p-DVCS and n-DVCS experiment status -Brief overview of the theory -Experiment setup -Analysis status Malek MAZOUZ LPSC Grenoble Hall A Collaboration MeetingJune 24 th 2005

GPDs properties, link to DIS and elastic form factors Generalized Parton distributions Link to DIS at =t=0 Link to form factors (sum rules) Access to quark angular momentum (Jis sum rule) Quark correlations !

Brief overview of the theory DVCS: Simplest hard exclusive process involving GPDs Purely real pQCD factorization theorem (Bjorken regime) Non perturbative description by Generalized Parton Distributions Perturbative description

Proton Target Proton t=-0.3 Target Proton Goeke, Polyakov and Vanderhaeghen Model: sin(Φ) term

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

p-DVCS and n-DVCS in Hall A DVCS on the proton : E Goal : Measure the absolute cross section of DVCS on proton (3 Q² values: 1.4, 1.9, 2.3 GeV²) and on neutron (Q²=1.9 GeV²) DVCS on the neutron : E Simplest access to the least known of GPDs: E First constraint of nucleon orbital angular momentum through model of E Check Handbag dominance & Test factorization Deduce Q² dependence and relative importance of leading twist and higher twists in helicity dependent cross-section Constrain GPDs …including Re(DVCS)

Experiment status s (GeV²) Q² (GeV²) P e (Gev/c) Θ e (deg) -Θ γ* (deg) proton neutron x Bj =0.364 Beam polarization was about 78% during the experiment E (p-DVCS) was finished in November 2004 (started in September) E (n-DVCS) was finished in December 2004 (started in November) (fb -1 )

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

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

High luminosity measurement Up to At ~1 meter from target (Θ γ* =15 degrees) Requires good electronics PMT G=10 4 x10electronics Low energy electromagnetic background

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.

Electronics Calorimeter trigger Not all the calorimeter channels are read for each event 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.

Analysis status - What is done Left HRS efficiency (detectors, tracking …) determined All good runs selected and total integrated luminosity extracted Calorimeter calibration done for almost all data Proton Array calibration done for a part of the data and still in progress Parameters of the wave form analysis and the clustering optimized Coincidence time of all detectors precisely adjusted All geometrical offsets taken into account

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

Analysis Status – Very preliminary Absolute cross sections necessary to extract helicity dependence of neutron

Analysis – Very preliminary Triple coincidence Missing mass 2 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

Analysis – Very preliminary Triple coincidence Missing mass 2 (background subtracted) LH2 target

Analysis – Fermi momentum effect Triple coincidence Missing mass 2 (background subtracted) LD2 target Very preliminary

π 0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Good way to control calorimeter calibration Missing mass 2 of ep eπ 0 x 3 possible reactions: ep eπ 0 p ep enρ +, ρ + π 0 π + ep e π 0 Δ + 2π production threshold Sigma = GeV 2 Sigma = 9.5 MeV

Analysis – Proton Array Calibration Used to calibrate the Proton Array

π 0 electroproduction - preliminary Invariant mass of 2 photons in the calorimeter Good way to control calorimeter calibration Missing mass 2 of ep eπ 0 x 3 possible reactions: ep eπ 0 p ep enρ +, ρ + π 0 π + ep e π 0 Δ + 2π production threshold Sigma = GeV 2 Sigma = 9.5 MeV

π0 electroproduction - preliminary Background subtraction Accidental π 0 Decorrelated photons π0π0

π0 electroproduction – background subtraction

Analysis – work in progress wave form analysis (detection) efficiency (almost done) Acquisition trigger efficiency Acceptance calculation Proton Array Calibration (almost done) Neutron detection efficiency in the Proton Array Implement neutron tagger analysis Evaluate Fermi motion consequences Study knock-out effects in the tagger (data + simulation) Proton experiment Neutron experiment

Conclusion -Requires wave form electronics - 10% of detector components almost unusable as expected after 3 months of data taking We have demonstrated that in Hall A with High Resolution spectrometer and a good calorimeter, we are able to measure: Real and Imaginary parts of DVCSBH interference: Work at precisely defined kinematics: Q 2, s and x Bj Absolute cross sections and cross section difference are determined with the precision of HRS (better than 5%) Analysis is in progress But Work at a luminosity up to Deep π 0 electroproduction cross-section almost finalized