International audienceRecent results on the Generalized Parton Distribution (GPD) program at Jefferson Lab (JLab) will be presented. The emphasis will be in the Hall A program aiming at measuring Q2−dependences of different terms of the Deeply Virtual Compton Scattering (DVCS) cross section. This is a fundamental step before one can extract GPD information from JLab DVCS data. The upcoming program in Hall A, using both a 6 GeV beam (≈ 2010) and a 11 GeV beam (≈ 2015) will also be described

Munoz Camacho, C.

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The GPD experimental program at Jefferson LabC. Munoz CamachoTo cite this version:C. Munoz Camacho. The GPD experimental program at Jefferson Lab. XIX InternationalBaldin Seminar on High Energy Physics Problems, Relativistic Nuclear Physics and QuantumChromodynamics, Sep 2008, Dubna, Russia. 2008. <in2p3-00344416>HAL Id: in2p3-00344416http://hal.in2p3.fr/in2p3-00344416Submitted on 4 Dec 2008HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.THE GPD EXPERIMENTAL PROGRAM AT JEFFERSON LABC. Mun˜oz Camacho1†(1) Laboratoire Physique Corpusculaire/CNRS-IN2P3, F-63177 Aubie`re, France† E-mail: munoz@jlab.orgAbstractRecent results on the Generalized Parton Distribution (GPD) program at JeffersonLab (JLab) will be presented. The emphasis will be in the Hall A program aimingat measuring Q2−dependences of different terms of the Deeply Virtual ComptonScattering (DVCS) cross section. This is a fundamental step before one can extractGPD information from JLab DVCS data. The upcoming program in Hall A, usingboth a 6 GeV beam (≈ 2010) and a 11 GeV beam (≈ 2015) will also be described.1 IntroductionMeasurements of electro-weak form factors determine nucleon spatial structure, and deepinelastic scattering (DIS) of leptons off the nucleon measures parton distributions, whichdetermine longitudinal momentum distributions. The demonstration by Ji [1], Radyushkin [2],and Mu¨ller et al. [3], of a formalism to relate the spatial and momentum distributions of thepartons allows the exciting possibility of determining spatial distributions of quarks and gluonsin the nucleon as a function of the parton wavelength. These new structure functions, nowcalled generalized parton distributions (GPD), became of experimental interest when it wasshown [1] that they are accessible through deep virtual Compton scattering (DVCS) and itsinterference with the Bethe-Heitler (BH) process (Fig. 1). The factorization proofs [4, 5]confirmed the connection between DVCS and DIS. Diehl et al. [6] showed that the twist-2and twist-3 contributions in the DVCS-BH interference terms (the first two leading ordersin a 1/√Q2 expansion) could be extracted independently from the azimuthal-dependenceof the helicity-dependent cross section. Burkardt [7] showed that the t-dependence of theGPDs is the Fourier conjugate to the transverse spatial distribution of quarks in the infinitemomentum frame as a function of momentum fraction. Ralston and Pire [8], Diehl [9] andBelitsky et al. [10] extended this interpretation to the general case of skewness ξ 6= 0.Figure 1: DVCS and BH processes: the two interfering contributions to the electroproductionof photons.12 Current experimental situationThese concepts stimulated an intense experimental effort in DVCS. The H1 [11, 12] andZEUS [13] collaborations measured the cross section for xB ≈ 10−3. The HERMES collabo-ration measured relative beam-helicity [14] and beam-charge asymmetries [15, 16]. Relativebeam-helicity [17] and longitudinal target [18] asymmetries were measured at the ThomasJefferson National Accelerator Facility (JLab) by the CLAS collaboration. The first ded-icated DVCS experiment ran in Hall A at JLab [19]. Helicity-correlated cross section ofelectroproduction of photons were measured with high statistical accuracy as a function ofQ2 (Fig. 2).The combination of Compton form factors (CFF) extracted from these data (Fig. 3) wasfound to be independent of Q2, within uncertainties. This exciting result strongly supportsthe twist-2 dominance of the imaginary part of the DVCS amplitude. GPDs can then beaccessed experimentally at the kinematics of JLab.A dedicated DVCS experiment on the neutron (E03-106) followed E00-110. Using a deu-terium target, E03-106 provided the first measurements of DVCS on the neutron, particularlysensitive to the least known GPD E, and DVCS on the deuteron [20]. A first