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Deeply Virtual Compton Scattering in JLAB Hall A

Deeply Virtual Compton Scattering in JLAB Hall A. Malek MAZOUZ. For JLab Hall A & DVCS collaborations. Hall A. 5 th ICPHP. LPSC Grenoble : mazzouz@lpsc.in2p3.fr. May 22 nd 2006. Generalized parton distributions: GPDs. Parton distribution via Deep inelastic scatering.

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Deeply Virtual Compton Scattering in JLAB Hall A

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  1. Deeply Virtual Compton Scattering in JLAB Hall A Malek MAZOUZ For JLab Hall A & DVCS collaborations Hall A 5th ICPHP LPSC Grenoble : mazzouz@lpsc.in2p3.fr May 22nd 2006

  2. Generalized parton distributions: GPDs Parton distribution via Deep inelastic scatering Form Factors via Elastic scaterring Generalized parton distribution viaDeep exclusive scaterring Two independent informations about the nucleon structure Link Mueller, Radyushkin, Ji

  3. x x+x x-x t 4 GPDs : For each quark flavor Generalized parton distributions: GPDs Probability|Ψ(x)|2 that a quark carries a fraction x of the nucleon momentum Parton distributions q(x), Δq(x) measured in inclusive reactions (D.I.S.) GPDsmeasure the CoherenceΨ*(x+ξ)Ψ(x-ξ) between a initial state with a quark carrying a fraction x+ξof the nucleon momentum and a final state with a quark carrying a fraction x-ξ Dependence in t : new wealth of physics to explore Mueller, Radyushkin, Ji

  4. Link to form factors (sum rules) Generalized Parton distributions Link to DIS at x=t=0 Access to quark angular momentum (Ji’s sum rule) GPDs properties, link to DIS and elastic form factors

  5. k’ k q’ p p’ GPDs How to access GPDs: DVCS Collins, Freund, Strikman Simplest hard exclusive process involving GPDs Bjorken regime pQCD factorization theorem Perturbative description (High Q² virtual photon) fraction of longitudinal momentum Non perturbative description by Generalized Parton Distributions

  6. The GPDs enter the DVCS amplitude as an integral over x : GPDs appear in the real part through a PP integral over x GPDs appear in the imaginary part but at the line x=ξ Deeply Virtual Compton Scattering

  7. What is done at JLab Hall A But using a polarized electron beam: Asymmetry appears in Φ The cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=ξ Purely real and fully calculable Small at Jlab enegies The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x Kroll, Guichon, Diehl, Pire …

  8. B contains twist 3 terms A contains twist 2 terms and is a linear combination of three GPD imaginary part evaluated at x=ξ cross-sectiondifference in the handbag dominance Pire, Diehl, Ralston, Belitsky, Kirchner, Mueller Γcontains BH propagators and some kinematics

  9. Twist-2 contribution(Γ.A.sinφ) dominate the total cross-section and cross-section difference. • Twist-2 term (A) and twist-3 term (B) have only log(Q2 ) dependence. Test of the handbag dominance Test of the handbag dominance To achieve this goal, an experiment was initiated at JLab Hall A on hydrogen target with high luminosity (1037 cm-2 s-1) and exclusivity. Another experiment on a deuterium target was initiated to measure DVCS on the neutron. The neutron contribution is very interesting since it will provide a direct measure of GPD E(less constrained!)

  10. Neutron Target Neutron Model: neutron Goeke, Polyakov and Vanderhaeghen t=-0.3

  11. Experiment kinematics E00-110 (p-DVCS) was finished in November 2004 (started in September) E03-106 (n-DVCS) was finished in December 2004 (started in November) xBj=0.364 (fb-1) proton neutron Beam polarization was about 75.3% during the experiment

  12. Experimental method Proton: (E00-110) Left High Resolution Spectrometer scattered electron Neutron: (E03-106) LH2 or (LD2) target Polarized beam Reaction kinematics is fully defined photon Scintillating paddles recoil nucleon (proton veto) Check of the recoil nucleon position Only for Neutron experiment Scintillator Array Electromagnetic Calorimeter (photon detection) (Proton Array)

  13. Proton Array (100 blocks) Calorimeter in the black box (132 PbF2 blocks) Proton Tagger (57 paddles)

  14. Mx2 cut =(Mp+Mπ)2 Contamination by N + mesons DVCS (Resonnant or not) Analysis - Selection of DVCS events Mp2

  15. π0 contamination subtraction One needs to do a π0 subtraction if the only (e,γ) system is used to select DVCS events. Symmetric decay: two distinct photons are detected in the calorimeter  No contamination Asymmetric decay: 1 photon carries most of the π0 energy  contamination because DVCS-like event.

