Probing the Light Pseudoscalar Window

Very light pseudoscalars can arise from the symmetry-breaking sector in many extensions of the Standard Model. If their mass is below 200 MeV, they can be long-lived and have interesting phenomenology. We discuss the experimental constraints on several models with light pseudoscalars, including one in which the pseudoscalar is naturally fermiophobic. Taking into account the stringent bounds from rare K and B decays, we find allowed parameter space in each model that may be accessible in direct production experiments. In particular, we study the photoproduction of light pseudoscalars at Jefferson Lab and conclude that a beam dump experiment could explore some of the allowed parameter space of these models.Comment: 22 pages, 4 figure

Anomalous Hall effect as a probe of the chiral order in spin glasses

Anomalous Hall effect arising from the noncoplanar spin configuration (chirality) is discussed as a probe of the chiral order in spin glasses. It is shown that the Hall coefficient yields direct information about the linear and nonlinear chiral susceptibilities of the spin sector, which has been hard to obtain experimentally from the standard magnetic measurements. Based on the chirality scenario of spin-glass transition, predictions are given on the behavior of the Hall resistivity of canonical spin glasses.Comment: Order estimate of the effect given, one reference added. To appear in Phys. Rev. Letter

First Principles Calculation of Anomalous Hall Conductivity in Ferromagnetic bcc Fe

We perform a first principles calculation of the anomalous Hall effect in ferromagnetic bcc Fe. Our theory identifies an intrinsic contribution to the anomalous Hall conductivity and relates it to the k-space Berry phase of occupied Bloch states. The theory is able to account for both dc and magneto-optical Hall conductivities with no adjustable parameters.Comment: 4 pages, 6 figures, author list correcte

Investigation of the Domain Wall Fermion Approach to Chiral Gauge Theories on the Lattice

We investigate a recent proposal to construct chiral gauge theories on the lattice using domain wall fermions. We restrict ourselves to the finite volume case, in which two domain walls are present, with modes of opposite chirality on each of them. We couple the chiral fermions on only one of the domain walls to a gauge field. In order to preserve gauge invariance, we have to add a scalar field, which gives rise to additional light mirror fermion and scalar modes. We argue that in an anomaly free model these extra modes would decouple if our model possesses a so-called strong coupling symmetric phase. However, our numerical results indicate that such a phase most probably does not exist. ---- Note: 9 Postscript figures are appended as uuencoded compressed tar file.Comment: 27p. Latex; UCSD/PTH 93-28, Wash. U. HEP/93-6

Anomalous Hall effect in ferromagnetic semiconductors

We present a theory of the anomalous Hall effect in ferromagnetic (Mn,III)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wavefunction upon traversing closed paths on the spin-split Fermi surface of a ferromagnetic state. It can be applied equally well to any itinerant electron ferromagnet. The quantitative agreement between our theory and experimental data in both (In,Mn)As and (Ga,Mn)As systems suggests that this disorder independent contribution to the anomalous Hall conductivity dominates in diluted magnetic semiconductors.Comment: 4 pages, 2 figure

On the Formation of Copper Linear Atomic Suspended Chains

We report high resolution transmission electron microscopy and classical molecular dynamics simulation results of mechanically stretching copper nanowires conducting to linear atomic suspended chains (LACs) formation. In contrast with some previous experimental and theoretical work in literature that stated that the formation of LACs for copper should not exist our results showed the existence of LAC for the [111], [110], and [100] crystallographic directions, being thus the sequence of most probable occurence.Comment: 4 pages, 3 figure

Supernova Simulations with Boltzmann Neutrino Transport: A Comparison of Methods

Accurate neutrino transport has been built into spherically symmetric simulations of stellar core collapse and postbounce evolution. The results of such simulations agree that spherically symmetric models with standard microphysical input fail to explode by the delayed, neutrino-driven mechanism. Independent groups implemented fundamentally different numerical methods to tackle the Boltzmann neutrino transport equation. Here we present a direct and detailed comparison of such neutrino radiation-hydrodynamical simulations for two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the Garching group. The former solves the Boltzmann equation directly by an implicit, general relativistic discrete angle method on the adaptive grid of a conservative implicit hydrodynamics code with second-order TVD advection. In contrast, the latter couples a variable Eddington factor technique with an explicit, moving-grid, conservative high-order Riemann solver with important relativistic effects treated by an effective gravitational potential. The presented study is meant to test both neutrino radiation-hydrodynamics implementations and to provide a data basis for comparisons and verifications of supernova codes to be developed in the future. Results are discussed for simulations of the core collapse and post-bounce evolution of a 13 solar mass star with Newtonian gravity and a 15 solar mass star with relativistic gravity.Comment: 23 pages, 13 figures, revised version, to appear in Ap

Exotic Meson Decay Widths using Lattice QCD

A decay width calculation for a hybrid exotic meson h, with JPC=1-+, is presented for the channel h->pi+a1. This quenched lattice QCD simulation employs Luescher's finite box method. Operators coupling to the h and pi+a1 states are used at various levels of smearing and fuzzing, and at four quark masses. Eigenvalues of the corresponding correlation matrices yield energy spectra that determine scattering phase shifts for a discrete set of relative pi+a1 momenta. Although the phase shift data is sparse, fits to a Breit-Wigner model are attempted, resulting in a decay width of about 60 MeV when averaged over two lattice sizes.Comment: 9 pages, 8 figures, RevTex4, minor change to Fig.

Staggered Fermions and Gauge Field Topology

Based on a large number of smearing steps, we classify SU(3) gauge field configurations in different topological sectors. For each sector we compare the exact analytical predictions for the microscopic Dirac operator spectrum of quenched staggered fermions. In all sectors we find perfect agreement with the predictions for the sector of topological charge zero, showing explicitly that the smallest Dirac operator eigenvalues of staggered fermions at presently realistic lattice couplings are insensitive to gauge field topology. On the smeared configurations, $4\nu$ eigenvalues clearly separate out from the rest on configurations of topological charge $\nu$, and move towards zero in agreement with the index theorem.Comment: LaTeX, 10 page

Low-lying fermion modes, topology and light hadrons in quenched QCD

We explore the properties of low lying eigenmodes of fermions in the quenched approximation of lattice QCD. The fermion action is a recently proposed overlap action and has exact chiral symmetry. We find that chiral zero-eigenvalue modes are localized in space and their positions correlate strongly with the locations (as defined through the density of pure gauge observables) of instantons of the appropriate charge. Nonchiral modes are also localized with peaks which are strongly correlated with the positions of both charges of instantons. These correlations slowly die away as the fermion eigenvalue rises. Correlators made of quark propagators restricted to these modes closely reproduce ordinary hadron correlators at small quark mass in many channels. Our results are in qualitative agreement with the expectations of instanton liquid models.Comment: 21 pages, Revtex, 21 postscript figures. COLO-HEP-45