172 research outputs found

### The functional role of contrast adaptation

Prolonged inspection of high contrast sinewave gratings increases the contrast required to detect gratings having a similar spatial frequency and orientation. The functional role of such adaptation has, however, in the past, eluded disclosure. We here show that 5 min adaptation to a 2 c/deg sinewave grating of 0.8 contrast changes the observer's ability to discriminate the contrast level of a subsequently presented grating of the same spatial frequency and orientation. Similar to the threshold elevation effect, the observers required more incremental contrast for background contrast levels between 0.1 and 0.4 following adaptation. However, for contrast levels above 0.5, the observers required less delta contrast, following adaptation, to correctly discriminate which of two gratings was incremented in contrast. A simple model for adaptation is proposed to account for the findings which is based on a shift in the semi-saturation constant of the detector's contrast-response function. According to this model, adaptation acts to linearize the underlying mechanism's response in the region near the prevailing contrast level

### Perturbative and non-perturbative studies of the SU(2)-Higgs model on lattices with asymmetric lattice spacings

We present a calculation of the O(g^2,\lambda) perturbative corrections to
the coupling anisotropies of the SU(2)-Higgs model on lattices with asymmetric
lattice spacings. These corrections are obtained by a one-loop calculation
requiring the rotational invariance of the gauge and Higgs boson propagators in
the continuum limit. The coupling anisotropies are also determined from
numerical simulations of the model on appropriate lattices. The one-loop
perturbation theory and the simulation results agree with high accuracy. It is
demonstrated that rotational invariance is also restored for the static
potential determined from space-space and space-time Wilson loops.Comment: 27pages, Latex, 7 figures (7 eps, 3 ps files), correction of
misprint

### Effective heavy-light meson energies in small-volume quenched QCD

We study effective energies of heavy-light meson correlation functions in
lattice QCD and a small volume of (0.2 fm)^4 to non-perturbatively calculate
their dependence on the heavy quark mass in the continuum limit. Our quenched
results obtained here constitute an essential intermediate step of a first
fully non-perturbative computation of the b-quark's mass in the static
approximation that has recently been presented as an application of a new
proposal to non-perturbatively renormalize the Heavy Quark Effective Theory.
The renormalization constant and the improvement coefficients relating the
renormalized current and subtracted quark mass are determined in the relevant
parameter region at weak couplings, which allows to perform the numerical
simulations at several, precisely fixed values of the renormalization group
invariant heavy quark mass in a range from 3 GeV to 15 GeV.Comment: 24 pages including figures and tables, latex2e; version published in
JHEP, small additions, results unchange

### A precise determination of $B_K$ in quenched QCD

The $B_K$ parameter is computed in quenched lattice QCD with Wilson twisted
mass fermions. Two variants of tmQCD are used; in both of them the relevant
$\Delta S = 2$ four-fermion operator is renormalised multiplicatively. The
renormalisation adopted is non-perturbative, with a Schroedinger functional
renormalisation condition. Renormalisation group running is also
non-perturbative, up to very high energy scales. In one of the two tmQCD
frameworks the computations have been performed at the physical $K$-meson mass,
thus eliminating the need of mass extrapolations. Simulations have been
performed at several lattice spacings and the continuum limit was reached by
combining results from both tmQCD regularisations. Finite volume effects have
been partially checked and turned out to be small. Exploratory studies have
also been performed with non-degenerate valence flavours. The final result for
the RGI bag parameter, with all sources of uncertainty (except quenching) under
control, is $\hat B_K =0.789 \pm 0.046$.Comment: 54 pages, 11 figure

### Flavour symmetry restoration and kaon weak matrix elements in quenched twisted mass QCD

We simulate two variants of quenched twisted mass QCD (tmQCD), with
degenerate Wilson quarks of masses equal to or heavier than half the strange
quark mass. We use Ward identities in order to measure the twist angles of the
theory and thus check the quality of the tuning of mass parameters to a physics
condition which stays constant as the lattice spacing is varied. Flavour
symmetry breaking in tmQCD is studied in a framework of two fully twisted and
two standard Wilson quark flavours, tuned to be degenerate in the continuum.
Comparing pseudoscalar masses, obtained from connected quark diagrams made of
tmQCD and/or standard Wilson quark propagators, we confirm that flavour
symmetry breaking effects, which are at most 5%, decrease as we approach the
continuum limit. We also compute the pseudoscalar decay constant in the
continuum limit, with reduced systematics. As a consequence of improved tuning
of the mass parameters at $\beta = 6.1$, we reanalyse our previous $B_K$
results. Our main phenomenological findings are $r_0 f_K = 0.421(7)$ and $\hat
B_K = 0.735(71)$.Comment: 41 pages, figures included, one reference added. Final version as
accepted for publication on Nucl.Phys.

### Non-perturbative Heavy Quark Effective Theory

We explain how to perform non-perturbative computations in HQET on the
lattice. In particular the problem of the subtraction of power-law divergences
is solved by a non-perturbative matching of HQET and QCD. As examples, we
present a full calculation of the mass of the b-quark in the combined static
and quenched approximation and outline an alternative way to obtain the B-meson
decay constant at lowest order. Since no excessively large lattices are
required, our strategy can also be applied including dynamical fermions.Comment: 27 pages including figures and tables, latex2e; version published in
JHEP, typos corrected and 1 reference adde

### Where does the hot electroweak phase transition end?

We give the nonperturbative phase diagram of the four-dimensional hot
electroweak phase transition. A systematic extrapolation $a \to 0$ is done. Our
results show that the finite temperature SU(2)-Higgs phase transition is of
first order for Higgs-boson masses $m_H<66.5 \pm 1.4$ GeV. The full
four-dimensional result agrees completely with that of the dimensional
reduction approximation. This fact is of particular importance, because it
indicates that the fermionic sector of the Standard Model (SM) can be included
perturbatively. We obtain that the Higgs-boson endpoint mass in the SM is $72.4
\pm 1.7$ GeV. Taking into account the LEP Higgs-boson mass lower bound excludes
any electroweak phase transition in the SM.Comment: LATTICE98(electroweak), presented by Z. Fodor. Latex, 3 pages, 3 figu
res. Comment line change

### Lattice QCD with light dynamical quarks

We report on the simulation of QCD with light dynamical quarks using the
two-step multi-boson (TSMB) algorithm.
In an exploratory study with two flavours of quarks at lattice spacing about
0.27 fm and with quark mass down to one sixth of the strange quark mass
eigenvalue spectra and autocorrelations have been studied.
Here we present results on the volume dependence as well as tests of possible
algorithmic improvements.Comment: 6 pages, Lattice2002(spectrum

- â€¦