The three-dimensional structure of nucleons (protons and neutrons) is embedded in so-called generalized parton distributions, which are accessible from deeply virtual Compton scattering. In this process, a high-energy electron is scattered off a nucleon by exchanging a virtual photon. Then, a highly energetic real photon is emitted from one of the quarks inside the nucleon, which carries information on the quark's transverse position and longitudinal momentum. By measuring the cross-section of deeply virtual Compton scattering, Compton form factors related to the generalized parton distributions can be extracted. Here, we report the observation of unpolarized deeply virtual Compton scattering off a deuterium target. From the measured photon-electroproduction cross-sections, we have extracted the cross-section of a quasifree neutron and a coherent deuteron. Due to the approximate isospin symmetry of quantum chromodynamics, we can determine the contributions from the different quark flavours to the helicity-conserved Compton form factors by combining our measurements with previous ones probing the proton's internal structure. These results advance our understanding of the description of the nucleon structure, which is important to solve the proton spin puzzle. The internal structure of the neutron has now been probed by highly energetic photons scattering off it. Combined with previous results for protons, these measurements reveal the contributions of quark flavours to the nucleon structure.

Understanding the internal structure of the nucleon is a key task in the domain of hadronic physics. Suggested in the 60th the approximate flavor SU(3) symmetry of QCD led to a remarkably successful classification of lowlying mesons and baryons. Many properties of baryons known at that time were transparently explained by the Constituent Quark Model (CQM) [1] that treats baryons as bound systems of three effective (constituent) quarks. CQM-based calculations predicted a rich spectrum of baryon resonances with widths varying from ∼ 80 to ∼ 400 MeV. Nevertheless, in spite of significant efforts, many of the predicted resonances still escape from reliable experimental identification (the so-called “missing resonances”). The Chiral Soliton Model (χSM) is an alternative picture of baryons. It treats them as space/flavor rotational excitations of a classical object a soliton of the chiral field. The model predicts the lowest-mass baryon multiplets to be the (8, 1/2) octet and the (10, 3/2) decuplet exactly as CQM does. χSM also predicts the existence of long-lived exotic particles [2]. Therefore the search for light-quark exotic states may provide critical benchmarks to examine two different approaches and to establish the connection between them. In this context the observation of a narrow enhancement at W ∼ 1.68 GeV in the γn → ηn excitation function (the so-called “neutron anomaly”) at GRAAL, CBELSA/TAPS, LNS and A2@MAMI C [3–7] might be quite important. Narrow structures at the same energy were also observed in Compton scattering on the neutron γn → γn [8] and in the beam asymmetry for the η photoproduction of the proton γp → ηp [9] (see alsociteann). The recent data on the beam asymmetry for Compton scattering on the proton γp → γp [11], the precise data for the γn → ηn [12] and π−p → π−p [13] reactions revealed two narrow structures at W ∼ 1.68 and W ∼ 1.72 GeV. The whole complex of experimental observations may signal the existence of one (N(1685)) or two (N(1685) and N(1726)) narrow nucleon resonances. The proper-

Observation of a narrow structure at W ∼ 1.68 GeV in the excitation functions of some photon- and pion-induced reactions may signal a new narrow isospin-1/2 N (1685) resonance. New data on the γ N → πηN reactions from GRAAL seems to reveal the signals of both N + (1685) and N 0 (1685) resonances.

Different interpretations of narrow structures at W ~ 1.68 and 1.72 GeV observed in several reactions are discussed. It is questionable whether interference phenomena could explain the whole complex of experimental findings. More probable hypotheses would be the existence of one or two narrow resonances N (1685) and N (1726) and/or the sub-threshold virtual K Σ and ω p production (cusps).

