Lai-Him Chan

The spin structure of the effective quark Hamiltonian and the hyperfine splittings of charmonium

Abstract We provide a simple classical derivation on the spin dependent part of the effective quark Hamiltonian. We suggest that the large S-state hyperfine splitting can be resolved by interpreting that the quark confinement potential is mainly due to an effective scalar exchange.

Determination of Charmed Hadron Masses and Electromagnetic Mass Differences in a Broken SU(8) Quark Model

Assuming that hadron mass differences result from intrinsic quark mass differences and two-body spin-spin interactions together with the Coulomb and magnetic interactions in an SU(8) model, we determine the charmed-hadron mass spectrum. A complete set of mass relations is derived for the baryon electromagnetic mass differences. In addition to the prediction of charmed-baryon mass differences a modified SU(6) relation is found to be well satisfied. D/sup +/ - D/sup 0/ and D-bar/sup +/ - D-bar/sup 0/ are estimated to be about 5.2 MeV and 3.7 MeV, respectively.

Is the nonlinear sigma model the m/sub sigma/. -->. infinity limit of the linear sigma model

We propose to investigate whether the SO(N) nonlinear sigma model is equivalent to the m/sub sigma/..-->..infinity limit of the linear sigma model by comparing the corresponding one-loop effective-action expansions up to the four-derivative terms and including the symmetry-breaking term. For this purpose we use a new background-field method to calculate the effective-action expansion directly. In the case of the linear sigma model, the renormalization procedure is implemented carefully before the m/sub sigma/..-->..infinity limit is taken. For the nonlinear sigma model we introduce a new and intuitive covariant treatment for the perturbation calculation of the field theory with nonlinear constraint. We do not find any noninvariant terms in either case. We show that the divergent parts of the effective Lagrangians due to m/sub ..pi../..-->..0, m/sub sigma/..-->..infinity, or N..-->..infinity are equivalent in the two models. However, the nonleading finite parts of the effective Lagrangians are different. Therefore, the two operations, taking the m/sub sigma/..-->..infinity limit and calculating the quantum corrections, do not commute. The origin of this difference may be a violation of decoupling.

Charmonium-the 1P1 state

Abstract Based on a model which gives agreement with the observed hyperfine and fine splitting of the charmonium states, we predict the mass of 1 p 1 to be 3372 MeV, substantially lower than any previous predicted values. We have also estimated branching ratios and decay rates for the production of this state via channels which become accessible because of its low mass.

INDUCED LORENTZ-VIOLATING CHERN-SIMONS TERM IN QED: UNCOVERING SHORT DISTANCE INTERACTION TERMS IN THE EFFECTIVE LAGRANGIAN WITHOUT THE SHADOW OF REGULARIZATION

We show that the correctly evaluated effective Lagrangian should include short-distance interaction terms which have been avoided under the protection of usual regularization and must be properly identified and reinstated if regularization is to be removed. They have special physical and mathematical significance as well as restoring gauge invariance and suppressing divergence in the effective Lagrangian. The rich structure of the short-distance interaction terms can open up challenging opportunities where the conventional regularization with rigid structure is unavailable and inappropriate. It becomes clear that gauge invariance is preserved with or without regularization and therefore there is no Lorentz-Violating Chern-Simons term in QED.