We have measured the induced polarization of the ${Lambda}(1116)$ in the reaction $eprightarrow eK^+{Lambda}$, detecting the scattered $e$ and $K^+$ in the final state along with the proton from the decay $Lambdarightarrow ppi^-$.The present study us
ed the CEBAF Large Acceptance Spectrometer (CLAS), which allowed for a large kinematic acceptance in invariant energy $W$ ($1.6leq W leq 2.7$ GeV) and covered the full range of the kaon production angle at an average momentum transfer $Q^2=1.90$ GeV$^2$.In this experiment a 5.50 GeV electron beam was incident upon an unpolarized liquid-hydrogen target. We have mapped out the $W$ and kaon production angle dependencies of the induced polarization and found striking differences from photoproduction data over most of the kinematic range studied. However, we also found that the induced polarization is essentially $Q^2$ independent in our kinematic domain, suggesting that somewhere below the $Q^2$ covered here there must be a strong $Q^2$ dependence. Along with previously published photo- and electroproduction cross sections and polarization observables, these data are needed for the development of models, such as effective field theories, and as input to coupled-channel analyses that can provide evidence of previously unobserved $s$-channel resonances.
The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant
model dependence, and there is limited direct experimental evidence for such corrections. We present the results of a new experimental technique for making direct $e^pm p$ comparisons, which has the potential to make precise measurements over a broad range in $Q^2$ and scattering angles. We use the Jefferson Lab electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton. The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of $Q^2$ and scattering angle. Nonetheless, this measurement yields a data sample for $e^pm p$ with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. The final ratio of positron to electron scattering: $R=1.027pm0.005pm0.05$ for $<Q^2>=0.206$ GeV$^2$ and $0.830leq epsilonleq 0.943$.
The reaction $gamma pto K^+Lambda$ has been investigated over the center-of-momentum energy, $W$, range from threshold up to 2.2 GeV in a tree-level effective Lagrangian model that incorporates most of the well-established baryon resonances with spin
s equal to or below 5/2. Four less well-established nucleon resonances of higher mass are also included. The fitted parameters consist, for each resonance included, of the products of the coupling strengths at the electromagnetic and strong interaction vertices and, for the less-established nucleon resonances, the total decay width. For the well-established nucleon resonances, the energy and momentum dependence of the widths is treated within a dynamical model that is normalized to give the empirical decay branching ratios on the resonance mass shells. For the less-established resonances, the total decay width is treated as a single parameter independent of the reaction kinematics. The model is used to fit recent data for the unpolarized differential cross section (CLAS), the induced hyperon polarization asymmetry, $P$ (CLAS, GRAAL, and SAPHIR), the beam spin asymmetry, $Sigma$ (LEPS), and the double polarization observables $C_x$ and $C_z$ (CLAS). Two different fits were obtained: one that incorporates SU(3) symmetry constraints on the Born contributions to the reaction amplitude and one in which these constraints are relaxed. Explicit numerical results are given only for the first fit since the two fits gave nearly identical results for the observables and the $chi^2$ per degree of freedom obtained with the second fit was only marginally better than that of the first fit ($<1%$ better). Results are presented for the fitted observables at several different energies and center-of-momentum (c.m.) frame kaon angles.
