6,790 research outputs found
Electroweak matrix elements at large Nc: matching quarks to mesons
I review some progress made on the problem of calculating electroweak
processes of mesons at low energy with the use of an approximation to large-Nc
QCD which we call the Minimal Hadronic Approximation. An update of results for
the matrix elements of the electroweak penguin operators Q7 and Q8 is also
given.Comment: 12 pages, 2 figures. Invited talk given at ``The Phenomenology of
Large-Nc QCD'', Arizona State U., January 9-11, 2002. A comment and a
reference adde
Matching Long and Short Distances in Large-Nc QCD
It is shown, with the example of the experimentally known Adler function,
that there is no matching in the intermediate region between the two asymptotic
regimes described by perturbative QCD (for the very short-distances) and by
chiral perturbation theory (for the very long-distances). We then propose to
consider an approximation of large-Nc QCD which consists in restricting the
hadronic spectrum in the channels with J^P quantum numbers 0^-, 1^-, 0^+ and
1^+ to the lightest state and treating the rest of the narrow states as a
perturbative QCD continuum; the onset of this continuum being fixed by
consistency constraints from the operator product expansion. We show how to
construct the low-energy effective Lagrangian which describes this
approximation. The number of free parameters in the resulting effective
Lagrangian can be reduced, in the chiral limit where the light quark masses are
set to zero, to just one mass scale and one dimensionless constant to all
orders in chiral perturbation theory. A comparison of the corresponding
predictions, to O(p^4) in the chiral expansion, with the phenomenologically
known couplings is also made.Comment: 35 pages, 9 figures, LaTeX. Added a couple of reference
LOOKING AT THE QCD CORRECTIONS FOR LARGE : AN EFFECTIVE LAGRANGIAN POINT OF VIEW
We discuss the QCD corrections to the large- electroweak contributions
to and to the process as two of the most
representative examples. This needs the construction of an effetive field
theory below the top quark. We discuss the issue of what scale is the
appropriate one at every stage and argue that, while matching corrections do
verify the simple prescription of taking in ,
logarithmic (i.e. ) corrections do not, and require the use of
the running in the corresponding renormalization group
equations. In particular we obtain the correction to the
non-universal contribution to the vertex.Comment: Latex with macro appended at the beginning of file. Talk given at the
Ringberg Workshop ``Perspectives for electroweak interactions in
collisions", Ringberg Castle, Germany, February 5--8, 1995. To appear in the
proceeding
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