Target and Beam-Target Spin Asymmetries in Exclusive Pion Electroproduction for Q\u3csup\u3e2\u3c/sup\u3e \u3e 1 GeV\u3csup\u3e2\u3c/sup\u3e . I. ep → eπ\u3csup\u3e+\u3c/sup\u3en

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π+ electroproduction reaction γ∗p→nπ+. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is 1.1 \u3c W \u3c 3 GeV and 1 \u3c Q2 \u3c 6 GeV2. Results were obtained for about 6000 bins in W, Q2, cos(θ∗), and φ∗. Except at forward angles, very large target-spin asymmetries are observed over the entire W region. Reasonable agreement is found with phenomenological fits to previous data for W \u3c 1.6 GeV, but very large differences are seen at higher values of W. A generalized parton distributions (GPD)-based model is in poor agreement with the data. When combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q2, for resonances with masses as high as 2.4 GeV. © 2017 American Physical Society

Differential cross sections and polarization observables from CLAS K* photoproduction and the search for new N* states

The reaction gamma p -\u3e K*(+)Lambda was measured using the CLAS detector for photon energies between the threshold and 3.9 GeV at the Thomas Jefferson National Accelerator Facility. For the first time, spin-density matrix elements have been extracted for this reaction. Differential cross sections, spin density matrix elements, and the Lambda recoil polarization are compared with theoretical predictions using the BnGa partial wave analysis. The main result is the evidence for significant contributions from N(1895) 1/2(-) and N(2100) 1/2(+) to the reaction. Branching ratios for decays into K*Lambda for these resonances and further resonances are reported. (C) 2017 The Author. Published by Elsevier B.V

Exclusive eta electroproduction at W \u3e 2 GeV with CLAS and transversity generalized parton distributions

The cross section of the exclusive eta electroproduction reaction ep -\u3e e\u27p\u27eta was measured at Jefferson Laboratorywith a 5.75 GeV electron beam and the CLAS detector. Differential cross sections d(4) sigma/dtdQ(2) dx(B)d phi(eta) and structure functions sigma(U) = sigma(T) + epsilon sigma(L), sigma(TT), and sigma(LT), as functions of t, were obtained over a wide range of Q(2) and x(B). The eta structure functions are compared with those previously measured for pi(0) at the same kinematics. At low t, both pi(0) and eta are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The pi(0) and eta data, when taken together, can facilitate the flavor decomposition of the transversity GPDs

Measurement of the Helicity Asymmetry \u3cem\u3eE\u3c/em\u3e in \u3cem\u3eω\u3c/em\u3e → \u3cem\u3eπ\u3c/em\u3e\u3csup\u3e+\u3c/sup\u3e\u3cem\u3eπ\u3c/em\u3e\u3csup\u3e−\u3c/sup\u3e\u3cem\u3eπ\u3c/em\u3e\u3csup\u3e0\u3c/sup\u3e Photoproduction

The double-polarization observable E was studied for the reaction γp → pω using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, ω → π+π−π0, using a circularly polarized tagged-photon beam with energies ranging from the ω threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the 3/2+ partial wave near threshold, which is identified with the subthreshold N(1720)3/2+ nucleon resonance. To describe the entire data set, which consisted of ω differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to nonresonant mechanisms, π exchange in the t channel was found to remain small across the analyzed energy range, while Pomeron t-channel exchange gradually grew from the reaction threshold to dominate all other contributions above W ≈ 2 GeV

Photon beam asymmetry Sigma in the reaction \u3c(gamma)over right arrow\u3e -\u3e p omega for E-gamma=1.152 to 1.876 GeV

Photon beam asymmetry Sigma measurements for omega photoproduction in the reaction gamma (gamma) over right arrow p -\u3e omega p are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between t-channel meson exchange and s- and u-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the Sigma data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the J(P)= 3/2(+) partial wave), as well as the resonant portions of the smaller partial waves with J(P)= 1/2(-), 3/2(-), and 5/2(+). (C) 2017 The Author(s). Published by Elsevier B.V

Target and beam-target spin asymmetries in exclusive pion electroproduction for Q(2) \u3e 1 GeV2. I. ep -\u3e e pi(+)n

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive pi(+) electroproduction reaction. gamma(*) p -\u3e n pi(+). The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is 1.1 \u3c W \u3c 3 GeV and 1 \u3c Q(2) \u3c 6 GeV2. Results were obtained for about 6000 bins in W, Q(2), cos(theta(*)), and phi(*). Except at forward angles, very large target-spin asymmetries are observed over the entire W region. Reasonable agreement is found with phenomenological fits to previous data for W \u3c 1.6 GeV, but very large differences are seen at higher values of W. A generalized parton distributions (GPD)-based model is in poor agreement with the data. When combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q(2), for resonances with masses as high as 2.4 GeV

Measurements of ep -\u3e e\u27 pi(+) pi(-) p\u27 cross sections with CLAS at 1.40 GeV \u3c W \u3c 2.0 GeV and 2.0GeV(2) \u3c Q(2) \u3c 5.0 GeV2

This paper reports new exclusive cross sections for ep -\u3e e\u27 pi(+) pi(-) p\u27 using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities 2.0 GeV2 \u3c Q(2) \u3c 5.0 GeV2 in the center-of-mass energy range 1.4 GeV \u3c W \u3c 2.0 GeV, which covers a large part of the nucleon resonance region. Using a model developed for the phenomenological analysis of electroproduction data, we see strong indications that the relative contributions from the resonant cross sections at W \u3c 1.74 GeV increase with Q(2). These data considerably extend the kinematic reach of previous measurements. Exclusive ep -\u3e e\u27 pi(+) pi(-) p\u27 cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV

Beam-target helicity asymmetry for γ→n→→π−p in the N*resonance region

We report the first beam-target double-polarization asymmetries in the γ þ nðpÞ → π− þ pðpÞ reaction spanning the nucleon resonance region from invariant mass W ¼ 1500 to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the E polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses and have led to significant revisions for several γnN* resonance photocouplings

Search for Θ+(1540)\Theta^+(1540) pentaquark in high statistics measurement of γp→Kˉ0K+n\gamma p \to \bar K^0 K^+ n at CLAS

The exclusive reaction $\gamma p \to \bar K^0 K^+ n$ was studied in the photon energy range between 1.6-3.8 GeV searching for evidence of the exotic baryon $\Theta^+(1540)\to nK^+$. The decay to $nK^+$ requires the assignment of strangeness $S=+1$ to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70 $pb^{-1}$. No evidence for the $\Theta^+$ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and $nK^+$ mass. The 95% CL upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.Comment: Submitted to Physical Review Letter