Measurement of the Beam Asymmetry Σ and the Target Asymmetry T in the Photoproduction of ω Mesons off the Proton using CLAS at Jefferson Laboratory

he photoproduction of ω mesons off the proton has been studied in the reaction γp → pω using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry T has been measured in photoproduction from the decay ω → π+π−π0, using a transversely polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant nonzero values are observed for these asymmetries, reaching about 30–40% in the third-resonance region. New measurements for the photon-beam asymmetry Σ are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional ω photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from s-channel resonance production were found in addition to t-channel exchange processes

Target and Beam Target Spin Asymmetries in Exclusive π+ and π- Electroproduction with 1.6 to 5.7 GeV Electrons

Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive π+ and quasiexclusive π- electroproduction were obtained from scattering of 1.6- to 5.7-GeV longitudinally polarized electrons from longitudinally polarized protons (for π+) and deuterons (for π-) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is 1.1 \u3c W \u3c 2.6 GeV and 0.05 \u3c Q2 \u3c 5 GeV2, with good angular coverage in the forward hemisphere. The asymmetry results were divided into approximately 40 000 kinematic bins for π+ from free protons and 15 000 bins for π- production from bound nucleons in the deuteron. The present results are found to be in reasonable agreement with fits to previous world data for W \u3c 1.7 GeV and Q2 \u3c 0.5 GeV2, with discrepancies increasing at higher values of Q2, especially for W \u3e 1.5 GeV. Very large target-spin asymmetries are observed for W \u3e 1.6 GeV. When combined with cross-section measurements, the present results can provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q2, for resonances with masses as high as 2.3 GeV

Transverse Polarization of Σ+ (1189) in Photoproduction on a Hydrogen Target in CLAS

Experimental results on the Σ+(1189) hyperon transverse polarization in photoproduction on a hydrogen target using the CLAS detector at Jefferson Laboratory are presented. The Σ+(1189) was reconstructed in the exclusive reaction γ+p→K0S+Σ+(1189) via the Σ+→pπ0 decay mode. The K0S was reconstructed in the invariant mass of two oppositely charged pions with the π0 identified in the missing mass of the detected pπ+π− final state. Experimental data were collected in the photon energy range Eγ=1.0 –3.5 GeV (√s range 1.66–2.73 GeV). We observe a large negative polarization of up to 95% . As the mechanism of transverse polarization of hyperons produced in unpolarized photoproduction experiments is still not well understood, these results will help to distinguish between different theoretical models on hyperon production and provide valuable information for the searches of missing baryon resonances

Exploring the Structure of the Bound Proton With Deeply Virtual Compton Scattering

In the past two decades, deeply virtual Compton scattering of electrons has been successfully used to advance our knowledge of the partonic structure of the free proton and investigate correlations between the transverse position and the longitudinal momentum of quarks inside the nucleon. Meanwhile, the structure of bound nucleons in nuclei has been studied in inclusive deep-inelastic lepton scattering experiments off nuclear targets, showing a significant difference in longitudinal momentum distribution of quarks inside the bound nucleon, known as the EMC effect. In this Letter, we report the first beam spin asymmetry (BSA) measurement of exclusive deeply virtual Compton scattering off a proton bound in 4He. The data used here were accumulated using a 6 GeV longitudinally polarized electron beam incident on a pressurized 4He gaseous target placed within the CLAS spectrometer in Hall-B at the Thomas Jefferson National Accelerator Facility. The azimuthal angle (ϕ) dependence of the BSA was studied in a wide range of virtual photon and scattered proton kinematics. The Q², xB, and t dependencies of the BSA on the bound proton are compared with those on the free proton. In the whole kinematical region of our measurements, the BSA on the bound proton is smaller by 20% to 40%, indicating possible medium modification of its partonic structure

Measurements of ep → e′π+π-p′ Cross Sections with CLAS at 1.40 GeV \u3c W \u3c 2.0 GeV and 2.0 GeV² \u3c Q² \u3c 5.0 GeV²

