N K and Delta K states in the chiral SU(3) quark model

The isospin I=0 and I=1 kaon-nucleon $S$, $P$, $D$, $F$ wave phase shifts are studied in the chiral SU(3) quark model by solving the resonating group method (RGM) equation. The calculated phase shifts for different partial waves are in agreement with the experimental data. Furthermore, the structures of the $\Delta K$ states with L=0, I=1 and I=2 are investigated. We find that the interaction between $\Delta$ and $K$ in the case of L=0, I=1 is attractive, which is not like the situation of the $NK$ system, where the $S$-wave interactions between $N$ and $K$ for both I=0 and I=1 are repulsive. Our numerical results also show that when the model parameters are taken to be the same as in our previous $NN$ and $YN$ scattering calculations, the $\Delta K$ state with L=0 and I=1 is a weakly bound state with about 2 MeV binding energy, while the one with I=2 is unbound in the present one-channel calculation.Comment: 14 pages, 6 figures. PRC70,064004(2004

Sigma_c Dbar and Lambda_c Dbar states in a chiral quark model

The S-wave Sigma_c Dbar and Lambda_c Dbar states with isospin I=1/2 and spin S=1/2 are dynamically investigated within the framework of a chiral constituent quark model by solving a resonating group method (RGM) equation. The results show that the interaction between Sigma_c and Dbar is attractive, which consequently results in a Sigma_c Dbar bound state with the binding energy of about 5-42 MeV, unlike the case of Lambda_c Dbar state, which has a repulsive interaction and thus is unbound. The channel coupling effect of Sigma_c Dbar and Lambda_c Dbar is found to be negligible due to the fact that the gap between the Sigma_c Dbar and Lambda_c Dbar thresholds is relatively large and the Sigma_c Dbar and Lambda_c Dbar transition interaction is weak.Comment: 7 pages,2 figures. arXiv admin note: text overlap with arXiv:nucl-th/0606056 by other author

Fireball/Blastwave Model and Soft Gamma-ray Repeaters

Soft gamma-ray repeaters are at determined distances and their positions are known accurately. If observed, afterglows from their soft gamma-ray bursts will provide important clues to the study of the so called "classical gamma-ray bursts". On applying the popular fireball/blastwave model of classical gamma-ray bursts to soft gamma-ray repeaters, it is found that their X-ray and optical afterglows are detectable. Monitoring of the three repeaters is solicited.Comment: Already published in 1998 in "Chinese Physics Letters", replaced with the published version. See astro-ph/0502452 for a more detailed versio

Transition Temperature of a Uniform Imperfect Bose Gas

We calculate the transition temperature of a uniform dilute Bose gas with repulsive interactions, using a known virial expansion of the equation of state. We find that the transition temperature is higher than that of an ideal gas, with a fractional increase K_0(na^3)^{1/6}, where n is the density and a is the S-wave scattering length, and K_0 is a constant given in the paper. This disagrees with all existing results, analytical or numerical. It agrees exactly in magnitude with a result due to Toyoda, but has the opposite sign.Comment: Email correspondence to [email protected] ; 2 pages using REVTe

GRB afterglows: deep Newtonian phase and its application

Gamma-ray burst afterglows have been observed for months or even years in a few cases. It deserves noting that at such late stages, the remnants should have entered the deep Newtonian phase, during which the majority of shock-accelerated electrons will no longer be highly relativistic. To calculate the afterglows, we must assume that the electrons obey a power-law distribution according to their kinetic energy, not simply the Lorentz factor.Comment: Poster at the 4th workshop "Gamma-Ray Bursts in the Afterglow Era" (Rome, 2004), accepted for publication in the proceedings. 4 pages, with 3 figures inserte

Dressing the electromagnetic nucleon current

A field-theory-based approach to pion photoproduction off the nucleon is used to derive a microscopically consistent formulation of the fully dressed electromagnetic nucleon current in an effective Lagrangian formalism. It is shown how the rigorous implementation of local gauge invariance at all levels of the reaction dynamics provides equations that lend themselves to practically manageable truncations of the underlying nonlinearities of the problem. The requirement of consistency also suggests a novel way of treating the pion photoproduction problem. Guided by a phenomenological implementation of gauge invariance for the truncated equations that has proved successful for pion photoproduction, an expression for the fully dressed nucleon current is given that satisfies the Ward-Takahashi identity for a fully dressed nucleon propagator as a matter of course. Possible applications include meson photo- and electroproduction processes, bremsstrahlung, Compton scattering, and $ee'$ processes off nucleons.Comment: 10 pages, 9 figure