constraint onthe orbital angular momentum of quarks inside the nucleon (Fig. 4) was obtained relying onthe VGG model of GPDs [21].3 Future DVCS program with CEBAF at 6 GeVA new experiment E07-007 was recently approved with the highest scientific rating (A) bythe JLab Program Advisory Committee (PAC-31) and will run in the next 2 years. E07-007will use the important lessons we learned in E00-110 to further test and explore the potentialof DVCS to measure GPDs.In E00-110 we managed to determine the photon electroproduction helicity-independent(unpolarized) cross section. This additional information was not in the initial goal of theE00-110 experiment, and we were only able to measure it at one single value of Q2 =2.3 GeV2 (Fig. 2). We found that the total cross section was much larger than the BHcontribution. The excess in the cross section is then coming from both the interference term(BH–DVCS) and the DVCS2 contribution. These contributions contain different kinds ofGPD combinations. The interference term is proportional to a linear combination of GPDintegrals, whereas the DVCS2 is related to a bilinear combination of GPD integrals.The harmonic φ structure of the DVCS cross section does not allow the independentdetermination of the BH-DVCS interference and the DVCS2 contributions.3.1 Interference and DVCS2 separationE07-007 was approved to measure the photon electroproduction helicity-independent crosssection as a function of Q2 and at different incident beam energies. The different beamenergy dependence of the DVCS2 and BH·DVCS interference term complement the azimuthalanalysis of the cross section in order to provide a complete separation of these two contribution.3.2 Scaling of the real part of the DVCS amplitudeThe Q2−dependence of these observables will provide a strong scaling test of the real partof the DVCS amplitude that will complement the one obtained in E00-110 from the helicity-2)4DVCS cross section (nb/GeV 2  = 1.9 GeV 2 Q 2  = 1.5 GeV 2 Q (deg)φ-0.0200.0290 180 270 (deg)φ-0.0200.0290 180 270 (deg)φ-0.0200.0290 180 270 (deg)φ-0.0200.0290 180 270-0.04-0.0200.020.04 E00-110Total fitTwist - 2-0.04-0.0200.020.04-0.04-0.0200.020.04-0.04-0.0200.020.04-0.0200.02-0.0300.03 2< t > = - 0.33 GeV  2< t > = - 0.28 GeV  2< t > = - 0.23 GeV  2< t > = - 0.17 GeV)4DVCS cross section (nb/GeVbeam helicity-independentbeam helicity-dependent (deg)φ90 180 270E00-110 2|BH+DVCS| 2|BH| 2 - |BH| 2|BH+DVCS| (deg)φ90 180 270 (deg)φ90 180 270 (deg)φ90 180 270E00-110Total fitTwist - 200.050.1-0.0200.02 2< t > = - 0.33 GeV  2< t > = - 0.28 GeV  2< t > = - 0.23 GeV  2< t > = - 0.17 GeVFigure 2: Data and fit to the helicity-dependent d4Σ/dQ2dxBdtdφ, and the helicity-independent d4σ/[dQ2dxBdtdφ] cross sections, as a function of the azimuthal angle φ betweenthe leptonic and hadronic planes. All bins are at 〈xB〉 = 0.36. Error bars show statistical un-certainties. Solid lines show total fits with one-σ statistical error bands. The green line is the|BH|2 contribution to d4σ. The first three rows show the helicity-dependent cross section forvalues ofQ2 = 1.5, 1.9 and 2.3 GeV2, respectively. The last row shows the helicity-independentcross section for Q2 = 2.3 GeV2.3)2-t (GeV0.15 0.2 0.25 0.3 0.35, E)H~ (H, IIm C012345 2 = 1.5 GeV2Q2 = 1.9 GeV2Q2 = 2.3 GeV2QVGG modelFigure 3: Extracted imaginary part of the twist-2 angular harmonic as function of t. Super-posed points are offset for visual clarity. Their error bars show statistical uncertainties.uJ-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1dJ-1-0.8-0.6-0.4-0.2-00.20.40.60.81HERMES Preliminaryp-DVCSJLab Hall An-DVCSGPDs from :Goeke et al., Prog. Part. Nucl. Phys. 47 (2001), 401.Code VGG (Vanderhaeghen, Guichon and Guidal) 0.14± = 0.18 5.0uJ + dJAHLT GPDs [36]Lattice QCDSF (quenched) [40]Lattice QCDSF (unquenched) [41]LHPC Lattice (connected terms) [42]Figure 4: Experimental constraint on Ju and Jd quark angular momenta from the presentn-DVCS results [20]. A similar constraint from the p-DVCS target spin asymmetry measuredby HERMES, and different lattice QCD based calculations are also shown.correlated cross section. Testing scaling on the real part of the DVCS amplitude is indeeda very stringent check. The imaginary part of the DVCS amplitude access GPDs along onekinematical line x = ±ξ. In JLab kinematics x ∈ [ξ, 1] and the twist-2 amplitude of DVCSdescribes the emission and absorption of a quark