210 research outputs found
Precision Upsilon Spectroscopy from Nonrelativistic Lattice QCD
The spectrum of the Upsilon system is investigated using the Nonrelativistic
Lattice QCD approach to heavy quarks and ignoring light quark vacuum
polarization. We find good agreement with experiment for the Upsilon(1S),
Upsilon(2S), Upsilon(3S) and for the center of mass and fine structure of the
chi_b states. The lattice calculations predict b-bbar D-states with center of
mass at (10.20 +/- 0.07 +/- 0.03)GeV. Fitting procedures aimed at extracting
both ground and excited state energies are developed. We calculate a
nonperturbative dispersion mass for the Upsilon(1S) and compare with
tadpole-improved lattice perturbation theory.Comment: 8 pages, latex, SCRI-94-57, OHSTPY-HEP-T-94-00
Quenched Chiral Artifacts for Wilson-Dirac Fermions
We examine artifacts associated with the chiral symmetry breaking induced
through the use of Wilson-Dirac fermions in lattice Monte Carlo computations.
For light quark masses, the conventional quenched theory can not be defined
using direct Monte Carlo methods due to the existence of nonintegrable poles in
physical quantities. These poles are associated with the real eigenvalue
spectrum of the Wilson-Dirac operator. We show how this singularity structure
can be observed in the analysis of both QED in two dimensions and QCD in four
dimensions.Comment: 32 pages (Latex) including 13 figures (EPS
Vector Correlators in Lattice QCD: methods and applications
We discuss the calculation of the leading hadronic vacuum polarization in
lattice QCD. Exploiting the excellent quality of the compiled experimental data
for the e^+e^- --> hadrons cross-section, we predict the outcome of
large-volume lattice calculations at the physical pion mass, and design
computational strategies for the lattice to have an impact on important
phenomenological quantities such as the leading hadronic contribution to
(g-2)mu and the running of the electromagnetic coupling constant. First, the
R(s) ratio can be calculated directly on the lattice in the threshold region,
and we provide the formulae to do so with twisted boundary conditions. Second,
the current correlator projected onto zero spatial momentum, in a Euclidean
time interval where it can be calculated accurately, provides a potentially
critical test of the experimental R(s) ratio in the region that is most
relevant for (g-2)mu. This observation can also be turned around: the vector
correlator at intermediate distances can be used to determine the lattice
spacing in fm, and we make a concrete proposal in this direction. Finally, we
quantify the finite-size effects on the current correlator coming from
low-energy two-pion states and provide a general parametrization of the vacuum
polarization on the torus.Comment: 16 pages, 9 figure files; corrected a factor 2 in Eq. (7) over the
published versio
Quenched Approximation Artifacts: A study in 2-dimensional QED
The spectral properties of the Wilson-Dirac operator in 2-dimensional QED
responsible for the appearance of exceptional configurations in quenched
simulations are studied in detail. The mass singularity structure of the
quenched functional integral is shown to be extremely compicated, with multiple
branch points and cuts. The connection of lattice topological charge and
exactly real eigenmodes is explored using cooling techniques. The lattice
volume and spacing dependence of these modes is studied, as is the effect of
clover improvement of the action. A recently proposed modified quenched
approximation is applied to the study of meson correlators, and the results
compared with both naive quenched and full dynamical calculations of the same
quantity.Comment: 34 pages (Latex) plus 9 embedded figures; title change
Anomalous Scale Dimensions from Timelike Braiding
Using the previously gained insight about the particle/field relation in
conformal quantum field theories which required interactions to be related to
the existence of particle-like states associated with fields of anomalous
scaling dimensions, we set out to construct a classification theory for the
spectra of anomalous dimensions. Starting from the old observations on
conformal superselection sectors related to the anomalous dimensions via the
phases which appear in the spectral decomposition of the center of the
conformal covering group we explore the possibility
of a timelike braiding structure consistent with the timelike ordering which
refines and explains the central decomposition. We regard this as a preparatory
step in a new construction attempt of interacting conformal quantum field
theories in D=4 spacetime dimensions. Other ideas of constructions based on the
- or the perturbative SYM approach in their relation to the
present idea are briefly mentioned.Comment: completely revised, updated and shortened replacement, 24 pages
tcilatex, 3 latexcad figure
New extended high temperature series for the N-vector spin models on three-dimensional bipartite lattices
High temperature expansions for the susceptibility and the second correlation
moment of the classical N-vector model (O(N) symmetric Heisenberg model) on the
sc and the bcc lattices are extended to order for arbitrary N. For
N= 2,3,4.. we present revised estimates of the critical parameters from the
newly computed coefficients.Comment: 11 pages, latex, no figures, to appear in Phys. Rev.
Effects of non-perturbatively improved dynamical fermions in QCD at fixed lattice spacing
We present results for the static inter-quark potential, lightest glueballs,
light hadron spectrum and topological susceptibility using a non-perturbatively
improved action on a lattice at a set of values of the bare
gauge coupling and bare dynamical quark mass chosen to keep the lattice size
fixed in physical units ( fm). By comparing these measurements with a
matched quenched ensemble, we study the effects due to two degenerate flavours
of dynamical quarks. With the greater control over residual lattice spacing
effects which these methods afford, we find some evidence of charge screening
and some minor effects on the light hadron spectrum over the range of quark
masses studied (). More substantial differences between
quenched and unquenched simulations are observed in measurements of topological
quantities.Comment: 53 pages, LaTeX/RevTeX, 16 eps figures; corrected clover action
expression and various typos, no results change
Limits on the Majorana neutrino mass in the 0.1 eV range
The Heidelberg-Moscow experiment gives the most stringent limit on the
Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements,
we set a lower limit on the half-life of the neutrinoless double beta decay in
76Ge of T_1/2 > 5.7 * 10^{25} yr at 90% C.L., thus excluding an effective
Majorana neutrino mass greater than 0.2 eV. This allows to set strong
constraints on degenerate neutrino mass models.Comment: 6 pages (latex) including 3 postscript figures and 2 table
CD8beta Endows CD8 with Efficient Coreceptor Function by Coupling T Cell Receptor/CD3 to Raft-associated CD8/p56lck Complexes
The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation
Double Beta Decay: Historical Review of 75 Years of Research
Main achievements during 75 years of research on double beta decay have been
reviewed. The existing experimental data have been presented and the
capabilities of the next-generation detectors have been demonstrated.Comment: 25 pages, typos adde
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