Photodisintegration of He4 into p+t

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 γ4He→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 ep→epπ0 beam spin asymmetries above the resonance region

The beam spin asymmetry (BSA) in the exclusive reaction e→p→epπ0 was measured with the CEBAF 5.77 GeV polarized electron beam and Large Acceptance Spectrometer (CLAS). The xB,Q2,t, and ϕ dependences of the π0 BSA are presented in the deep inelastic regime. The asymmetries are fitted with a sinϕ function and their amplitudes are extracted. Overall, they are of the order of 0.04–0.11 and roughly independent of t. This is the signature of a nonzero longitudinal-transverse interference. The implications concerning the applicability of a formalism based on generalized parton distributions, as well as the extension of a Regge formalism at high photon virtualities, are discussed

Polarized structure function σLT′ for 1H(e→,e′K+)Λ in the nucleon resonance region

The first measurements of the polarized structure function σLT′ for the reaction 1H(e→,e′K+)Λ in the nucleon resonance region are reported. Measurements are included from threshold up to W=2.05 GeV for central values of Q2 of 0.65 and 1.00 GeV2, and nearly the entire kaon center-of-mass angular range. σLT′ is the imaginary part of the longitudinal-transverse response and is expected to be sensitive to interferences between competing intermediate s-channel resonances, as well as resonant and nonresonant processes. The results for σLT′ are comparable in magnitude to previously reported results from CLAS for σLT, the real part of the same response. An intriguing sign change in σLT′ is observed in the high Q2 data at W≈1.9 GeV. Comparisons to several existing model predictions are shown

Differential cross sections for γ+p→K++Y for Λ and Σ0 hyperons

High-statistics cross sections for the reactions γ+p→K++Λ and γ+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 −0.8

Search for Medium Modifications of the ρ Meson

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, ρ, ω, and ϕ, are observed via their decay to e+e−, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the ρ meson. The ρ mass spectrum is extracted from the data on various nuclei,2 H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson

First Measurement of the Double Spin Asymmetry in e⃗ p⃗ →e′π+n in the Resonance Region

The double spin asymmetry in the ep→e′π+n reaction has been measured for the first time in the resonance region for four-momentum transfer Q2=0.35–1.5GeV2. Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH3 target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity- 1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q2 of the helicity asymmetry A1 for the F15(1680) resonance than expected from the analysis of the unpolarized data

Coherent photoproduction of π+ from 3He

We have measured the differential cross section for the γHe3→π+t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid 3He target. The differential cross sections for the γ3He→π+t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model

The ep→e′pη Reaction at and above the S11(1535) Baryon Resonance

New cross sections for the reaction ep→epη are reported for total center of mass energy W=1.5–1.86GeV and invariant momentum transfer Q2=0.25–1.5(GeV/c)2. This large kinematic range allows extraction of important new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. Newly observed structure at W∼1.65GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance

First measurement of beam-recoil observables C(x) and C(z) in hyperon photoproduction

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions γ⃗+p→K++Λ⃗ and γ⃗+p→K++Σ⃗0. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) detector at the Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85θK+c.m.Cz, was found to be near unity for a wide range of energy and kaon production angles. The associated transverse polarization coefficient Cx is smaller than Cz by a roughly constant difference of unity. Most significantly, the total Λ polarization vector, including the induced polarization P, has magnitude consistent with unity at all measured energies and production angles when the beam is fully polarized. For the Σ0 this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results

Measurement of the proton spin structure function g1(x;Q2) for Q2 from 0.15-GeV2 to 1.6-GeV2 with CLAS

Double-polarization asymmetries for inclusive ep scattering were measured at Jefferson Lab using 2.6 and 4.3 GeV longitudinally polarized electrons incident on a longitudinally polarized NH3 target in the CLAS detector. The polarized structure function g1(x,Q2) was extracted throughout the nucleon resonance region and into the deep inelastic regime, for Q2=0.15–1.64   GeV2. The contributions to the first moment Γ1(Q2)=∫g1(x,Q2) dx were determined up to Q2=1.2   GeV2. Using a parametrization for g1 in the unmeasured low x regions, the complete first moment was estimated over this Q2 region. A rapid change in Γ1 is observed for Q2\u3c1   GeV2, with a sign change near Q2=0.3   GeV2, indicating dominant contributions from the resonance region. At Q2=1.2   GeV2 our data are below the perturbative QCD evolved scaling value