Monte Carlo renormalization-group investigation of the two-dimensional O(4) sigma model.
An improved Monte Carlo renormalization-group method is used to determine the ..beta.. function of the two-dimensional O(4) sigma model. While for (inverse) couplings ..beta..approx. >2.2 one obtains agreement with asymptotic scaling according to asymptotic freedom, one can see deviations from it at smaller couplings. They are, however, consistent with the behavior of the correlation length, indicating ''scaling'' according to the full ..beta.. function. These results contradict recent claims that the model has a critical point at finite coupling.

title: Biased Metropolis-heat-bath algorithm for fundamental-adjoint SU(2) lattice gauge theory
abstract: For SU(2) lattice gauge theory with the fundamental-adjoint action an efficient heat-bath algorithm is not known so that one had to rely on Metropolis simulations supplemented by overrelaxation. Implementing a novel biased Metropolis-heat-bath algorithm for this model, we find improvement factors in the range 1.45 to 2.06 over conventionally optimized Metropolis simulations. If one optimizes further with respect to additional overrelaxation sweeps, the improvement factors are found in the range 1.3 to 1.8.

title: Divergent chiral condensate in the quenched Schwinger model
abstract: We calculate numerically the eigenvalue distribution of the overlap Dirac operator in the quenched Schwinger model on a lattice. The distribution does not fit any of the three universality classes of spontaneous chiral-symmetry breaking, and its strong volume dependence indicates that the chiral condensate in the quenched theory is an ill-defined and divergent quantity. When we reweight configurations with the Dirac determinant to study the theory with N{sub f}=1, we obtain a distribution of eigenvalues that is well-behaved and consistent with the theory of explicit symmetry breaking due to the anomaly.

title: Study of chiral symmetry in quenched QCD using the overlap Dirac operator
abstract: We compute fermionic observables relevant to the study of chiral symmetry in quenched QCD using the overlap Dirac operator for a wide range of fermion mass. We use analytical results to disentangle the contribution from exact zero modes and simplify our numerical computations. Details concerning the numerical implementation of the overlap Dirac operator are presented.

title: Infrared and ultraviolet properties of the Landau gauge quark propagator
abstract: We present a current summary of a program to study the quark propagator using lattice QCD. We use the Overlap and “Asqtad” quark actions on a number of lattice ensembles. to assess systematic errors. We comment on the place of this work amongst studies of QCD Green's functions in other formulations. A preliminary calculation of the running quark mass is presented.

title: Search for symmetry restoration in the fundamental SU(2) Higgs model
abstract: The phase structure of the fundamental SU(2) Higgs model at finite temperature is investigated close to the zero-temperature Higgs transition by Monte Carlo methods. In the confined phase at zero temperature we find a crossover from a confined to a deconfined region that remains quite sharp on lattices of temporal extent NT = 4 even close to the Higgs transition. On asymmetric lattices the Higgs transition shifts slightly compared to the zero-temperature, symmetric lattice, giving indirect evidence for symmetry restoration at high temperature.

title: Comparison of 20 exclusive reactions at large t.
abstract: We report a study of 20 exclusive reactions measured at the AGS at 5.9 GeV/[ital c] incident momentum, 90[degree] center of mass. This experiment confirms the strong quark flow dependence of two-body hadron-hadron scattering at large angle. At 9.9 GeV/[ital c] an upper limit had been set for the ratio of cross sections for ([ital [bar p]p][r arrow][ital [bar p]p])/([ital pp][r arrow][ital pp]) at 90[degree] c.m. with the ratio less than 4%. The present experiment was performed at lower energy to gain sensitivity, but was still within the fixed angle scaling region. A ratio [ital R]([ital [bar p]p]/[ital pp])[approx]1/40 was measured at 5.9 GeV/[ital c], 90[degree] c.m. in comparison to a ratio near 1.7 for small angle scattering. In addition, many other reactions were measured, often for the first time at 90[degree] c.m. in the scaling region, using beams of [pi][sup [plus minus]], [ital K][sup [plus minus]], [ital p] and [ital [bar p]] on a hydrogen target. There are similar large differences in cross sections for other reactions: [ital R]([ital K][sup [minus]][ital p][r arrow][pi][sup +][Sigma][sup [minus]]/[ital K][sup [minus]][ital p][r arrow][pi][sup [minus]][Sigma][sup +])[approx]1/12, for example. The relative magnitudes of the different cross sections are consistent with the dominance of quark interchangemore » in these 90[degree] reactions, and indicate that pure gluon exchange and quark-antiquark annihilation diagrams are much less important. The angular dependence of several elastic cross sections and the energy dependence at a fixed angle of many of the reactions are also presented.« less

