHC holds combinaison which should vanish if NPE holds • exchange of unnatural parity particlein the t-channel ω decay study (4)

  33. If SCHC then : and  decay study (cont.) But : and

  34. Same helicity amplitudes than the ones that come into play in R extraction Theory : 0 if SCHC 1 if SCHC SCHC   Observation : So one can’t extract R.

  35. R extraction (cont.) Suppose SCHC holds, then : One has found : Then : But : So :

  36. 0 meson production and JML model

  37. <W> = 2.1 GeV <W> = 2.8 GeV Comparison with JML model (cont.) • Within this model, • Data still favor a t-dependent πγωform factor. • π0 exchange dominant for the whole Q2 range. • σT dominant for the whole Q2 range.

  38. Exclusive elastic reactions e- e- e- ou M Deep exclusive reactions e- Q2 = -t Form Factors => transverseposition x, t, ξ p p p p e- Deep inclusive reactions e- e- • Generalized • Parton Distributions • correlations ! • quark angular momentum x = xB e- X * p • Parton Distributions • longitudinal momentum fraction • quarks contribution to nucleon spin Motivations High Q2 reactions give access to internal dynamics of the nucleon

  39. GPD formalism (cont.) Link with FF and PD •  0  access to correlations , x,  dependence  quark longitudinal impulsion t dependence  quark transverse impulsion Ji sum rule :

  40. VGG (GPDs) model x,t, dependence parametrization: 2 parameters : bvalenceet bsea In fact Q2non infinite Corrections to leading order: In pratice : non perturbative effects at small Q2 averaged by « frozing » the strong coupling constantS to 0.56

  41. Contribution of L from VGG (GPDs) model (cont.)

  42. xB=0.31 xB=0.38 s (g*p  pr) (mb) Q2 (GeV2) Q2 (GeV2) xB=0.45 xB=0.52 Q2 (GeV2) Q2 (GeV2) 0 meson results at 4.2 GeV (Regge) C. Hadjidakis et al.

  43. xB=0.38 xB=0.31 s (g*Lp  prL) (mb) Q2 (GeV2) Q2 (GeV2) xB=0.45 xB=0.52 Q2 (GeV2) Q2 (GeV2) 0 meson results at 4.2 GeV (GPDs) C. Hadjidakis et al.

  44. G parity definition

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