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A future n-DVCS experiment in Hall A Malek MAZOUZ October 15 th 2007 What was done in E 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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E 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 = 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 : E 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. arXiv

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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 : E 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 E 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 E 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 E 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 E (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., E

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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), 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) LH LH LD LD Total LD2 Luminosity (fb -1 ) fb fb -1

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Projected results previous systematics new systematics The error bars are computed with the E 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) LH (approved) LH (approved) LD LD 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 E 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) LH LH LD LD Total LD2 Luminosity Same kinematics than E 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 E Prediction from F. Cano and B. Pire. Eur. Phys. J. A19, 423 (2004) Experimental results + E03-106

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E 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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