A Wilson-Yukawa model with a chiral spectrum in 2D

We summarize our recent study of the fermion spectrum in a fermion-scalar 2D model with a chiral $U(1)_L \times U(1)_R$ global symmetry. This model is obtained from a two-cutoff lattice formulation of a 2D U(1) chiral gauge theory, in the limit of zero gauge coupling. The massless fermion spectrum found deep in the vortex phase is undoubled and chiral.Comment: 3 pages, LaTeX, uses espcrc2.sty. To appear in proceedings of Lattice 97, Edinbugh, Scotlan

Testing for Adverse Selection into Private Medical Insurance

We develop a test for adverse selection and use it to examine privatehealth insurance markets. In contrast to earlier papers that consider apurely private system or a system in which private insurance supplementsa public system, we focus our attention on a system where privately fundedhealth care is substitutive of the publicly funded one. Using a model ofcompetition among insurers, we generate predictions about the correlationbetween risk and the probability of taking private insurance under bothsymmetric information and adverse selection. These predictions constitutethe basis for our adverse selection test. The theoretical model is also usefulto conclude that the setting that we focus on is especially attractive to testfor adverse selection. Using the British Household Panel Survey, we findevidence that adverse selection is present in this market. We develop a test for adverse selection and use it to examine privatehealth insurance markets. In contrast to earlier papers that consider apurely private system or a system in which private insurance supplementsa public system, we focus our attention on a system where privately fundedhealth care is substitutive of the publicly funded one. Using a model ofcompetition among insurers, we generate predictions about the correlationbetween risk and the probability of taking private insurance under bothsymmetric information and adverse selection. These predictions constitutethe basis for our adverse selection test. The theoretical model is also usefulto conclude that the setting that we focus on is especially attractive to testfor adverse selection. Using the British Household Panel Survey, we find evidence that adverse selection is present in this market

Finite-size scaling for the left-current correlator with non-degenerate quark masses

We study the volume dependence of the left-current correlator with non-degenerate quark masses to next-to-leading order in the chiral expansion. We consider three possible regimes: all quark masses are in the $\epsilon$-regime, all are in the $p$-regime and a mixed-regime where the lighest quark masses satisfy $m_v \Sigma V \leq 1$ while the heavier $m_s \Sigma V \gg 1$. These results can be used to match lattice QCD and the Chiral Effective Theory in a large but finite box in which the Compton wavelength of the lightest pions is of the order of the box size. We consider both the full and partially-quenched results.Comment: 27 pages, 4 figure

A note on the practical feasibility of domain-wall fermions

Domain-wall fermions preserve chiral symmetry up to terms that decrease exponentially when the lattice size in the fifth dimension is taken to infinity. The associated rates of convergence are given by the low-lying eigenvalues of a simple local operator in four dimensions. These can be computed using the Ritz functional technique and it turns out that the convergence tends to be extremely slow in the range of lattice spacings relevant to large-volume numerical simulations of lattice QCD. Two methods to improve on this situation are discussed.Comment: 14 pages, talk given by P. H. at the workshop on {\em Current theoretical problems in lattice field theory}, Ringberg, German

Finite Volume Scaling of Pseudo Nambu-Goldstone Bosons in QCD

We consider chiral perturbation theory in a finite volume and in a mixed regime of quark masses. We take N_l light quarks near the chiral limit, in the so-called epsilon-regime, while the remaining N_h quarks are heavier and in the standard p-regime. We compute in this new mixed regime the finite-size scaling of the light meson correlators in the scalar, pseudoscalar, vector and axial vector channels.Using the replica method, we easily extend our results to the partially quenched theory. With the help of our results, lattice QCD simulations with 2+1 flavors can safely investigate pion physics with very light up and down quark masses even in the region where the pion's correlation length overcomes the size of the space-time lattice.Comment: 32 pages, 6 figures, published versio

On the index theorem for Wilson fermions

We consider a Wilson-Dirac operator with improved chiral properties. We show that, for arbitrarily rough gauge fields, it satisfies the index theorem if we identify the zero modes with the small real eigenvalues of the fermion operator and use the geometrical definition of topological charge. This is also confirmed in a numerical study of the quenched Schwinger model. These results suggest that integer definitions of the topological charge based on counting real modes of the Wilson operator are equivalent to the geometrical definition. The problem of exceptional configurations and the sign problem in simulations with an odd number of dynamical Wilson fermions are briefly discussed.Comment: Latex, 18 pages, 6 figure