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-Further, for the charged mode, we differentiate between the cases where the final K − track 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 + * channels with pKK 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.
Background: Much less is known about neutron structure than that of the proton due to the absence of free neutron targets. Neutron information is usually extracted from data on nuclear targets such as deuterium, requiring corrections for nuclear binding and nucleon off-shell effects. These corrections are model dependent and have significant uncertainties, especially for large values of the Bjorken scaling variable x. As a consequence, the same data can lead to different conclusions, for example, about the behavior of the d quark distribution in the proton at large x.Purpose: The Barely Off-shell Nucleon Structure (BONuS) experiment at Jefferson Lab measured the inelastic electron-deuteron scattering cross section, tagging spectator protons in coincidence with the scattered electrons. This method reduces nuclear binding uncertainties significantly and has allowed for the first time a (nearly) model-independent extraction of the neutron structure function F2(x, Q 2 ) in the resonance and deep-inelastic regions.Method: A novel compact radial time projection chamber was built to detect protons with momentum between 70 and 150 MeV/c and over a nearly 4π angular range. For the extraction of the free-neutron structure function F n 2 , spectator protons at backward angles (> 100• relative to the momentum transfer) and with momenta below 100 MeV/c were selected, ensuring that the scattering took place on a nearly free neutron. The scattered electrons were detected with Jefferson Lab's CLAS spectrometer, with data taken at beam energies near 2, 4 and 5 GeV. Results:The extracted neutron structure function F n 2 and its ratio to the inclusive deuteron structure function F d 2 are presented in both the resonance and deep-inelastic regions for momentum transfer squared Q 2 between 0.7 and 5 GeV 2 /c 2 , invariant mass W between 1 and 2.7 GeV/c 2 , and Bjorken x between 0.25 and 0.6 (in the DIS region). The dependence of the semi-inclusive cross section on the spectator proton momentum and angle is investigated, and tests of the spectator mechanism for different kinematics are performed.Conclusions: Our data set on the structure function ratio F n 2 /F d 2 can be used to study neutron resonance excitations, test quark-hadron duality in the neutron, develop more precise parametrizations of structure functions, as well as investigate binding effects (including possible mechanisms for the nuclear EMC effect) and provide a first glimpse of the asymptotic behavior of d/u at x → 1.
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the eta photoproduction reaction gamma p -> eta p. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich-Bonn model to examine the case for the existence of a narrow N* resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the Eobservable from that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances. (C) 2016 Published by Elsevier B.V
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