334 research outputs found
Quenched lattice calculation of the B --> D l nu decay rate
We calculate, in the continuum limit of quenched lattice QCD, the form factor
that enters in the decay rate of the semileptonic decay B --> D l nu. Making
use of the step scaling method (SSM), previously introduced to handle two scale
problems in lattice QCD, and of flavour twisted boundary conditions we extract
G(w) at finite momentum transfer and at the physical values of the heavy quark
masses. Our results can be used in order to extract the CKM matrix element Vcb
by the experimental decay rate without model dependent extrapolations.Comment: 5 pages, 4 figures, accepted for publication on Phys. Lett. B,
corrected one typ
The three-loop beta function of SU(N) lattice gauge theories with Wilson fermions
We calculate the third coefficient of the lattice beta function associated
with the Wilson formulation for both gauge fields and fermions. This allows us
to evaluate the three-loop correction (linear in ) to the relation
between the lattice Lambda-parameter and the bare coupling , which is
important in order to verify asymptotic scaling predictions. Our calculation
also leads to the two-loop relation between the coupling renormalized in the
MSbar scheme and .
The original version of this paper contained a numerical error in one of the
diagrams, which has now been corrected. The calculations, as well as the layout
of the paper have remained identical, but there are some important changes in
the numerical results.Comment: One 14-page LaTeX file, one PostScript file containing 2 figures.
Corrected a numerical error in one of the diagrams. The calculations, as well
as the layout of the paper have remained unaffected, but there are some
important changes in the numerical result
Higgs mediated flavor violating top quark decays t --> u_i H, u_i gamma, u_i gamma gamma, and the process gamma gamma --> t c in effective theories
The rare top quark couplings and ()
induced at the one-loop level by a flavor violating vertex are studied
within the context of an effective Yukawa sector that incorporates
-invariant operators of up to dimension six. Data on the
recently observed mixing are employed to constrain the
vertex, which is then used to predict the , , and decays, as well as the reaction in the context of the ILC. It is found that
the and decays can reach sizable branching
ratios as high as and , respectively. As for the
decay, it can have a branching ratio of that
is about 6 orders of magnitude larger than the standard model prediction,
which, however, is still very small to be detected. As for production, it
is found that, due to the presence of a resonant effect in the convoluted cross
section , about events may be produced at the ILC for a value of the
Higgs mass near to the top mass.Comment: 5 pages and 3 figure
Brief review on semileptonic B decays
We concisely review semileptonic B decays, focussing on recent progress on
both theoretical and experimental sides.Comment: 18 pages, 2 figures; version to be published in Mod. Phys. Lett.
Form Factors from QCD Light-Cone Sum Rules
We derive new QCD sum rules for and form factors. The
underlying correlation functions are expanded near the light-cone in terms of
-meson distribution amplitudes defined in HQET, whereas the -quark mass
is kept finite. The leading-order contributions of two- and three-particle
distribution amplitudes are taken into account. From the resulting light-cone
sum rules we calculate all B\to \Dst form factors in the region of small
momentum transfer (maximal recoil). In the infinite heavy-quark mass limit the
sum rules reduce to a single expression for the Isgur-Wise function. We compare
our predictions with the form factors extracted from experimental B\to \Dst l
\nu_l decay rates fitted to dispersive parameterizations.Comment: 20 pages, 6 figures; one reference, one figure and several comments
added; version to appear in European Physical Journal
Non-perturbative determination of improvement b
We present our preliminary results of the non-perturbative determination of the valence mass dependent coefficients bA - bP and bm as well as the ratio ZPZm=ZA entering the flavour non-singlet PCAC relation in lattice QCD with Nf = 3 dynamical flavours. We apply the method proposed in the past for quenched approximation and Nf = 2 cases, employing a set of finite-volume ALPHA configurations with Schrödinger functional boundary conditions, generated with O(a) improved Wilson fermions and the tree-level Symanzik-improved gauge action for a range of couplings relevant for simulations at lattice spacings of about 0.09 fm and below
The (LATTICE) QCD Potential and Running Coupling: How to Accurately Interpolate between Multi-Loop QCD and the String Picture
We present a simple parameterization of a running coupling constant, defined
via the static potential, that interpolates between 2-loop QCD in the UV and
the string prediction in the IR. Besides the usual \Lam-parameter and the
string tension, the coupling depends on one dimensionless parameter,
determining how fast the crossover from UV to IR behavior occurs (in principle
we know how to take into account any number of loops by adding more
parameters). Using a new Ansatz for the LATTICE potential in terms of the
continuum coupling, we can fit quenched and unquenched Monte Carlo results for
the potential down to ONE lattice spacing, and at the same time extract the
running coupling to high precision. We compare our Ansatz with 1-loop results
for the lattice potential, and use the coupling from our fits to quantitatively
check the accuracy of 2-loop evolution, compare with the Lepage-Mackenzie
estimate of the coupling extracted from the plaquette, and determine Sommer's
scale much more accurately than previously possible. For pure SU(3) we
find that the coupling scales on the percent level for .Comment: 47 pages, incl. 4 figures in LaTeX [Added remarks on correlated vs.
uncorrelated fits in sect. 4; corrected misprints; updated references.
Infrared behavior of the gluon propagator in lattice Landau gauge: the three-dimensional case
We evaluate numerically the three-momentum-space gluon propagator in the
lattice Landau gauge, for three-dimensional pure-SU(2) lattice gauge theory
with periodic boundary conditions. Simulations are done for nine different
values of the coupling , from (strong coupling) to (in the scaling region), and for lattice sizes up to . In the
limit of large lattice volume we observe, in all cases, a gluon propagator
decreasing for momenta smaller than a constant value . From our data
we estimate MeV. The result of a gluon propagator
decreasing in the infrared limit has a straightforward interpretation as
resulting from the proximity of the so-called first Gribov horizon in the
infrared directions.Comment: 14 pages, BI-TP 99/03 preprint, correction in the Acknowledgments
section. To appear in Phys.Rev.
Non-perturbative renormalization of QCD
In these lectures, we discuss different types of renormalization problems in
QCD and their non-perturbative solution in the framework of the lattice
formulation. In particular the recursive finite size methods to compute the
scale-dependence of renormalized quantities is explained. An important
ingredient in the practical applications is the Schr\"odinger functional. It is
introduced and its renormalization properties are discussed.
Concerning applications, the computation of the running coupling and the
running quark mass are covered in detail and it is shown how the
-parameter and renormalization group invariant quark mass can be
obtained. Further topics are the renormalization of isovector currents and
non-perturbative Symanzik improvement.Comment: 49 pages, lectures at Schladming-9
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