199 research outputs found
Remarks on sum rules in the heavy quark limit of QCD
We underline a problem existing in the heavy quark limit of QCD concerning
the rates of semileptonic B decays into P-wave mesons, where (wide states) or (narrow states). The leading order
sum rules of Bjorken and Uraltsev suggest , in contradiction with experiment. The same trend follows also from a sum
rule for the subleading curent matrix element correction .
The problem is made explicit in relativistic quarks models \`a la Bakamjian and
Thomas, that give a transparent physical interpretation of the latter as due,
not to a force, but to the Wigner rotation of the light quark spin.
We point out moreover that the selection rule for decay constants of states, , predicts, assuming the model of factorization,
the opposite hierarchy .Comment: Contribution to the International Europhysics Conference on HEP,
Budapest, July 2001 (presented by L. Oliver); 5 page
Phenomenological discussion of decays in QCD improved factorization approach
Trying a global fit of the experimental branching ratios and CP-asymmetries
of the charmless decays according to QCD factorization, we find it
impossible to reach a satisfactory agreement, the confidence level (CL) of the
best fit is smaller than .1 %. This failure reflects the difficulty to
accommodate several large experimental branching ratios of the strange
channels. Furthermore, experiment was not able to exclude a large direct CP
asymmetry in , which is predicted very small by QCD
factorization. Proposing a fit with QCD factorization complemented by a
charming-penguin inspired model we reach a best fit which is not excluded by
experiment (CL of about 8 %) but is not fully convincing.
These negative results must be tempered by the remark that some of the
experimental data used are recent and might still evolve significantly.Comment: 8 pages, 2 figures (requires epsfig, psfrag),talk presented at the
XXXVIIIth Rencontres de Moriond: Electroweak Interactions and Unified
Theories,Les Arcs, France, March 15-22, 2003. To be published in the
Proceeding
Nuclear models on a lattice
We present the first results of a quantum field approach to nuclear models
obtained by lattice techniques. Renormalization effects for fermion mass and
coupling constant in case of scalar and pseudoscalar interaction lagrangian
densities are discussed.Comment: 4 pages - 7 figures ; Invited talk to QCD 05: 12th International QCD
Conference, 4-9 Jul 2005, Montpellier, France ; To appear in Nucl. Phys. B
(Proc. Suppl.
B_s-\bar{B_s} mixing with a chiral light quark action
We study the mixing amplitude in Standard Model by
computing the relevant hadronic matrix element in the static limit of lattice
HQET with the Neuberger light quark action. In the quenched approximation, and
after matching to the scheme in QCD, we obtain .Comment: 6 pages, 3 figures, talk presented at Lattice 2005 (Heavy quark
physics
Indirect lattice evidence for the Refined Gribov-Zwanziger formalism and the gluon condensate in the Landau gauge
We consider the gluon propagator at various lattice sizes and
spacings in the case of pure SU(3) Yang-Mills gauge theories using the Landau
gauge fixing. We discuss a class of fits in the infrared region in order to
(in)validate the tree level analytical prediction in terms of the (Refined)
Gribov-Zwanziger framework. It turns out that an important role is played by
the presence of the widely studied dimension two gluon condensate
. Including this effect allows to obtain an acceptable fit up to
1 \'{a} 1.5 GeV, while corroborating the Refined Gribov-Zwanziger prediction
for the gluon propagator. We also discuss the infinite volume extrapolation,
leading to the estimate . As a byproduct, we can
also provide the prediction obtained at
the renormalization scale .Comment: 17 pages, 10 figures, updated version, accepted for publication in
Phs.Rev.
Instanton traces in lattice gluon correlation functions
Strong coupling constant computed in Landau gauge and MOM renormalization
scheme from lattice two and three gluon Green Functions exhibits an unexpected
behavior in the deep IR, showing a maximum value around . We
analise this coupling below this maximum within a semiclassical approach, were
gluon degrees of freedom at very low energies are described in terms of the
classical solutions of the lagrangian, namely instantons. We provide some new
results concerning the relationship between instantons and the low energy
dynamics of QCD, by analising gluon two- and three-point Green functions
separately and with the help of a cooling procedure to eliminate short range
correlations.Comment: 4 pages, talk given at XXXX Rencontres de Moriond on QCD and Hadronic
Interactions, La Thuile (Italy
The dynamical origin of the refinement of the Gribov-Zwanziger theory
In recent years, the Gribov-Zwanziger action was refined by taking into
account certain dimension 2 condensates. In this fashion, one succeeded in
bringing the gluon and the ghost propagator obtained from the GZ model in
qualitative and quantitative agreement with the lattice data. In this paper, we
shall elaborate further on this aspect. First, we shall show that more
dimension 2 condensates can be taken into account than considered so far and,
in addition, we shall give firm evidence that these condensates are in fact
present by discussing the effective potential. It follows thus that the
Gribov-Zwanziger action dynamically transforms itself into the refined version,
thereby showing that the continuum nonperturbative Landau gauge fixing, as
implemented by the Gribov-Zwanziger approach, is consistent with lattice
simulations.Comment: 36 pages, 4 figure
The Infrared Behaviour of the Pure Yang-Mills Green Functions
We study the infrared behaviour of the pure Yang-Mills correlators using
relations that are well defined in the non-perturbative domain. These are the
Slavnov-Taylor identity for three-gluon vertex and the Schwinger-Dyson equation
for ghost propagator in the Landau gauge. We also use several inputs from
lattice simulations. We show that lattice data are in serious conflict with a
widely spread analytical relation between the gluon and ghost infrared critical
exponents. We conjecture that this is explained by a singular behaviour of the
ghost-ghost-gluon vertex function in the infrared. We show that, anyhow, this
discrepancy is not due to some lattice artefact since lattice Green functions
satisfy the ghost propagator Schwinger-Dyson equation. We also report on a
puzzle concerning the infrared gluon propagator: lattice data seem to favor a
constant non vanishing zero momentum gluon propagator, while the Slavnov-Taylor
identity (complemented with some regularity hypothesis of scalar functions)
implies that it should diverge.Comment: 25 pages, 7 figures; replaced version with some references adde and
an enlarged discussion of the non-renormalization theorem; second replacement
with improved figures and added reference
Asymptotic behavior of the ghost propagator in SU3 lattice gauge theory
We study the asymptotic behavior of the ghost propagator in the quenched
SU(3) lattice gauge theory with Wilson action. The study is performed on
lattices with a physical volume fixed around 1.6 fm and different lattice
spacings: 0.100 fm, 0.070 fm and 0.055 fm. We implement an efficient algorithm
for computing the Faddeev-Popov operator on the lattice. We are able to
extrapolate the lattice data for the ghost propagator towards the continuum and
to show that the extrapolated data on each lattice can be described up to
four-loop perturbation theory from 2.0 GeV to 6.0 GeV. The three-loop values
are consistent with those extracted from previous perturbative studies of the
gluon propagator. However the effective \Lambda_{\ms} scale which reproduces
the data does depend strongly upon the order of perturbation theory and on the
renormalization scheme used in the parametrization. We show how the truncation
of the perturbative series can account for the magnitude of the dependency in
this energy range. The contribution of non-perturbative corrections will be
discussed elsewhere.Comment: 26 pages, 7 figure
- …