We present measurements of the masses and decay widths of the baryonic states $Sigma_{c}(2455)^{0/++}$ and $Sigma_{c}(2520)^{0/++}$ using a data sample corresponding to an integrated luminosity of 711 fb$^{-1}$ collected with the Belle detector at th
e KEKB $e^{+}e^{-}$ asymmetric-energy collider operating at the $Upsilon(4S)$ resonance. We report the mass differences with respect to the $Lambda_{c}^{+}$ baryon $M(Sigma_{c}(2455)^{0})-M(Lambda_{c}^{+}) = 167.29pm0.01pm0.02$ MeV/$c^{2}$, $M(Sigma_{c}(2455)^{++})-M(Lambda_{c}^{+}) = 167.51pm0.01pm0.02$ MeV/$c^{2}$, $M(Sigma_{c}(2520)^{0})-M(Lambda_{c}^{+}) = 231.98pm0.11pm0.04$ MeV/$c^{2}$, $M(Sigma_{c}(2520)^{++})-M(Lambda_{c}^{+}) = 231.99pm0.10pm0.02$ MeV/$c^{2}$, and the decay widths $Gamma(Sigma_{c}(2455)^{0}) = 1.76pm0.04^{+0.09}_{-0.21}$ MeV/$c^{2}$, $Gamma(Sigma_{c}(2455)^{++}) = 1.84pm0.04^{+0.07}_{-0.20}$ MeV/$c^{2}$, $Gamma(Sigma_{c}(2520)^{0}) = 15.41pm0.41^{+0.20}_{-0.32}$ MeV/$c^{2}$, $Gamma(Sigma_{c}(2520)^{++}) = 14.77pm0.25^{+0.18}_{-0.30}$ MeV/$c^{2}$, where the first uncertainties are statistical and the second are systematic. The isospin mass splittings are measured to be $M(Sigma_{c}(2455)^{++})-M(Sigma_{c}(2455)^{0})=0.22pm0.01pm0.01$ MeV/$c^{2}$ and $M(Sigma_{c}(2520)^{++})-M(Sigma_{c}(2520)^{0})=0.01pm0.15pm0.03$ MeV/$c^{2}$. These results are the most precise to date.
Magnetic excitations in a Ni$_4$ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility ($chi_text{bulk}$) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments ex
hibits three modes at energy transfers of $hbaromega=0.5$, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependences of the inelastic neutron scattering data and $chi_text{bulk}$ can be well reproduced by an effective spin Hamiltonian consisted of intra-molecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and Zeeman term. Under a hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system.
Frustrated magnetic interactions in a quasi-two-dimensional [111] slab of pyrochlore lattice were studied. For uniform nearest neighbor (NN) interactions, we show that the complex magnetic problem can be mapped onto a model with two independent degre
es of freedom, tri-color and binary sign. This provides a systematic way to construct the complex classical spin ground states with collinear and coplanar bi-pyramid spins. We also identify `partial but extended zero-energy excitations amongst the ground states. For nonuniform NN interactions, the coplanar ground state can be obtained from the collinear bi-pyramid spin state by collectively rotating two spins of each tetrahedron with an angle, $alpha$, in an opposite direction. The latter model with $alpha sim 30^circ$ fits the experimental neutron data from SCGO well.
Within a dynamical coupled-channels model which has already been fixed from analyzing the data of the pi N -> pi N and gamma N -> pi N reactions, we present the predicted double pion photoproduction cross sections up to the second resonance region, W
< 1.7 GeV. The roles played by the different mechanisms within our model in determining both the single and double pion photoproduction reactions are analyzed, focusing on the effects due to the direct gamma N -> pi pi N mechanism, the interplay between the resonant and non-resonant amplitudes, and the coupled-channels effects. The model parameters which can be determined most effectively in the combined studies of both the single and double pion photoproduction data are identified for future studies.
Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr$_2$O$_4$ where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spin
s surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a N{e}el state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr$_2$O$_4$ resolves conflicting magnetic interactions.
We have performed a dynamical coupled-channels analysis of available p(e,epi)N data in the region of W < 1.6 GeV and Q^2 < 1.45 (GeV/c)^2. The channels included are gamma^* N, pi N, eta N, and pi pi N which has pi Delta, rho N, and sigma N components
. With the hadronic parameters of the model determined in our previous investigations of pi N --> pi N, pi pi N reactions, we have found that the available data in the considered W < 1.6 GeV region can be fitted well by only adjusting the bare gamma^* N --> N^* helicity amplitudes for the lowest N^* states in P33, P11, S11 and D13 partial waves. The sensitivity of the resulting parameters to the amount of data included in the analysis is investigated. The importance of coupled-channels effect on the p(e,e pi)N cross sections is demonstrated. The meson cloud effects, as required by the unitarity conditions, on the gamma^* N --> N^* form factors are also examined. Necessary future developments, both experimentally and theoretically, are discussed.
We compute dilepton invariant mass spectra from the decays of rho mesons produced by photon reactions off nuclei. Our calculations employ a realistic model for the rho photoproduction amplitude on the nucleon which provides fair agreement with measur
ed cross sections. Medium effects are implemented via an earlier constructed rho propagator based on hadronic many-body theory. At incoming photon energies of 1.5 -3 GeV as used by the CLAS experiment at JLAB, the average density probed for iron targets is estimated at about half saturation density. At the pertinent rho-meson 3-momenta the predicted medium effects on the rho propagator are rather moderate. The resulting dilepton spectra approximately agree with recent CLAS data.
Within the relativistic quantum field theory, we analyze the differences between the $pi N$ reaction models constructed from using (1) three-dimensional reductions of Bethe-Salpeter Equation, (2) method of unitary transformation, and (3) time-ordered
perturbation theory. Their relations with the approach based on the dispersion relations of S-matrix theory are dicusssed.
We study coherent pion production in neutrino-nucleus scattering in the energy region relevant to the recent neutrino oscillation experiments. Our approach is based on the combined use of the Sato-Lee model and the Delta-hole model. Our initial numer
ical results are compared with the recent data from K2K and SciBooNE.
As a step toward performing a complete coupled-channels analysis of the world data of pi N, gamma^* N --> pi N, eta N, pi pi N reactions, the pi N --> pi pi N reactions are investigated starting with the dynamical coupled-channels model developed in
Phys. Rev. C76, 065201 (2007). The channels included are pi N, eta N, and pi pi N which has pi Delta, rho N, and sigma N resonant components. The non-resonant amplitudes are generated from solving a set of coupled-channels equations with the meson-baryon potentials defined by effective Lagrangians. The resonant amplitudes are generated from 16 bare excited nucleon (N^*) states which are dressed by the non-resonant interactions as constrained by the unitarity condition. The data of total cross sections and pi N and pi pi invariant mass distributions of pi^+ p --> pi^+ pi^+ n, pi^+ pi^0p and pi^- p --> pi^+ pi^- n, pi^- pi^0 n, pi^0 pi^0 n reactions from threshold to the invariant mass W = 2 GeV can be described to a very large extent. We show the importance of the coupled-channels effects and the strong interference between the contributions from the pi Delta, sigma N, and rho N channels. The large interference between the resonant and non-resonant amplitudes is also demonstrated. Possible future developements are discussed.
