Baryon Masses in Chiral Perturbation Theory with Infrared Regularization

The baryon masses are examined in SU(3) chiral perturbation theory to third order using the recently proposed infrared regularization scheme. Fourth order is estimated by evaluating the dominant diagram. With this regularization the magnitude of the loop integrals is reduced so that the convergence of the series appears to be better than in the heavy baryon approach.Comment: The original third order calculation is supplemented by an estimate of fourth order using just the dominant diagram. The convergence still appears to be better than in the heavy baryon approach. To be published in Phys. Rev. C. 15 pages latex, 2 postscript figure

On the effects of (partial) quenching on penguin contributions to K-> pi pi

Recently, we pointed out that chiral transformation properties of strong penguin operators change in the transition from unquenched to (partially) quenched QCD. As a consequence, new penguin-like operators appear in the (partially) quenched theory, along with new low-energy constants, which should be interpreted as a quenching artifact. Here, we extend the analysis to the contribution of the new low-energy constants to the K^0 -> pi^+ pi^- amplitude, at leading order in chiral perturbation theory, and for arbitrary (momentum non-conserving) kinematics. Using these results, we provide a detailed discussion of the intrinsic systematic error due to this (partial) quenching artifact. We also give a simple recipe for the determination of the leading-order low-energy constant parameterizing the new operators in the case of strong $LR$ penguins.Comment: 17 pages, 1 figure, minor correction

Aspects of Nucleon Compton Scattering

We consider the spin-averaged nucleon forward Compton scattering amplitude in heavy baryon chiral perturbation theory including all terms to order ${\cal O} (q^4)$. The chiral prediction for the spin-averaged forward Compton scattering amplitude is in good agreement with the data for photon energies $\omega \le 110$ MeV. We also evaluate the nucleon electric and magnetic Compton polarizabilities to this order and discuss the uncertainties of the various counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5

Scaling tests of the improved Kogut-Susskind quark action

Improved lattice actions for Kogut-Susskind quarks have been shown to improve rotational symmetry and flavor symmetry. In this work we find improved scaling behavior of the rho and nucleon masses expressed in units of a length scale obtained from the static quark potential, and better behavior of the Dirac operator in instanton backgrounds.Comment: 4 pages, 4 figures, Revte