403 research outputs found
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
Spatio-temporal pattern recognizers using spiking neurons and spike-timing-dependent plasticity.
It has previously been shown that by using spike-timing-dependent plasticity (STDP), neurons can adapt to the beginning of a repeating spatio-temporal firing pattern in their input. In the present work, we demonstrate that this mechanism can be extended to train recognizers for longer spatio-temporal input signals. Using a number of neurons that are mutually connected by plastic synapses and subject to a global winner-takes-all mechanism, chains of neurons can form where each neuron is selective to a different segment of a repeating input pattern, and the neurons are feed-forwardly connected in such a way that both the correct input segment and the firing of the previous neurons are required in order to activate the next neuron in the chain. This is akin to a simple class of finite state automata. We show that nearest-neighbor STDP (where only the pre-synaptic spike most recent to a post-synaptic one is considered) leads to "nearest-neighbor" chains where connections only form between subsequent states in a chain (similar to classic "synfire chains"). In contrast, "all-to-all spike-timing-dependent plasticity" (where all pre- and post-synaptic spike pairs matter) leads to multiple connections that can span several temporal stages in the chain; these connections respect the temporal order of the neurons. It is also demonstrated that previously learnt individual chains can be "stitched together" by repeatedly presenting them in a fixed order. This way longer sequence recognizers can be formed, and potentially also nested structures. Robustness of recognition with respect to speed variations in the input patterns is shown to depend on rise-times of post-synaptic potentials and the membrane noise. It is argued that the memory capacity of the model is high, but could theoretically be increased using sparse codes
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
K-->pipi amplitudes from lattice QCD with a light charm quark
We compute the leading-order low-energy constants of the DeltaS=1 effective
weak Hamiltonian in the quenched approximation of QCD with up, down, strange,
and charm quarks degenerate and light. They are extracted by comparing the
predictions of finite volume chiral perturbation theory with lattice QCD
computations of suitable correlation functions carried out with quark masses
ranging from a few MeV up to half of the physical strange mass. We observe a
large DeltaI=1/2 enhancement in this corner of the parameter space of the
theory. Although matching with the experimental result is not observed for the
DeltaI=1/2 amplitude, our computation suggests large QCD contributions to the
physical DeltaI=1/2 rule in the GIM limit, and represents the first step to
quantify the role of the charm quark-mass in K-->pipi amplitudes.Comment: 4 pages, 1 figure. Minor modifications. Final version to appear on
PR
Non-perturbative renormalisation of left-left four-fermion operators with Neuberger fermions
We outline a general strategy for the non-perturbative renormalisation of
composite operators in discretisations based on Neuberger fermions, via a
matching to results obtained with Wilson-type fermions. As an application, we
consider the renormalisation of the four-quark operators entering the Delta S=1
and Delta S=2 effective Hamiltonians. Our results are an essential ingredient
for the determination of the low-energy constants governing non-leptonic kaon
decays.Comment: 14 pages, 3 figure
Correlation functions at small quark masses with overlap fermions
We report on recent work on the determination of low-energy constants
describing Delta{S}=1 weak transitions, in order to investigate the origins of
the Delta{I}=1/2 rule. We focus on numerical techniques designed to enhance the
statistical signal in three-point correlation functions computed with overlap
fermions near the chiral limit.Comment: Talk presented at Lattice2004(weak), Fermilab, 21-26 June 2004, 3
pages, 2 figure
The role of dissatisfaction and per capita income in explaining self-employment across 15 European countries
This paper deals with explaining the sizable differences in the rate of self-employment (business ownership) across 15 European countries in the period 1978-2000, within a framework of occupational choice, focusing on the influence of dissatisfaction and of per capita income. Using two different measures of dissatisfaction, in addition to the level of economic development and controlling for several other variables, we find that, in addition to a negative and significant impact of per capita income, dissatisfaction at the level of societies has a positive and significant influence on self-employment levels. Both dissatisfaction with life and dissatisfaction with the way democracy works are found to influence self-employment. It is concluded that these are proxies for job dissatisfaction and at the same time represent other negative 'displacements' known to promote self-employment. The findings indirectly point at the potential importance of push factors within the incentive structures of modern economies
On the determination of low-energy constants for transitions
We present our preliminary results for three-point correlation functions
involving the operators entering the effective Hamiltonian with
an active charm quark, obtained using overlap fermions in the quenched
approximation. This is the first computation carried out for valence quark
masses small enough so as to permit a matching to Quenched Chiral Perturbation
Theory in the -regime. The commonly observed large statistical
fluctuations are tamed by means of low-mode averaging techniques, combined with
restrictions to individual topological sectors. We also discuss the matching of
the resulting hadronic matrix elements to the effective low-energy constants
for transitions. This involves (a) finite-volume corrections
which can be evaluated at NLO in Quenched Chiral Perturbation Theory, and (b)
the short-distance renormalization of the relevant four-quark operators in
discretizations based on the overlap operator. We discuss perturbative
estimates for the renormalization factors and possible strategies for their
non-perturbative evaluation. Our results can be used to isolate the
long-distance contributions to the rule, coming from physics
effects around the intrinsic QCD scale.Comment: 11 pages, 2 figures, talks presented at Lattice 2005 (Weak matrix
elements
- âŠ