Search Results (6 total)

The recoil proton polarization was measured in the ²H(e→,e^′p→)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility. The electron kinematics were centered on the quasielastic peak (x_Bj≈1) and included three values of the squared four-momentum transfer, Q²=0.43,1.00 and 1.61 (GeV/c)². For Q²=0.43 and 1.61 (GeV/c)², the missing momentum, p_m, was centered at zero, whereas for Q²=1.00  (GeV/c)² two values of p_m were chosen: 0 and 174  MeV/c. At low p_m, the Q² dependence of the longitudinal polarization, P_z^′, is not well described by a state-of-the-art calculation. Further, at higher p_m, a 3.5σ discrepancy was observed in the transverse polarization, P_x^′. Understanding the origin of these discrepancies is important to confidently extract the neutron electric form factor from the analogous ²H(e→,e^′n→)p experiment.

The first complete measurements of the angular distributions of the two-body deuteron photodisintegration differential cross section at photon energies above 1.6 GeV were performed at the Thomas Jefferson National Accelerator Facility. The results show a persistent forward-backward asymmetry up to E_γ=2.4 GeV, the highest-energy measured in this experiment. The Hard Rescattering and the Quark-Gluon string models are in fair agreement with the results.

We present measurements of the recoil proton polarization for the ¹H(γ→,p→)π⁰ reaction for θ_c.m.^π=60°–135° and for photon energies up to 4.1 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. Various theoretical models are compared with the results. No evidence for hadron helicity conservation is observed. Models that employ factorization are not favored. It appears from the strong angular dependence of the induced polarization at photon energies of 2.5 and 3.1 GeV that a relatively high spin resonance or background amplitude might exist in this energy region.

We present measurements of the ratio of the proton elastic electromagnetic form factors, μ_pG_Ep/G_Mp. The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. [Phys. Rev. Lett. 84, 1398 (2000)], that the form factor ratio decreases significantly from unity above Q²=1 GeV².

We present measurements of the recoil proton polarization for the d(γ→,p→)n reaction at θ_c.m.  =  90° for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization p_y vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer C_x does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(γ,p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.

The ³He(γ,π^±p) reactions were measured simultaneously over a tagged photon energy range of 800<~E_γ<~1120 MeV, well above the Δ resonance region. An analysis was performed to kinematically isolate Δ knockout events from conventional Δ photoproduction events, and a statistically significant excess of π⁺p events was identified, consistent with Δ⁺⁺ knockout. Two methods were used to estimate the ΔNN probability in the ³He ground state corresponding to the observed knockout cross section. The first gave a lower probability limit of 1.5±0.6±0.5 %; the second yielded an upper limit of about 2.6%.