Pion and photon induced reactions on the nucleon in a unitary model

We present a relativistic calculation of pion scattering, pion photoproduction and Compton scattering on the nucleon in the energy region of the Delta-resonance (upto 450 MeV photon lab energy), in a unified framework which obeys the unitarity constraint. It is found that the recent data on the cross section for nucleon Compton scattering determine accurately the parameters of the electromagnetic nucleon-Delta coupling. The pion-photoproduction partial-wave amplitudes, calculated with parameters fitted to the pion-nucleon and Compton scattering, agree well with the recent Amdt analysis.</p

Compton scattering on the proton and light nuclei in the Delta-resonance region

Microscopic calculations of Compton scattering on the free proton and light nuclei are presented. For the description of Compton scattering on the proton the conventional K-matrix approach and the "Dressed K-Matrix" model are introduced. The latter approach can be used to calculate polarizabilities as well as Compton scattering for photon energies upto 1 GeV since it obeys the symmetry properties which are appropriate in the different energy regions. In particular, crossing symmetry, gauge invariance and unitarity are satisfied. The extent of violation of analyticity (causality) is used as an expansion parameter. Coherent Compton scattering on light nuclei at 200-300 MeV is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the Delta-resonance. Modifications of the properties of the Delta-resonance due to the nuclear medium are accounted for through the self-energy operator of the Delta. The dominant medium effects such as the Pauli blocking effects in the decay width, effective nucleon mass and particle-hole excitations in the pion propagator axe consistently included in nuclear matter

Complete Anatomy of B -> K*ll and its angular distribution

We present a complete and optimal set of observables for the exclusive 4-body B meson decay B -> K*(->K pi) l+l- in the low dilepton mass region, that contains a maximal number of clean observables. This basis of observables is built in a systematic way. We show that all the previously defined observables and any observable that one can construct, can be expressed as a function of this basis. This set of observables contains all the information that can be extracted from the angular distribution in the cleanest possible way. We provide explicit expressions for the full and the uniangular distributions in terms of this basis. The conclusions presented here can be easily extended to the large-q^2 region. We study the sensitivity of the observables to right-handed currents and scalars. Finally, we present for the first time all the symmetries of the full distribution including massive terms and scalar contributions.Comment: 37 pages, 12 Figures. Corrected typo in Eqs. (29) and (44). Results and conclusions unchange

Cross sections and electromagnetic response functions for radiative proton capture in pd -&gt; He-3+e(+)e(-)

Exclusive differential cross sections of virtual-photon radiative proton capture in pd --> He-3 + e(+)e(-) at a proton energy of 190 MeV have been measured for the first time. The leptonic-angle dependence of the measured cross section is exploited in order to determine the electromagnetic response functions in the time-like region. The data are compared to a relativistic gauge-invariant impulse approximation and a Faddeev calculation. Differences between theoretical predictions and data are discussed in terms of enhanced magnetic radiation possibly originating from virtual Delta excitations

Photoproduction of electron-positron pairs on the proton in the resonance region

Production of lepton pairs in the gamma p-->e(-)e(+)p reaction is studied at photon energies up to 1 GeV. We show that even if lepton charge is not measured there are extensive kinematical regimes where the nuclear process dominates over the Bethe-Heitler contribution. The decomposition is performed of the unpolarized cross section for virtual Compton scattering in terms of response functions. These are expressed in terms of a polarization density matrix of the virtual photon and are shown to be sensitive to the properties of baryon resonances in the first and the second resonance regions. In the analysis a unitary K-matrix model is used, based on an effective Lagrangian including nucleon, mesons, and baryon resonances with masses up to 1.7 GeV. Results of the model are compared with data for real-photon Compton scattering and predictions are given for observables in photoproduction of lepton pairs