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A future n-DVCS experiment in Hall A Malek MAZOUZ October 15 th 2007 What was done in E03-106 Motivations Experimental upgrades Projected Results DVCS meeting Faculté des Sciences de Monastir – Tunisia

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Deeply Virtual Compton Scattering 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 at the line x=±ξ k k q GPDs pp factorization Simplest hard exclusive process involving GPDs Mueller, Radyushkin, Ji Collins, Freund, Strikman

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What could be done at JLab Hall A The cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=±ξ The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x Purely real and fully calculable Twist-3 term Kroll, Guichon, Diehl, Pire … Twist-2 term Bilinear combinations of GPDs

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Expression of the cross-section difference Combination to be extracted from the data GPDs n-DVCS experiments provide different linear combinations of GPDs than p-DVCS experiments. n-DVCS experiments have different flavor sensitivity than p-DVCS experiments. A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).

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E03-106 experimental apparatus and kinematics LH 2 / LD 2 target Polarized Electron Beam Scattered Electron Left HRS Electromagnetic Calorimeter DVCS events are identified with M X 2 Beam energy = 5.75 GeV Beam polarization = 75% Beam current = ~ 4 μA Luminosity = 4. 10 37 cm -2.s -1 nucleon -1

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I.A. and DVCS on coherent deuteron Impulse approximation : With a Deuterium target one can have 3 different DVCS processes p n d n d d d d p-DVCSd-DVCSn-DVCS (incoherent) (coherent)(incoherent) Access deuteron GPDs overlap < 3% Final state interaction effects between a pn pair should be small probability

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p-DVCS and n-DVCS accidentals MN2MN2 M N 2 +t/2 d-DVCS Analysis method Contamination by M x 2 cut = (M N +M π ) 2 N + mesons (Resonnant or not)

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Extraction of observables MC sampling Luminosity with Under the M X 2 cut : A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002). A. Kirchner, D. Muller, Eur. Phys. J. C32, 347 (2004). Fit of : by :

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What was done : E03-106 results Deuteron moments compatible with zero at large |t| Neutron moments are small and compatible with zero Results can constrain GPD models (and therefore GPD E) - Large systematic error due to the uncertainty on the relative calibration between the LH2 and the LD2 data. - Large systematic error due to the uncertainty on the contamination of the DVCS-like π 0 channel. M. Mazouz et al., submitted to PRL. arXiv0709.0450

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n-DVCS is sensitive to Jd p-DVCS is sensitive to Ju Complementarity between neutron and proton measurements What was done : E03-106 results Model dependent extraction of J u and J d

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What is interesting to do now k k q GPDs pp factorization Already determined in E03-106 Sensitive to x=± ±…

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Expression of the unpolarized cross section GPDs In the nucleon case : A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).

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What was NOT done in E03-106 and why Cannot be separated with a φ analysis Six twist-2 coefficients to be extracted from the data The large systematic error (>50%) of E03-106 did not allow a significant extraction of unpolarized cross sections. The DVCS 2 and the interference terms have a similar φ dependence and therefore cannot be separated.

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We propose to perform a new n-DVCS experiment at the same kinematics than E03-106 (Q 2 =1.9 GeV 2 and x B =0.36) but with 2 different beam energies (4.82 GeV and 6 GeV) and with 5% systematic errors. Interference and DVCS 2 Separation The Γ factors depend on the beam energy while the GPD combinations do not. Different beam energies allow to separate the DVCS 2 and the interference terms C. Muňoz Camacho et al., E07-007.

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Accessible observables Neutron GPDs integral Determine twice as accurately. Measure the polarized cross sections of exclusive π 0 electroproduction on the neutron and coherent deuteron. Linear combinations : Bilinear combinations : Deutron GPDs integral ? Linear combinations : Bilinear combinations :

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Model Calculation for n-DVCS VGG model : Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401. M. Vanderhaeghen, P.A.M. Guichon, M. Guidal, Phys. Rev. D 60 (1999), 094017. GPD Ě contribution 30% In VGG model, the DVCS 2 contribution is larger than the interference one. DVCS 2 /BH 2 depends on the beam energy

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Experimental upgrades We will use the experimental apparatus of the previous DVCS experiments with a few modifications and upgrades ( already planed for the approved p-DVCS experiment) 50% expanded calorimeter (13 x 16 blocks instead of 11 x 12 blocks) Improve the π 0 contamination subtraction at high -t No recoil detectors will be used Reduction of the experimental dead time Faster electronics to improve the data record and transfer New trigger logic (higher threshold + specific trigger for π 0 ) Detection of enough π 0 to subtract correctly the contamination and determine the π 0 electro-production polarized cross sections. Remove the systematic error due to the uncertainty on the contamination of DVCS-like π 0 channel.