title: Search for direct CP violation in nonleptonic decays of charged Xi and lambda hyperons
abstract: A search for direct CP violation in the nonleptonic decays of hyperons has been performed. In comparing the product of the decay parameters, ), in terms of an asymmetry parameter, A(XiLambda), between hyperons and antihyperons in the charged Xi-->Lambdapi and Lambda-->ppi decay sequence, we found no evidence of direct CP violation. The parameter A(XiLambda) was measured to be 0.012+/-0.014.

title: Dynamics of multidimensional generalization of Bianchi type-IX cosmological models.
abstract: We investigate the dynamics of an 11-dimensional homogeneous cosmological model. We assume that the t = const hypersurfaces are products of a 3-dimensional Bianchi type-IX space and a 7-dimensional torus. Most results of our investigation hold when the 7-dimensional torus is replaced by an m-dimensional torus T/sup m/. We show that for a large class of vacuum solutions the physical space expands while the microspace contracts providing a natural mechanism of dimensional reduction. Matter satisfying a simple barotropic equation of state always breaks the process of dynamical dimensional reduction. With special attention we study the behavior of our model close to the initial singularity. In contrast with the 4-dimensional Bianchi type-IX cosmological model the Kasner solution always describes an approach to the initial singularity. We study the transition from the Kasner regime to the oscillatory regime. We show that matter does not significantly change this property. We have found some exact solutions; they describe isotropic physical space, and axially symmetric physical space. We also investigate high-temperature quantum effects and their effects on the dynamics of our model. We show that the high-temperature quantum corrections do not depend on the geometrical properties of space, provided it is compact. The high-temperature quantummore » corrections can be described by a macroscopic equation of state satisfied by radiation.« less

title: Polarization of Omega - hyperons produced in 800 GeV proton-beryllium collisions.
abstract: The polarization of 103 211 [Omega][sup [minus]] hyperons produced in 800 GeV proton-beryllium inclusive reactions has been measured. Between 0.3[lt][ital x][sub [ital F]][lt]0.7 and 0.5[lt][ital p][sub [ital t]][lt]1.3 GeV/[ital c], the [Omega][sup [minus]] polarization is found to be consistent with zero, with a mean value of [minus]0.01[plus minus]0.01 at [l angle][ital x][sub [ital F]][r angle]=0.5 and [l angle][ital p][sub [ital t]][r angle]=0.95 GeV/[ital c]. This behavior is similar to that of [bar [Lambda]][sup 0], which also does not have any quarks in common with the incident proton, but is different from [bar [Xi]][sup +], which is significantly polarized in the same kinematic region.

title: Comparison of exclusive reactions at large t.
abstract: Cross sections or upper limits are reported for 12 meson-baryon and two baryon-baryon reactions for an incident momentum of 9.9 GeVc, near 90/sup 0/ c.m.: ..pi../sup +- /p..-->..p..pi../sup +- /,pp/sup +- /,..pi../sup +0//sup +- /,K/sup +/)Sigma)/sup +- /, (..lambda../sup 0/)Sigma)/sup 0/)K/sup 0/;K/sup +- /p..-->..pK/sup +- /;p/sup +- /p..-->..pp. By studying the flavor dependence of the different reactions, we have been able to isolate the quark-interchange mechanism as dominant over gluon exchange and quark-antiquark annihilation