Search

We review recent developments in strangeness photo- and electro- production off the proton and neutron, as investigated using CLAS in Hall B at Jefferson Lab. By measuring sufficient spin observables one can decompose the reaction mechanism into elementary amplitudes. We discuss progress toward this end in recent data from CLAS, including cross sections and spin observables. We next discuss new results on the mass distribution of the Λ(1405), which shows signs of being a composite meson-baryon object of mixed isospin. The work on other hyperons such as the Ξ resonances will be mentioned, and future prospects for the CLAS program outlined. [ABSTRACT FROM AUTHOR]

The Excited Baryon Analysis Center (EBAC), which is associated with the Theory Group at Jefferson Laboratory, was initiated in 2006. Its main goal is to extract and interpret properties of nucleon resonances (N*) from the world data of meson production reactions induced by pions, photons and electrons. We review the main accomplishments of the center since then and sketch its near future perspectives. [ABSTRACT FROM AUTHOR]

The CLAS Collaboration is using the p(e, e′K+p)π- reaction to measure the induced polarization of the electroproduced Λ(1116). In this experiment a 5.499-GeV electron beam was incident upon an unpolarized liquid-hydrogen target. The CEBAF Large Acceptance Spectrometer (CLAS) was used to detect the scattered electron, the kaon, and the decay proton from the Λ hyperon. CLAS allowed for a large kinematic acceptance in Q2(0.75≤Q2≤3.5 GeV2) and W(1.6≤W≤3.0 GeV), as well as the kaon center-of-mass scattering angle. The goal is to map out the kinematic dependencies for the induced polarization in order to provide new constraints on models of K-hyperon production. Along with previously published photo- and electroproduction cross sections and polarization observables from CLAS, LEPS, SAPHIR, and GRAAL, these new induced polarization data are needed in coupled-channel analyses to search for previously unobserved s-channel resonances. Preliminary polarization results are presented. [ABSTRACT FROM AUTHOR]

Exclusive pi0 electroproduction from nucleons at large photon virtuality can be described in terms of Generalized Parton Distributions, particularly the chiral odd subset. Chiral odd GPDs are related to transversity and can be accessed experimentally for various special choices of observables. This is accomplished by choosing C-parity odd and chiral odd combinations of t-channel exchange quantum numbers. These GPDs are calculated in a spectator model, and are constrained by boundary functions. Alternatively, the production amplitudes correspond to C-odd Regge exchanges with final state interactions. The helicity structure of the virtual photoproduction amplitudes provides relations between the partonic description via GPDs and the hadronic, Regge description of C-odd processes, all via quark helicity flip. GPDs, in general, are analytic functions of energy variables. Their integrals over unobservable parton momenta thereby satisfy Dispersion Relations (DR). Using DR’s could allow the real part of the amplitudes, the integrated GPDs, to be extracted from the more easily measured imaginary parts. However, at non-zero momentum transfer DRs require integration over unphysical regions of the variables. We show that the relevant unphysical region of the non-forward DRs is considerable. This will vitiate the efforts to avoid the actual measurement of the real parts more directly. [ABSTRACT FROM AUTHOR]

In a nonperturbative approach we have made extensive and comprehensive calculations of the processes of vector meson electroproduction. We here confront our results with recent data on the elastic processes of ρ and [lowercase_phi_synonym] electroproduction. The description is unified for all vector mesons, without free parameters, covering 5 scales of magnitude in cross sections, exhibiting a universality that is determined by properties of the overlap of photon and vector meson wave functions. [ABSTRACT FROM AUTHOR]

Exclusive [lowercase_phi_synonym] electroproduction and proton-dissociative diffractive photoproduction of J/ψ mesons has been studied in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 65 pb-1 and 112 pb-1, respectively. The [lowercase_phi_synonym] cross section was measured as a function of Q2, W, t and helicity angle θh in the kinematic range of 2 < Q2 < 70 GeV2, 35 < W < 145 GeV and |t| < 0.6 GeV2. The J/ψ photoproduction cross section was measured for |t| up to 20 GeV2 in the range of 50 < W < 150 GeV as a function of |t| and W. Both measurements are compared to other vector meson cross sections and predictions from theoretical models. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]