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Exclusive electroproduction of the r + on the proton at CLAS

Exclusive electroproduction of the r + on the proton at CLAS. Outline: Physics motivations: GPDs CLAS experiment: e1-dvcs Data analysis: r + cross section. Ahmed FRADI, IPN Orsay. Bosen Workshop 2007. n. Large Q 2 , small t. Mesons : s L. Meson. g*. t. -2x. F( z). x+ x.

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Exclusive electroproduction of the r + on the proton at CLAS

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

  2. Large Q2, small t Mesons : sL Meson g* t -2x F( z) x+x x-x ~ ~ H,E,H,E(x,x,t) n(=p+D) Physics motivations: GPDs (Generalized Parton Distributions)(Ji, Radyushkin, Collins, Strikman, Frankfurt,…) For the electroproduction of mesons, the reactionamplitude can be factorized in 3 parts: • 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 amplitudef(z). the dominant process is the handbag diagram  • -1<x<1 • 0<x<1 t=D2 ~ ~ p

  3. “Ordinary” parton distributions Ji’s sum rule 2Jq =  x(H+E)(x,ξ,0)dx x (nucleon spin) H(x,0,0) = q(x), H(x,0,0) = Δq(x) ~ GPDs are not completely unknown t γ, π, ρ, ω… -2ξ Exclusive scattering x+ξ x-ξ ~ ~ H, H, E, E (x,ξ,t) Elastic form factors x • H(x,ξ,t)dx = F1(t)(Dirac FF)( ξ) • E(x,ξ,t)dx = F2(t) (Pauli FF)( ξ) Deep inelastic scattering Elastic scattering X.Ji,Phys.Rev.Lett.78,610(1997); Phys.Rev.D55,7114(1997)

  4. Separate flavors Hq,Eq : r0 g* r+ g* D-term NO D-term p p p n Physics motivations : interest of r+ CLAS analysis S. Morrow/M. Guidal (almost finalized) Vector meson H,E r+ (e p e p r0) r0 : 2/3Hu+1/3Hd r+ : Hu - Hd D-term The GPDs H(x,x,t) and E(x,x,t) must satisfy a polynomiality rule: the nthx moment of GPDs must be a polynomial in x 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 introduced1 to take into account this missing power x n+1 . (1) M.Polyakov and C.Weiss,Phys.Rev. D60,114017(1999) D-term: must bean odd function of x. D-term can be interpreted as an isoscalar scalar(0+) meson contribution

  5. Prediction for the cross section:GPDs (VGG) s r+≈ sr0 /5 M.Vanderhaeghen,P.A.M. Guichon and M.Guidal,Phys.rev.D 60 094017 (1999)

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

  7. e1 - dvcs (February-June 2005) e pe nr+e’ np+p0 n e p+ g g Detected in CLAS Missing mass Beam energy = 5.75 GeV .1< xB < .8 Q2 up to5 GeV2 Integrated Luminosity ≈40fb-1

  8. Cross section s(g*p  n p+ p0) Np+p0(Q2, xB) 1 s(Q2, xB) = g* p  n p+ p0 GV(Q2, xB) Acc(Q2, xB) Lint DQ2DxB Acc(Q2, xB): Acceptance of the CLAS detector. GV(Q2, xB) : the virtual photon flux . Lint : integrated luminosity ≈ 40fb-1. DQ2DxB :bin width .

  9. Channel selection e pe nr+e’ np+p0 nep+ g g N p+p0 gginvariant mass Neutron missing mass

  10. N p+p0 Counts/20 MeV 100% e1-dvcs statistics p+p0 invariant mass =√( p p+ + p p0)2

  11. Np+p0(Q2, xB) 1 s(Q2, xB) = g* p  n p+ p0 GV(Q2, xB) Acc(Q2, xB) Lint DQ2DxB Cross section s(g*p  n p+ p0)

  12. MC Acceptance calculation in 7D Acc 120 million events generated with a realistic generator and simulated with a GEANT CLAS simulator. Acc(Q2,xB,t,…) = rec(Q2,xB,t,…) / gen(Q2,xB,t,…)

  13. Acc Q2 xb -t W p+p0 invariant mass F  p+ Cosp+ Kinematicalvariables( Data+ Simulation: r+ + phase space)

  14. Cross section s(g*p  n p+ p0) Np+p0(Q2, xB) 1 s(Q2, xB) = g* p  n p+ p0 GV(Q2, xB) Acc(Q2, xB) Lint DQ2DxB

  15. Reduced cross section :g*pn p+p0 Arbitrary units Statistical errors only Preliminary

  16. Cross section s(g*p  n p+ p0) Background subtraction Cross section s(g*p  n r+)

  17. Background subtraction: for each (Q2,xB) bin s g* p →n r+ (Q2,xB) Fit :5 parameters Skewed BreitWigner : 4 parameters r+ normalisation r+ mass r+ width r+ skew parameter Phase space:simulation 1 parameter (background) ds /d IM [p+p0 ] total fit result IM [p+p0 ] (GeV)

  18. Fit to p+p0 invariant mass for each (Q2,xB,t) bin ds/dt(Q2,xB,t) Good fits unexpected peaks !

  19. To-do list • Improve the fits to the p+p0 invariant mass. • Extract sg* p →n r+ and ds / dt • Separate the longitudinal( s L) from the tranverse(sT) cross section by fitting the cosHS+ and relying on SCHC(« s-channel helicity conservation » ). • Comparison with theory: GPDs(VGG),Regge approach(JML),…

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