Dermatitis herpetiformis treated with diaminodiphenylsulphone (DADPS)

No Abstract

Amplitude determination for MM -> MM, M= pi, K and cross-sections for gamma gamma -> pi^+pi^-, pi^0 pi^0 in a chiral model

Recently Dai and Pennington have performed a comprehensive analysis of essentially all pion and kaon pair production data from two-photon collisions below 1.5 GeV, including all high statistics results from Belle, as well as the older data from Mark II at SLAC, CELLO at DESY, and Crystal Ball at SLAC. Imposing the basic constraints required by analyticity, unitarity, and crossing symmetry and making use of Low's low energy theorem for QED, they are able to extract the final-state strong-interaction scattering amplitudes for the intermediate pi pi->pi pi and pi pi-> K\bar K reactions in a model-independent fashion. In addition, they provide good fits to the respective gamma gamma-> pi pi cross-sections that are known in the low-energy sector in the restricted angular range, | cos theta|< 0.6-0.8. Using the parameters obtained in this fashion, these authors contruct the \gamma\gamma->pi pi cross-sections integrated over the full angular range. In this work, we use a version of chiral perturbation theory developed by Oller and Oset to evaluate the final-state strong-interaction amplitudes directly theoretically and we compare our thus obtained low-energy QCD-based results directly with the amplitudes extracted by Dai and Pennington. We also calculate the gamma gamma -> pi pi cross-sections (integrated over the full angular range) and compare these with those obtained by Dai and Pennington. This calculation thus gives a more detailed insight into the fit of chiral perturbation theory, not just to the measured gamma gamma-> pi pi cross-sections, as is usually presented, but rather to a higher level of detail through the available analysis of the experimental data for the underlying final-state strong-interaction meson-meson scattering amplitudes pi pi-> pi pi and pi pi-> K\bar K themselves. The fits appear to be reasonable over the energy range considered.Comment: 17 pages, 5 figure

Pyoderma gangrenosum complicating a simple skin graft

No Abstract

Mass Generation in the Supersymmetric Nambu--Jona--Lasinio Model in an External Magnetic Field

The mass generation in the (3+1)-dimensional supersymmetric Nambu-Jona-Lasinio model in a constant magnetic field is studied. It is shown that the external magnetic field catalyzes chiral symmetry breaking.Comment: LaTeX file, 6 pages. Talk given at the International Seminar dedicated to the memory of Dmitrij Volkov "Supersymmetry and Quantum Field Theory", Kharkov, Ukraine, January 5-7, 199

Two regularizations - two different models of Nambu-Jona-Lasinio

Two variants of the Nambu--Jona-Lasinio model -- the model with 4-dimensional cutoff and the model with dimensionally-analytical regularization -- are systematically compared. It is shown that they are, in essence, two different models of light-quark interaction. In the mean-field approximation the distinction becomes apparent in a behavior of scalar amplitude near the threshold. For 4-dimensional cutoff the pole term can be extracted, which corresponds to sigma-meson. For dimensionally-analytical regularization the singularity of the scalar amplitude is not pole, and this singularity is quite disappeared at some value of the regularization parameter. Still more essential distinction of these models exists in the next-to-leading order of mean-field expansion. The calculations of meson contributions in the quark chiral condensate and in the dynamical quark mass demonstrate, that these contributions though their relatively smallness can destabilize the Nambu--Jona-Lasinio model with 4-dimensional cutoff. On the contrary, the Nambu--Jona-Lasinio model with dimensionally-analytical regularization is stabilized with the next-to-leading order, i.e. the value of the regularization parameter shifts to the stability region, where these contributions decrease.Comment: 14 pages; Journal version; parameter fixing procedure is modifie

Nambu-Jona-Lasinio Models Beyond the Mean Field Approximation

Inspired by the model of Nambu and Jona-Lasinio, various Lagrangians are considered for a system of interacting quarks. Employing standard techniques of many-body theory, the scalar part of the quark self-energy is calculated including terms up to second-order in the interaction. Results obtained for the single-particle Green's function are compared with those which only account for the mean-field or Hartree-Fock term in the self-energy. Depending on the explicit form of the Lagrangian, the second-order contributions range between 4 and 90 percent of the leading Hartree-Fock term. This leads to a considerable momentum dependence of the self-energy and the effective mass of the quarks.Comment: 17 page

Relativistic Kinetic Equations for Electromagnetic, Scalar and Pseudoscalar Interactions

We derive the kinetic equations for both the covariant and equal-time Wigner functions of Dirac particles with electromagnetic, scalar and pseudoscalar interactions. We emphasize the constraint equations for the spinor components in the equal-time formulation.Comment: 12 pages, no figures, revte