The internal structure of nucleons (protons and neutrons) remains one of the greatest outstanding problems in modern nuclear physics. By scattering high-energy electrons off a proton we are able to resolve its fundamental constituents and probe their momenta and positions. Here we investigate the dynamics of quarks and gluons inside nucleons using deeply virtual Compton scattering (DVCS)—a highly virtual photon scatters off the proton, which subsequently radiates a photon. DVCS interferes with the Bethe-Heitler (BH) process, where the photon is emitted by the electron rather than the proton. We report herein the full determination of the BH-DVCS interference by exploiting the distinct energy dependences of the DVCS and BH amplitudes. In the regime where the scattering is expected to occur off a single quark, measurements show an intriguing sensitivity to gluons, the carriers of the strong interaction.

We present deeply virtual π^{0} electroproduction cross-section measurements at x_{B}=0.36 and three different Q^{2} values ranging from 1.5 to 2  GeV^{2}, obtained from Jefferson Lab Hall A experiment E07-007. The Rosenbluth technique is used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component and, thus, is far from the asymptotic limit predicted by perturbative quantum chromodynamics. Nonetheless, an indication of a nonzero longitudinal contribution is provided by the measured interference term σ_{LT}. Results are compared with several models based on the leading-twist approach of generalized parton distributions (GPDs). In particular, a fair agreement is obtained with models in which the scattering amplitude includes convolution terms of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. This experiment, together with previous extensive unseparated measurements, provides strong support to the exciting idea that transversity GPDs can be accessed via neutral pion electroproduction in the high-Q^{2} regime.

First measurement of the beam asymmetry $\ensuremath{\Sigma}$ for Compton scattering off the proton in the energy range ${E}_{\ensuremath{\gamma}}=0.85--1.25\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$ is presented. The data reveal two narrow structures at ${E}_{\ensuremath{\gamma}}=1.036$ and ${E}_{\ensuremath{\gamma}}=1.119 \mathrm{GeV}$. They may signal narrow resonances with masses near 1.68 and $1.72\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$, or they may be generated by the sub-threshold $K\ensuremath{\Lambda}$ and $\ensuremath{\omega}p$ production. Their decisive identification requires additional theoretical and experimental efforts.

Disintegration of 12C nuclei by tagged photons of 700–1500 MeV energy at the GRAAL facility has been studied by means of the LAGRANγE detector with a wide angular acceptance. The energy and momentum distributions of produced neutrons and protons as well as their multiplicity distributions were measured and compared with corresponding distributions calculated with the RELDIS model based on the intranuclear cascade and Fermi break-up models. It was found that eight fragments are created on average once per about 100 disintegration events, while a complete fragmentation of 12C into 12 nucleons is observed typically only once per 2000 events. Measured multiplicity distributions of produced fragments are well described by the model. The measured total photoabsorption cross section on 12C in the same energy range is also reported.

First measurement of the beam asymmetry Sigma for Compton scattering off the proton in the energy range E-gamma = 0.85-1.25 GeV is presented. The data reveal two narrow structures at E-gamma = 1.036 and E-gamma = 1.119 GeV. They may signal narrow resonances with masses near 1.68 and 1.72 GeV, or they may be generated by the subthreshold K Lambda and omega p production. Their decisive identification requires additional theoretical and experimental efforts.

The \( \Sigma\) beam asymmetry in \( \eta^{\prime}\) photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around \( 90^{\circ}\). The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.

We report on new measurements of the beam asymmetry for \omega\ photo-production on proton and neutron in Hydrogen and Deuterium targets from the GRAAL collaboration. The beam asymmetry values are extracted from the reaction threshold (E_{\gamma} = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement and provide important constraints for the determination of resonant state contributions to the \omega\ production mechanism. First experimental results on the deuteron allow the extraction of the \Sigma\ beam asymmetry on quasi-free nucleons. Comparison of the results for free and quasi-free kinematics on the proton shows a generally reasonable agreement, similar to the findings in pseudo-scalar meson photo-production reactions. For the first time measurements on quasi-free neutrons are available, showing that both the strength and the angular distributions of the beam asymmetry are sensibly different from the results on the proton target.

