67 research outputs found
The invisible renormalon
We study the structure of renormalons in the Heavy Quark Effective Theory, by expanding the heavy quark propagator in powers of 1/m_Q. We demonstrate that the way in which renormalons appear depends on the regularisation scheme used to define the effective theory. In order to investigate the relation between ultraviolet renormalons and power divergences of matrix elements of higher-dimensional operators in the heavy quark expansion, we perform calculations in dimensional regularisation and in three different cut-off regularisation schemes. In the case of the kinetic energy operator, we find that the leading ultraviolet renormalon which corresponds to a quadratic divergence, is absent in all but one (the lattice) regularisation scheme. The nature of this ``invisible renormalon'' remains unclear
QCD factorization for exclusive, non-leptonic B meson decays: General arguments and the case of heavy-light final states
We provide a rigorous basis for factorization for a large class of non-leptonic two-body -meson decays in the heavy-quark limit. The factorization formula incorporates elements of the naive factorization approach and the hard-scattering approach, but allows us to compute systematically radiative (``non-factorizable'') corrections to naive factorization for decays such as and . We discuss the factorization formula for a general final state from a general point of view. We then consider factorization for decays into heavy-light final states (such as ) in more detail, including a proof of the factorization formula at two-loop order. Explicit results for the leading QCD corrections to factorization are presented and compared to existing measurements of branching fractions and final-state interaction phases
Radiative corrections to leptonic decays using infinite-volume reconstruction
Lattice QCD calculations of leptonic decay constants have now reached
sub-percent precision so that isospin-breaking corrections, including QED
effects, must be included to fully exploit this precision in determining
fundamental quantities, in particular the elements of the
Cabibbo-Kobayashi-Maskawa (CKM) matrix, from experimental measurements. A
number of collaborations have performed, or are performing, such computations.
In this paper we develop a new theoretical framework, based on Infinite-Volume
Reconstruction (IVR), for the computation of electromagnetic corrections to
leptonic decay widths. In this method, the hadronic correlation functions are
first processed theoretically in infinite volume, in such a way that the
required matrix elements can be determined non-perturbatively from lattice QCD
computations with finite-volume uncertainties which are exponentially small in
the volume. The cancellation of infrared divergences in this framework is
performed fully analytically. We also outline how this IVR treatment can be
extended to determine the QED effects in semi-leptonic kaon decays with a
similar degree of accuracy
First exploratory calculation of the long-distance contributions to the rare kaon decays <i>K</i> →π ℓ<sup>+</sup>ℓ<sup>-</sup>
The rare decays of a kaon into a pion and a charged lepton/antilepton pair
proceed via a flavour changing neutral current and therefore may only be
induced beyond tree level in the Standard Model. This natural suppression makes
these decays sensitive to the effects of potential New Physics. The CP
conserving decay channels however are dominated by a
single photon exchange; this involves a sizeable long-distance hadronic
contribution which represents the current major source of theoretical
uncertainty. Here we outline our methodology for the computation of the
long-distance contributions to these rare decay amplitudes using lattice QCD
and present the numerical results of the first exploratory studies of these
decays in which all but the disconnected diagrams are evaluated. The domain
wall fermion ensembles of the RBC and UKQCD collaborations are used, with a
pion mass of and a kaon mass of . In particular we determine the form factor, , of the
decay from the lattice at small values of
, obtaining for the
three values of respectively.Comment: 40 pages, 14 figures, 4 table
An ``Improved" Lattice Study of Semi-leptonic Decays of D-Mesons
We present results of a lattice computation of the matrix elements of the
vector and axial-vector currents which are relevant for the semi-leptonic
decays and . The computations are
performed in the quenched approximation to lattice QCD on a
lattice at , using an -improved fermionic action. In the limit
of zero lepton masses the semi-leptonic decays and are described by four form factors: and ,
which are functions of , where is the four-momentum transferred
in the process. Our results for these form factors at are:
f^+_K(0)=0.67 \er{7}{8} , V(0)=1.01 \err{30}{13} , A_1(0)=0.70
\err{7}{10} , A_2(0)=0.66 \err{10}{15} , which are consistent with the most
recent experimental world average values. We have also determined the
dependence of the form factors, which we find to be reasonably well described
by a simple pole-dominance model. Results for other form factors, including
those relevant to the decays \dpi and \drho, are also given.Comment: 41 pages, uuencoded compressed postscript file containing 14 figures,
LaTeX, Edinburgh Preprint 94/546 and Southampton Preprint SHEP 93/94-3
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