This paper reports new exclusive cross sections for ep → e′π+π-p′ using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities 2.0GeV2 2 \u3c 5.0GeV2 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 Q2. These data considerably extend the kinematic reach of previous measurements. Exclusive ep → e′π+π-p′ cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV

Target and Beam-Target Spin Asymmetries in Exclusive Pion Electroproduction for Q² \u3e 1 GeV² . II. ep→ eπºp

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π0 electroproduction reaction γ∗p→pπ0, expanding an analysis of the γ∗p→nπ+ reaction from the same experiment. 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 ranges covered are 1.1 \u3c W \u3c 3 GeV and 1 \u3c Q2\u3c 6 GeV2 . Results were obtained for about 5700 bins in W, Q2, cos(θ∗) , and ϕ∗. The beam-target asymmetries were found to generally be greater than zero, with relatively modest ϕ∗ dependence. The target asymmetries exhibit very strong ϕ∗ dependence, with a change in sign occurring between results at low W and high W , in contrast to π+ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for W \u3c 1.6 GeV, but significant differences are seen at higher W . When combined with cross-sectional measurements, as well as π+ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q2, for resonances with masses as high as 2.4 GeV

Exclusive η Electroproduction at W \u3e 2 GeV with CLAS and Transversity Generalized Parton Distributions

The cross section of the exclusive η electroproduction reaction ep -\u3e e\u27p\u27 η was measured at Jefferson Laboratory with a 5.75 GeV electron beam and the CLAS detector. Differential cross sections d4σ /dtdQ2 dxBd φη and structure functions σU=σT + εσL, σTT, and σLT, as functions of t, were obtained over a wide range of Q2 and xB. The η structure functions are compared with those previously measured for π0 at the same kinematics. At low t, both π0 and eta are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The π0 and η data, when taken together, can facilitate the flavor decomposition of the transversity GPDs

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

Measurement of Deeply Virtual Compton Scattering Off \u3csup\u3e4\u3c/sup\u3eHe with the CEBAF Large Acceptance Spectrometer at Jefferson Lab

We report on the measurement of the beam spin asymmetry in the deeply virtual Compton scattering off 4He using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 6 GeV longitudinally polarized electron beam incident on a pressurized 4He gaseous target. We detail the method used to ensure the exclusivity of the measured reactions, in particular the upgrade of CLAS with a radial time projection chamber to detect the low-energy recoiling 4He nuclei and an inner calorimeter to extend the photon detection acceptance at forward angles. Our results confirm the theoretically predicted enhancement of the coherent (e4He→e′4Heγ′) beam spin asymmetries compared to those observed on the free proton, while the incoherent (e4He→ e′p′γ′X′) asymmetries exhibit a 30% suppression. From the coherent data, we were able to extract, in a model-independent way, the real and imaginary parts of the only 4He Compton form factor, HA, leading the way toward 3D imaging of the partonic structure of nuclei

Target and Double Spin Asymmetries of Deeply Virtual π\u3csup\u3e0\u3c/sup\u3e Production With a Longitudinally Polarized Proton Target and CLAS

The target and double spin asymmetries of the exclusive pseudoscalar channel e→p→→epπ0 were measured for the first time in the deep-inelastic regime using a longitudinally polarized 5.9 GeV electron beam and a longitudinally polarized proton target at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS). The data were collected over a large kinematic phase space and divided into 110 four-dimensional bins of Q2, xB, −t and ϕ. Large values of asymmetry moments clearly indicate a substantial contribution to the polarized structure functions from transverse virtual photon amplitudes. The interpretation of experimental data in terms of generalized parton distributions (GPDs) provides the first insight on the chiral-odd GPDs HT and ET, and complement previous measurements of unpolarized structure functions sensitive to the GPDs HT and ET. These data provide a crucial input for parametrizations of essentially unknown chiral-odd GPDs and will strongly influence existing theoretical calculations based on the handbag formalism. © 2017 The Author(s