Browsing by Author "Anderson, M. D."

Unpolarized and beam-polarized fourfold cross sections ðd4σ=dQ2dxBdtdϕÞ for the ep → e0 p0 γ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 (Q2; xB; t) bins over the widest phase space ever explored in the valencequark region. Several models of generalized parton distributions (GPDs) describe the data well at most of our kinematics. This increases our confidence that we understand the GPD H, expected to be the dominant contributor to these observables. Through a leading-twist extraction of Compton form factors, these results support the model predictions of a larger nucleon size at lower quark-momentum fraction xB.

The beam-spin asymmetry, , for the reaction γ d → pn has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins, between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, θc.m., between 25◦ and 160◦. These are the first measurements of beam-spin asymmetries at θc.m. = 90◦ for photon-beam energies above 1.6 GeV, and the first measurements for angles other than θc.m. = 90◦. The angular and energy dependence of is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.

A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6 GeV electron beam, a longitudinally polarized proton target, and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep → e0 p0 γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q2, xB, t, and ϕ, for 166 four-dimensional bins. In the framework of generalized parton distributions, at leading twist the t dependence of these asymmetries provides insight into the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center. These results also bring important and necessary constraints for the existing parametrizations of chiral-even generalized parton distributions.

The reaction 𝛾+𝑝→𝐾++𝛴+𝜋 was used to determine the invariant mass distributions or “line shapes” of the 𝛴+⁢𝜋−, 𝛴−⁢𝜋+, and 𝛴0⁢𝜋0 final states, from threshold at 1328 MeV/𝑐2 through the mass range of the 𝛬⁡(1405) and the 𝛬⁡(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95<𝑊<2.85 GeV. The three mass distributions differ strongly in the vicinity of the 𝐼=0 𝛬⁡(1405), indicating the presence of substantial 𝐼=1 strength in the reaction. Background contributions to the data from the 𝛴0⁡(1385) and from 𝐾*⁢𝛴 production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the Math output error threshold. A best fit to all the data was obtained after including a phenomenological 𝐼=1, 𝐽𝑃=1/2− amplitude with a centroid at 1394±20 MeV/𝑐2 and a second 𝐼=1 amplitude at 1413±10 MeV/𝑐2. The centroid of the 𝐼=0 𝛬⁡(1405) strength was found at the 𝛴⁢𝜋 threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/𝑐2.

We report the measurement of near-threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range Q2 from 2 to 4.5 GeV2 and W from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles E0+ and S0+ in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors Gπ0p 1 (Q2) and Gπ0p 2 (Q2). The data are compared to these generalized form factors and the results for Gπ0p 1 (Q2) are found to be in good agreement with the LCSR predictions, but the level of agreement with Gπ0p 2 (Q2) is poor.

The inclusive polarized structure functions of the proton and deuteron, g1p and g1d, 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 lab angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual DIS kinematics, Q^2>1 GeV^2 and the final-state invariant mass W>2 GeV, the ratio of polarized to unpolarized structure functions g1/F1 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 QCD, 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.