with different momentum fractions. The realpart of the DVCS amplitude is related to GPD integrals over their full domain −1 < x < 1.Over the region x ∈ [−ξ, ξ] the twist-2 amplitude correspond to the emission of a quark-antiquark pair from the initial proton, and in the region x ∈ [−1,−ξ] one has emission andreabsorption of antiquarks. Therefore, testing the scaling properties of GPD integrals teststhe twist-2 dominance of DVCS over a wide variety of partonic configurations.43.3 Separation of the longitudinal pi0 electroproduction cross sec-tionIn addition to these fundamental measurements on DVCS, E07-007 will concurrently measurethe pi0 electroproduction cross section for different beam energies. We successfully measuredthe pi0 electroproduction cross section in E00-110. However, since we only ran at one incidentbeam energy, the longitudinal and transverse separation of the cross section could not be done.E07-007 will allow the first separation of the longitudinal cross section, using the Rosenbluthtechnique. A factorization theorem holds for the longitudinal pi0 electroproduction crosssection that relates it, at sufficiently large Q2, to a different flavor combination of GPDs. Thischannel is an essential element to make a flavor decomposition of GPDs, which is impossibleto obtain from DVCS on the proton alone.E07-007 will test the Q2−dependence of the longitudinal cross section and check if thefactorization is applicable at the Q2 values currently accessible experimentally (around Q2 ∼2 GeV2). If this test turns out positive, very interesting and complementary information onGPDs will be available from this channel.3.4 E08-025: Rosenbluth-like separation on the neutronAn equivalent DVCS program at 6 GeV using a deuterium target was approved earlier thisyear and will run concurrently with E07-007. It will allow to separate the interference andDVCS2 terms on neutron observables. This will complement E07-007 on the proton.4 12 GeV programThe experimental GPD program is at the heart of the scientific motivation of the majorJefferson Lab upgrade to 12 GeV.E12-06-114 [22] is a wide experimental program of DVCS high precision cross-sectionmeasurements in Hall A with CEBAF at 12 GeV, enthusiastically approved by JLab PAC-30.With polarized 6.6, 8.8, and 11 GeV beams incident on the liquid hydrogen target, we willprovide a major survey of proton DVCS in almost the entire kinematic range accessible withCEBAF at 12 GeV (Fig. 5).References[1] X.-D. Ji, Phys. Rev. Lett. 78, 610 (1997).[2] A. V. Radyushkin, Phys. Rev. D56, 5524 (1997).[3] D. Mueller et al., Fortschr. Phys. 42, 101 (1994).[4] J. C. Collins and A. Freund, Phys. Rev. D59, 074009 (1999).[5] X.-D. Ji and J. Osborne, Phys. Rev. D58, 094018 (1998).[6] M. Diehl, T. Gousset, B. Pire, and J. P. Ralston, Phys. Lett. B411, 193 (1997).[7] M. Burkardt, Phys. Rev. D62, 071503 (2000).[8] J. P. Ralston and B. Pire, Phys. Rev. D66, 111501 (2002).5Bjx0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9)2 (GeV2 Q05102<4 GeV2W 11 GeV≤beamUnphysical with E= 6.6 GeVbeamE= 8.8 GeVbeamE= 11.0 GeVbeamE= 5.75 GeVbeamEDVCS measurements in Hall A/JLabFigure 5: Approved DVCS kinematics for H(e, e′γ)p measurements in Hall A with 3, 4, and5 pass beams of CEBAF at 12 GeV. The diamond shapes trace the approximate acceptanceof the Hall A High Resolution Spectrometer in each setting. The boundary of the unphysicalregion corresponds to the maximum possible Q2 at a given xB for 11 GeV. This correspondsto 180◦ electron scattering, equivalent to θq = 0◦. The points at EBeam = 5.75 GeV wereobtained in E00-110.[9] M. Diehl, Eur. Phys. J. C25, 223 (2002).[10] A. V. Belitsky, X.-D. Ji, and F. Yuan, Phys. Rev. D69, 074014 (2004).[11] A. Aktas et al., Eur. Phys. J. C44, 1 (2005).[12] C. Adloff et al., Phys. Lett. B 517, 47 (2001).[13] S. Chekanov et al., Phys. Lett. B573, 46 (2003).[14] A. Airapetian et al., Phys. Rev. Lett. 87, 182001 (2001).[15] F. Ellinghaus, Nucl. Phys. A711, 171 (2002).[16] A. Airapetian et al., (2006).[17] S. Stepanyan et al., Phys. Rev. Lett. 87, 182002 (2001).[18] S. Chen et al., Phys. Rev. Lett. 97, 072002 (2006).[19] C. Mun˜oz Camacho et al., Phys. Rev. Lett. 97, 262002 (2006).[20] M. Mazouz et al., Phys. Rev. Lett. 99, 242501 (2007).[21] M. Vanderhaeghen, P. A. M. Guichon, and M. Guidal, Phys. Rev. D60, 094017 (1999).[22] J. Roche et al., (2006).6

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