Beam-helicity asymmetries in double-charged-pion photoproduction on the proton.

Beam-helicity asymmetries for the two-pion-photoproduction reaction gammap-->ppi(+)pi(-) have been studied for the first time in the resonance region for center-of-mass energies between 1.35 and 2.30 GeV. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer using circularly polarized tagged photons incident on an unpolarized hydrogen target. Beam-helicity-dependent angular distributions of the final-state particles were measured. The large cross-section asymmetries exhibit strong sensitivity to the kinematics and dynamics of the reaction. The data are compared with the results of various phenomenological model calculations, and show that these models currently do not provide an adequate description for the behavior of this new observable.

First measurement of transferred polarization in the exclusive ep-->e'K+Lambda--> reaction.

The first measurements of the transferred polarization for the exclusive e-->p-->e(')K+Lambda--> reaction have been performed at Jefferson Laboratory using the CLAS spectrometer. A 2.567 GeV beam was used to measure the hyperon polarization over Q2 from 0.3 to 1.5 (GeV/c)(2), W from 1.6 to 2.15 GeV, and over the full K+ center-of-mass angular range. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A nonrelativistic quark-model interpretation of our data suggests that the ssmacr; quark pair is produced with spins predominantly antialigned. Implications for the validity of the most widely used quark-pair creation operator are discussed.

Spin, parity, and nature of the xi(1620) resonance.

Using a unitary extension of chiral perturbation theory with a lowest-order s-wave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness S=-2 sector. A scattering-matrix pole is found around 1605 MeV which corresponds to an s-wave Xi resonance with J(P)=1/2(-). We identify this resonance with the Xi(1620) state, quoted by the Particle Data Group with I=1/2 but with unknown spin and parity. The addition of the S=-2 state to the recently computed Lambda(1670), Sigma(1620), and N(1535) states completes the octet of J(P)=1/2(-) resonances dynamically generated in this chiral unitary approach.