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

Measurement of the Differential Cross Section for the Reaction γn →π⁻p from Deuterium

We report a measurement of the differential cross section for the γn →π⁻p process from the CLAS detector at Jefferson Laboratory in Hall B for photon energies between 1.0 and 3.5 GeV and pion center-of-mass (c.m.) angles ( c.m.) between 50° and 115°. We confirm a previous indication of a broad enhancement around a c.m. energy (√ s) of 2.1 GeV at c.m. = 90° in the scaled differential cross section s7 dσdt and a rapid falloff in a center-of-mass energy region of about 400 MeV following the enhancement. Our data show an angular dependence of this enhancement as the suggested scaling region is approached for c.m. from 70° degrees to 105°

Electroexcitation of Nucleon Resonances from CLAS Data on Single Pion Electroproduction

We present results on the electroexcitation of the low mass resonances Δ(1232)P33 , N(1440)P11 , N(1520)D13 , and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Δ(1232)P33 show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2⩽6 GeV2 . For the Roper resonance, N(1440)P11, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ∗p → N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision. For the N(1535)S11, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D13 clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 \u3e1 GeV2, confirming a long-standing prediction of the constituent quark model

Differential Cross Sections for + p → K⁺ + Y for Λ and Σ⁰ Hyperons

High-statistics cross sections for the reactions + p → K⁺ + Λ and + p → K⁺ + Σ⁰ have been measured using CLAS at Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85 \u3c cos θ Kc.m. \u3c +0.95. In the K⁺ + Λ channel we confirm a resonance-like structure near W=1.9 GeV at backward kaon angles. The position and width of this structure change with angle, indicating that more than one resonance is likely playing a role. The K⁺ + Λ channel at forward angles and all energies is well described by a t-channel scaling characteristic of Regge exchange, whereas the same scaling applied to the K⁺ + Σ⁰ channel is less successful. Several existing theoretical models are compared to the data, but none provide a good representation of the results

Measurement of Coherent ɸ-Meson Photoproduction from the Deuteron at Low Energies

The cross section and decay angular distributions for the coherent ɸ-meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t = (pᵧ - pɸ)2 =-2 GeV2/c2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a rescattering model. In a framework of vector meson dominance, the data are consistent with the total ɸ-N cross section σɸN at about 10 mb. If vector meson dominance is violated, a larger σɸN is possible by introducing a larger t slope for the ɸN → ɸN process than that for the γN → ɸN process. The decay angular distributions of the phi are consistent with helicity conservation

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, GpE, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of GpEfrom 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 (ε) and momentum transfer (Q2) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ε at Q2=1.45 GeV2. This measurement is consistent with the size of the form factor discrepancy at Q2 ≈ 1.75 GeV2and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV2

Electron Scattering from High-Momentum Neutrons in Deuterium

We report results from an experiment measuring the semiinclusive reaction 2H(e,e′ps) in which the proton ps is moving at a large angle relative to the momentum transfer. If we assume that the proton was a spectator to the reaction taking place on the neutron in deuterium, the initial state of that neutron can be inferred. This method, known as spectator tagging, can be used to study electron scattering from high-momentum (off-shell) neutrons in deuterium. The data were taken with a 5.765 GeV electron beam on a deuterium target in Jefferson Laboratory\u27s Hall B, using the CEBAF large acceptance spectrometer. A reduced cross section was extracted for different values of final state missing mass W∗, backward proton momentum →ps, and momentum transfer Q2. The data are compared to a simple plane wave impulse approximation (PWIA) spectator model. A strong enhancement in the data observed at transverse kinematics is not reproduced by the PWIA model. This enhancement can likely be associated with the contribution of final state interactions (FSI) that were not incorporated into the model. Within the framework of the simple spectator model, a “bound neutron structure function” Feff2n was extracted as a function of W∗ and the scaling variable x∗ at extreme backward kinematics, where the effects of FSI appear to be smaller. For ps \u3e 0.4 GeV/c, where the neutron is far off-shell, the model overestimates the value of Feff2n in the region of x∗ between 0.25 and 0.6. A dependence of the bound neutron structure function on the neutron\u27s “off-shell-ness” is one possible effect that can cause the observed deviation

Photodisintegration of \u3csup\u3e4\u3c/sup\u3eHE into \u3ci\u3ep\u3c/i\u3e+\u3ci\u3et\u3c/i\u3e

The two-body photodisintegration of 4He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid 4He target. This is the first measurement of the photodisintegration of 4He above 0.4 GeV. The differential cross sections for the γ 4He →pt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6−1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of 3He that demonstrated the great importance of three-body mechanisms in the energy region 0.5−0.8 GeV

Measurement of Direct fₒ(980) Photoproduction on the Proton

We report on the results of the first measurement of exclusive f0(980) meson photoproduction on protons for Eγ = 3.0–3.8  GeV and −t = 0.4–1.0  GeV2 . Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The resonance was detected via its decay in the π+π− channel by performing a partial wave analysis of the reaction γp→pπ+π− . Clear evidence of the f0(980) meson was found in the interference between P and S waves at M π+π− ∼1  GeV. The S -wave differential cross section integrated in the mass range of the f0(980) was found to be a factor of about 50 smaller than the cross section for the ρ meson. This is the first time the f0(980) meson has been measured in a photoproduction experiment