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Calorimeter Calibration Elastic calibration H(e,ep) : will be performed periodically. We have 2 independent methods to continuously check and correct the calorimeter calibration 1 st method : missing mass of D(e,eπ - )X reaction 2 nd method : Invariant mass of 2 detected photons in the calorimeter (π 0 ) Calorimeter blocks with radiation damage will be cured with blue light. Will help to keep a good resolution. Frequent swap of LH 2 and LD 2 target Remove the systematic error due to the uncertainty on the relative calibration between LH 2 et LD 2 data

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Projected results TargetBeam energy (GeV) LH2 4.825250 LH2 6.001750 LD2 4.8213500 LD2 6.0013500 Total LD2 Luminosity (fb -1 ) 27000 fb -1 7000 fb -1

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Projected results previous systematics new systematics The error bars are computed with the E03-106 experimental resolution

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Requested kinematics and beam time TargetBeam energy (GeV) k (GeV/c) q (GeV) W 2 (GeV 2 ) Θ e (deg) -Θ γ* (deg) Requested beam time (hours) LH2 4.82 2.012.734.2625.6016.07 90 (approved) LH2 6.00 3.192.734.2618.1318.45 30 (approved) LD2 4.82 2.012.734.2625.6016.07 200 LD2 6.00 3.192.734.2618.1318.45 200 400 Total requested beam time Beam current = 4 μA

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Summary The results of the DVCS experiments in Hall A prove that, with our experimental apparatus, we are able to measure the DVCS polarized cross sections and extract GPDs from these quantities. Unfortunately, this measurement was not very significant in E03-106 because of very large systematics… But we know how to remove these systematics in a future experiment. n-DVCS experiments appear as a mandatory step towards a better knowledge of the nucleon structure. The unpolarized n-DVCS cross section represents a valuable source of information about GPDs (GPDs integral) A future n-DVCS experiment will also provide interesting results about d-DVCS and π 0 electroproduction on the neutron and deuteron.

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END Of the first part

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Proposed kinematics and luminosity TargetBeam energy (GeV) LH2 4.825250 LH2 6.001750 LD2 4.8213500 LD2 6.0013500 27000 Total LD2 Luminosity Same kinematics than E03-106 but with 2 beam energies (fb -1 )

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Extraction of observables Same extraction procedure than the previous experiment one: Fit of the experimental distribution by a Monte Carlo simulation within a global analysis involving a binning on Additional binning on the beam energy

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Analog Ring Sampler 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.

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Analysis method Adjust the calibration and the resolution of H 2 and D 2 data relatively to each other. Systematic error due to the relative calibration between H 2 and D 2 data Add the Fermi motion to the target nucleon for H 2 data Nb of counts Invariant mass (GeV) Variation of calibration coefficients during the experiment due to radiation damage. Calibration variation (%) Calorimeter block number Solution : extrapolation of elastic coefficients assuming a linearity between the received radiation dose and the gain variation By selecting n(e,eπ-)p events, one can predict the energy deposit in the calorimeter using only the cluster position. a minimisation between the measured and the predicted energy gives a better calibration.

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Analysis method Add the Fermi motion to the target nucleon for H 2 data Subtract the π° contamination

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Analysis method Substract H 2 data from D 2 data The π° contamination is treated as a systematic error

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Exclusivity and helicity signal

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sin(φ) and sin(2φ) moments Results are coherent with the fit of a single sin(φ) contribution

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n-DVCS results from E03-106

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d-DVCS results from E03-106 Prediction from F. Cano and B. Pire. Eur. Phys. J. A19, 423 (2004) Experimental results + E03-106

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E03-106 results

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π 0 to subtract π 0 contamination subtraction M x 2 cut =(M p +M π ) 2 H 2 data Subtraction of 0 contamination (1 in the calorimeter) is obtained from a phase space simulation which weight is adjusted to the experimental 0 cross section (2 in the calorimeter).

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Model Calculation for n-DVCS

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Projected results

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Model Calculation for d-DVCS Cano-Pire model : F. Cano and B. Pire, Eur. Phys. J. A 10 (2004), 423.

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Model Calculation for d-DVCS

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Projected results (KIN 1)

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Projected results (KIN 3)

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Projected results

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Resolution effect

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Calorimeter energy calibration We have 2 independent methods to check and correct the calorimeter calibration 1 st method : missing mass of D(e,eπ - )X reaction Mp2Mp2 By selecting n(e,eπ-)p events, one can predict the energy deposit in the calorimeter using only the cluster position. a minimisation between the measured and the predicted energy gives a better calibration.

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Calorimeter energy calibration 2 nd method : Invariant mass of 2 detected photons in the calorimeter (π 0 ) π 0 invariant mass position check the quality of the previous calibration for each calorimeter region. Corrections of the previous calibration are possible. Nb of counts Invariant mass (GeV)

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