We report on new measurements of the beam asymmetry for. photoproduction on protons and neutrons in hydrogen and deuterium targets from the GRAAL Collaboration. The beam asymmetry values are extracted from the reaction threshold (E-gamma = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement, leading to the world's most precise measurements of the beam asymmetry for omega photoproduction off free protons. First experimental results on the deuteron allow the extraction of the Sigma beam asymmetry on quasifree nucleons. The beam asymmetry angular distributions obtained for the free and the quasifree kinematics show the same behavior, similar to the findings in pseudoscalar meson photoproduction reactions. First results of the beam asymmetry on the quasifree neutrons are presented, showing different strengths and angular distributions from the results on the proton target.

Differential and total cross section measurements on eta' photoproduction were published by the CLAS Collaboration (M. Dugger et al., Phys. Rev. Lett. 96, 062001 (2006) and M. Williams et al., Phys.Rev.C80, 045213 (2009)) for center-of-mass energies from near the threshold up to 2.84 GeV, and by the CB-ELSA-TAPS Collaboration (V. Crede et al., Phys. Rev. C80, 055202 (2009)) up to 2.36 GeV and also making a precise threshold scan of the differential cross section in the 1446 -1527.4 MeV gamma beam energy range. However, the wide information about reaction cross sections are not sufficient to understand the role of resonances involved in the process. Different theoretical works stressed the importance to have also polarization observables in order to solve the ambiguity in the choice of the parameters used in their models.We present the analysis of the eta' photoproduction off the proton, identifying the meson via the gamma gamma, pi(0)pi(0)eta, and pi(+)pi(-)eta decay modes by using the GRAAL apparatus; and we show the preliminary GRAAL results on the beam asymmetry Sigma from the threshold (1.446 GeV) up to 1.5 GeV.

The physics case and an experimental overview of the MOLLER (Measurement Of a Lepton Lepton Electroweak Reaction) experiment at the 12 GeV upgraded Jefferson Lab are presented. A highlight of the Fundamental Symmetries subfield of the 2007 NSAC Long Range Plan was the SLAC E158 measurement of the parity-violating asymmetry A_PV in polarized electron-electron (Møller) scattering. The proposed MOLLER experiment will improve on this result by a factor of five, yielding the most precise measurement of the weak mixing angle at low or high energy anticipated over the next decade. This new result would be sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as ∼ 10^-3· G_F from as yet undiscovered dynamics beyond the Standard Model. The resulting discovery reach is unmatched by any proposed experiment measuring a flavor- and CP-conserving process over the next decade, and yields a unique window to new physics at MeV and multi-TeV scales, complementary to direct searches at high energy colliders such as the Large Hadron Collider (LHC). The experiment takes advantage of the unique opportunity provided by the upgraded electron beam energy, luminosity, and stability at Jefferson Laboratory and the extensive experience accumulated in the community after a round of recent successfully completed parity-violating electron scattering experiments

The only recent η′ photoproduction data off proton available in literature are the differential and total cross sections published by the CLAS and CB-ELSA-TAPS Collaborations. However, the wide information about reaction cross sections are not sufficient to understand the role of resonances involved in the process. Different theoretical works stressed the importance to measure also polarization observables in order to solve the ambiguities in the choice of the parameters used in their models. We present the analysis of η′ photoproduction off the proton analysis, identifying the investigated meson by the π+π−η, π0π0η, and γγ decay modes by using the GRAAL apparatus; and we show the preliminary GRAAL results on the beam asymmetry Σ at beam energy of 1475 MeV.