  16. Mx2 cut =(Mp+Mπ)2 π0 to subtract Still to check the exclusivity under the missing mass cut ! π0 contamination subtraction

  17. Missing mass2 with PA check Contamination < 3% Check of the exclusivity One can predictfor each (e,γ) eventthe Proton Array block where the missing proton is supposed to be (assuming DVCS event).

  18. MC sampling MC sampling Extraction of observables Q2 independent MC includes real radiative corrections (external+internal) A B

  19. Extraction of observables • π0Contribution is small • twist-3 contribution is small 2 bins in (Q2,t) out of 15 Acceptance included in fit

  20. Check of the handbag dominance A(twist-2) and B(twist-3) for a full bin<t>=0.25 GeV2 Strong indication for the validity of twist-3 approximation and the handbag dominance

  21. By subtracting proton contribution from deuterium, one should access to the neutron and coherent deuteron contributions. Q2= 1.9 GeV2 <t>= -0.3 GeV2 DVCS on the neutron and the deuteron Same exclusivity check as before The number of detected π0 with hydrogen and deuterium target (same kinematics) shows that: In our kinematics π0 come essentially from proton in the deuterium π0 asymmetry is small No π0 subtraction needed for neutron and coherent deuteron

  22. 1st cut 2nd cut It is clear that there are two contributions with different sign : DVCS on the neutron and DVCS on the deuteron The same extraction method (with an additional binning on the Mx2) will be applied on this data to have at least the twist-2 terms. DVCS on the neutron and the deuteron - Preliminary Q2= 1.9 GeV2 <t>= -0.3 GeV2 Mx2 upper cut

  23. With High Resolution spectrometer and a good calorimeter, we are able to measure the Helicity dependence of the nucleon. Work at precisely defined kinematics: Q2 , s and xBj Work at a high luminosity All tests of Handbag dominance give positive results : No Q2 dependence of twist-2 and twist-3 terms. Twist-3 contribution is small. Accurate extraction of a linear combination of GPDS (twist-2 terms) Conclusion High statistics extraction of the total cross-section (another linear combination of GPD!) Analysis in progress to extract the neutron and deuteron contribution

  24. Proton Target Proton Model: Goeke, Polyakov and Vanderhaeghen t=-0.3

  25. Electronics 1 GHz Analog Ring Sampler (ARS) x 128 samples x 289 detector channels Sample each PMT signal in 128 values (1 value/ns) Extract signal properties (charge, time) with a wave form Analysis. Allows to deal with pile-up events.

  26. Electronics Not all the calorimeter channels are read for each event Calorimeter trigger Following HRS trigger, stop ARS. 30MHz trigger FADC digitizes all calorimeter signals in 85ns window. - Compute all sums of 4 adjacent blocks. - Look for at least 1 sum over threshold - Validate or reject HRS trigger within 340 ns Not all the Proton Array channels are read for each event

  27. Calorimeter resolution and calibration • Time resolution < 1ns for all detectors • Energy resolution of the calorimeter : - Photon position resolution in the calorimeter: 2mm Invariant mass of 2 photons in the calorimeter σ= 9MeV Detecting π0 in the calorimeter checks its calibration

  28. PMT G=104 x10 electronics High luminosity measurement Up to At ~1 meter from target (Θγ*=18 degrees) Low energy electromagnetic background Requires good electronics

  29. Analysis status – preliminary Sigma = 0.6ns Time difference between the electron arm and the detected photon 2 ns beam structure Selection of events in the coincidence peak Determination of the missing particle (assuming DVCS kinematics) Time spectrum in the predicted block (LH2 target) Sigma = 0.9ns Check the presence of the missing particle in the predicted block (or region) of the Proton Array

  30. Analysis – preliminary Triple coincidence Missing mass2 of H(e,e’γ)x for triple coincidence events Background subtraction with non predicted blocks Proton Array and Proton Veto are used to check the exclusivity and reduce the background

  31. π0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Sigma = 9.5 MeV Good way to control calorimeter calibration Sigma = 0.160 GeV2 Missing mass2 of epeπ0x 2π production threshold 2 possible reactions: epeπ0p epenρ+ , ρ+ π0 π+

  32. Missing mass2 with LD2 target

  33. Time spectrum in the tagger (no Proton Array cuts)

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