Running Masses in the Nucleon and its Resonances

An overarching scientific challenge for the coming decade is to discover the meaning of confinement, its relationship to dynamical chiral symmetry breaking (DCSB) - the origin of visible mass - and the connection between them. In progressing toward meeting this challenge, significant progress has been made using continuum methods in QCD. For example, a novel understanding of gluon and quark confinement and its consequences has begun to emerge from quantum field theory; a clear picture is being drawn of how hadron masses emerge dynamically in a universe with light quarks; and ground-state hadron wave functions with a direct connection to QCD are becoming available, which reveal that quark-quark correlations are crucial in hadron structure. There is growing experimental support for this body of predictions in both elastic and nucleon-to-resonance-transition form factors.Comment: 10 pages, 6 figures. Contribution to the proceedings of NSTAR2015, the 10th International Workshop on the Physics of Excited Nucleons, 25-28 May 2015, Suita Campus, Osaka University, Osaka, Japa

Towards a Resolution of the Proton Form Factor Problem: New Electron and Positron Scattering Data

There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p). from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (epsilon) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing epsilon at Q(2) = 1.45 GeV2. This measurement is consistent with the size of the form factor discrepancy at Q(2) approximate to 1.75 GeV2 and with hadronic calculations including nucleon and Delta intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV2

Data Analysis Techniques, Differential Cross Sections, and Spin Density Matrix Elements for the Reaction ᵞp → ϕp

High-statistics measurements of differential cross sections and spin density matrix elements for the reaction ᵞp → ϕp have been made using the CLAS detector at Jefferson Lab. We cover center-of-mass energies (√s) from 1.97 to 2.84 GeV, with an extensive coverage in the ϕ production angle. The high statistics of the data sample made it necessary to carefully account for the interplay between the ϕ natural lineshape and effects of the detector resolution, that are found to be comparable in magnitude. We study both the charged-(ϕ → K+ K-) and neutral( ϕ → (KOSKOL) K K- decay modes of the ϕ. Further, for the charged mode, we differentiate between the cases where the final K-is directly detected or its momentum reconstructed as the total missing momentum in the event. The two charged-mode topologies and the neutral-mode have different resolutions and are calibrated against each other. Extensive usage is made of kinematic fitting to improve the reconstructed ϕ mass resolution. Our final results are reported in 10- and mostly 30-MeV-wide √s bins for the charged-and the neutral-modes, respectively. Possible effects from K+Λ* channels with p K K- final states are discussed. These present results constitute the most precise and extensive ϕ photoproduction measurements to date and in conjunction with the ω photoproduction results recently published by CLAS, will greatly improve our understanding of low energy vector meson photoproduction

Beam Asymmetry Σ for π+ and π0 Photoproduction on the Proton for Photon Energies From 1.102 to 1.862 GeV

Beam asymmetries for the reactions ˠp -\u3e pπ0 and ˠp -\u3e nπ+have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged, linearly polarized photon beam with energies from 1.102-1.862 GeV. A Fourier moment technique for extracting beam asymmetries from experimental data is described. The results reported here possess greater precision and finer energy resolution than previous measurements. Our data for both pion reactions appear to favor the SAID and Bonn-Gatchina scattering analyses over the older Mainz MAID predictions. After incorporating the present set of beam asymmetries into the world database, exploratory fits made with the SAID analysis indicate that the largest changes from previous fits are for properties of the Δ(1700)3/2- and Δ(1905) 5/2+ states

Induced Polarization of Λ1116 in Kaon Electroproduction

We have measured. the induced polarization of the Λ (1116) in the reaction ep →e′K+Λ , detecting the scattered e′ and K+ in the final state along with the proton from the decay Λ → pπ− . The present study used the CEBAF Large Acceptance Spectrometer (CLAS), which allowed for a large kinematic acceptance in invariant energy W (1.6≤ W ≤ 2.7 GeV) and covered the full range of the kaon production angle at an average momentum transfer Q2 = 1.90GeV2 . In this experiment a 5.50-GeV electron beam was incident upon an unpolarized liquid-hydrogen target. We have mapped out the W and kaon production angle dependencies of the induced polarization and found striking differences from photoproduction data over most of the kinematic range studied. However, we also found that the induced polarization is essentially Q2 independent in our kinematic domain, suggesting that somewhere below the Q2 covered here there must be a strong Q2 dependence. Along with previously published photo- and electroproduction cross sections and polarization observables, these data are needed for the development of models, such as effective field theories, and as input to coupled-channel analyses that can provide evidence of previously unobserved s -channel resonances

Precision measurements of g(1) of the proton and of the deuteron with 6 GeV electrons

The inclusive polarized structure functions of the proton and deuteron, g(1)(p) and g(1)(d) , were measured with high statistical precision using polarized 6 GeV electrons incident on a polarized ammonia target in Hall B at Jefferson Laboratory. Electrons scattered at laboratory angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual deep inelastic region kinematics, Q(2) \u3e 1 GeV2 and the final-state invariant mass W \u3e 2 GeV, the ratio of polarized to unpolarized structure functions g(1)/F-1 is found to be nearly independent of Q(2) at fixed x. Significant resonant structure is apparent at values of W up to 2.3 GeV. In the framework of perturbative quantum chromodynamics, the high-W results can be used to better constrain the polarization of quarks and gluons in the nucleon, as well as high-twist contributions

Measurement of Target and Double-Spin Asymmetries for the ep → eπ+(n) Reaction in the Nucleon Resonance Region at Low Q²

We report measurements of target- and double-spin asymmetries for the exclusive channel →e→p → eπ+(n) in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH3 target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3, and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low Q2 range from 0.0065 to 0.35(GeV/c)2. The Q2 access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as 6°. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID, and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models

Measurement of Two-Photon Exchange Effect by Comparing Elastic e ± p Cross Sections

Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four-momentum transfer (Q2) . Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab\u27s Hall B by passing the 5.6-GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron-positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm-long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons, we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ , the TPE correction to the elastic-scattering cross section, at Q2 ≈ 0.85 and 1.45 GeV2 over a large range of virtual photon polarization ɛ . Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a nonzero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q2 ≈ 2 GeV2 . These data are consistent with an increase in R2γ with decreasing ɛ at Q2 ≈ 0.85 and 1.45 GeV2 . There are indications of a slight increase in R2γ with Q2

Single and Double Spin Asymmetries for Deeply Virtual Compton Scattering Measured with CLAS and a Longitudinally Polarized Proton Target

Single-beam, single-target, and double spin asymmetries for hard exclusive electroproduction of a photon on the proton →e→p~ → e\u27p\u27γ are presented. The data were taken at Jefferson Lab using the CEBAF large acceptance spectrometer and a longitudinally polarized 14NH3 target. The three asymmetries were measured in 165 four-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of generalized parton distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H and ~H Compton form factors, which give insight into the electric and axial charge distributions of valence quarks in the proton

Measurement of the Generalized Form Factors Near Threshold Via ˠ*p→nπ+ at High Q²

We report on the first measurement of the F2 structure function of the neutron from the semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to ≲ 100  MeV/c and their angles to ≳ 100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The Fn2 data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65 \u3c Q2 2, with uncertainties from nuclear corrections estimated to be less than a few percent. These measurements provide the first determination of the neutron to proton structure function ratio Fn2 / Fp2 at 0.2 ≲ x ≲0.8 with little uncertainty due to nuclear effects