A. Acha, S. Abrahamyan, Z. Ahmed, H. Albataineh, K. Aniol, D. S. Armstrong, W. Armstrong, J. Arrington, T. Averett, B. Babineau, S.L.Bailey, J. Barber, A. Barbieri, A. Beck, V. Bellini, R. Beminiwattha, H. Benaoum, J. Benesch, F. Benmokhtar, P. Bertin, T. Bielarski, W. Boeglin, P. Bosted, F. Butaru, E. Burtin, J. Cahoon, A. Camsonne, M. Canan, P. Carter, C.C. Chang, G. D. Cates, Y.-C. Chao, C. Chen, J.-P. Chen, Seoho Choi, E. Chudakov, E. Cisbani, 17 B. Craver, F. Cusanno, ∗ M. M. Dalton, R. De Leo, K. de Jager, 48 W. Deconinck, 7 P. Decowski, D. Deepa, X. Deng, A. Deur, D. Dutta, A. Etile, C. Ferdi, R. J. Feuerbach, J.M. Finn, † D. Flay, G. B. Franklin, M. Friend, S. Frullani, E. Fuchey, 39 S.A. Fuchs, K. Fuoti, F. Garibaldi, E. Gasser, R. Gilman, A. Giusa, A. Glamazdin, L.E. Glesener, J. Gomez, J. Grames, K. Grimm, C. Gu, O. Hansen, J. Hansknecht, O. Hen, D. W. Higinbotham, R. S. Holmes, T. Holmstrom, C. J. Horowitz, J. Hoskins, J. Huang, T.B. Humensky, C. E. Hyde, 6 H. Ibrahim, F. Itard, C.-M. Jen, E. Jensen, X. Jiang, G. Jin, S. Johnston, J. Katich, L.J. Kaufman, A. Kelleher, K. Kliakhandler, P.M. King, A. Kolarkar, S. Kowalski, E. Kuchina, K. S. Kumar, L. Lagamba, D. Lambert, P. LaViolette, J. Leacock, J. Leckey IV, J. H. Lee, 30 J. J. LeRose, D. Lhuillier, R. Lindgren, N. Liyanage, N. Lubinsky, J. Mammei, F. Mammoliti, D.J. Margaziotis, P. Markowitz, M. Mazouz, K. McCormick, A. McCreary, D. McNulty, D.G. Meekins, L. Mercado, Z.-E. Meziani, R. W. Michaels, M. Mihovilovic, B. Moffit, P. Monoghan, N. Muangma, C. Muñoz-Camacho, S. Nanda, V. Nelyubin, D. Neyret, N. Nuruzzaman, Y. Oh, K. Otis, A. Palmer, D. Parno, K. D. Paschke, S. K. Phillips, M. Poelker, R. Pomatsalyuk, M. Posik, M. Potokar, K. Prok, A.J.R. Puckett, X. Qian, Y. Qiang, ‡ B. Quinn, A. Rakhman, P. E. Reimer, B. Reitz, S. Riordan, J. Roche, § P. Rogan, G. Ron, G. Russo, K. Saenboonruang, A. Saha, † B. Sawatzky, A. Shahinyan, 40 R. Silwal, J. Singh, S. Sirca, K. Slifer, R. Snyder, P. Solvignon, P. A. Souder, ¶ M. L. Sperduto, R. Subedi, M.L. Stutzman, R. Suleiman, V. Sulkosky, C. M. Sutera, W. A. Tobias, W. Troth, G. M. Urciuoli, P. Ulmer, A. Vacheret, A. Voutier, B. Waidyawansa, D. Wang, K. Wang, J. Wexler, A. Whitbeck, R. Wilson, B. Wojtsekhowski, X. Yan, H. Yao, Y. Ye, Z. Ye, 48 V. Yim, L. Zana, X. Zhan, J. Zhang, Y. Zhang, X. Zheng, V. Ziskin, and P. Zhu

The 2014-scheduled 12 GeV upgrade of JLab will allow for a whole new range of experiments, extending our knowledge of nucleon structure. The program for the nucleon form factors and elastic scattering parity-violating asymmetry experiments is presented here, along with the benefits of the new large-acceptance forward spectrometer (Super BigBite, SBS) to be installed in Hall A. (C) 2012 Elsevier B.V. All rights reserved.

We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.