66 research outputs found
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
Electroweak Hadronic Contributions to the muon g-2
We reanalyze the two-loop electroweak hadronic contributions to the muon g-2
that may be enhanced by large logarithms. The present evaluation is improved
over those already existing in the literature by the implementation of the
current algebra Ward identities and the inclusion of the correct short-distance
QCD behaviour of the relevant hadronic Green's function.Comment: 29 pages, 6 figures, LaTeX. Typos fixe
Decay of pseudoscalars into lepton pairs and large-Nc QCD
The counterterm combination that describes the decay of pseudoscalar mesons
into charged lepton pairs at lowest order in chiral perturbation theory is
considered within the framework of QCD in the limit of a large number of
colours Nc. When further restricted to the lowest meson dominance approximation
to large-Nc QCD, our results agree well with the available experimental data.Comment: 5 pages, 2 figure
New nonrenormalization theorems for anomalous three point functions
Nonrenormalization theorems involving the transverse, i.e. non anomalous,
part of the correlator in perturbative QCD are proven. Some of their
consequences and questions they raise are discussed.Comment: 14 pages. People added in the acknowledgements. Minor changes to
match version to appear in journa
Disposition of valganciclovir during continuous renal replacement therapy in two lung transplant recipients
Objectives To determine whether valganciclovir 450 mg every 48 h for cytomegalovirus (CMV) prophylaxis provides appropriate ganciclovir exposure in solid organ transplant recipients during continuous renal replacement therapy (CRRT). Patients and methods Ganciclovir pharmacokinetics was intensively studied in two lung transplant recipients under valganciclovir 450 mg every 48 h over one dosing interval. In vitro experiments using blank whole blood spiked with ganciclovir further investigated exchanges between plasma and erythrocytes. Results Ganciclovir disposition was characterized by apparent total body clearance of 3.3 and 5.8 L/h, terminal half-life of 16.9 and 14.1 h, and apparent volume of distribution of 60.3 and 104.9 L in Patients 1 and 2, respectively. The observed sieving coefficient was 1.05 and 0.96, and the haemofiltration clearance was 3.3 and 3.1 L/h. In vitro experiments confirmed rapid efflux of ganciclovir from red blood cells into plasma, increasing the apparent efficacy of haemofiltration. Conclusions A valganciclovir dosage of 450 mg every 48 h appears adequate for patients under CRRT requiring prophylaxis for CMV infection, providing concentration levels in the range reported for 900 mg once daily dosing outside renal failur
Analyticity, crossing and the absorptive parts of the one-loop contributions to the quark-quark-gluon gauge boson four-point function
Starting from the known one-loop result for the -annihilation
process with
massless quarks we employ analyticity and crossing to determine the absorptive
parts of the corresponding one-loop contributions in Deep Inelastic Scattering
(DIS) and in the Drell-Yan process (DY). Whereas the
absorptive parts generate a non-measurable phase factor in the
-annihilation channel one obtains measurable phase effects from the
one-loop contributions in the deep inelastic and in the Drell-Yan case. We
compare our results with the results of previous calculations where the
absorptive parts in DIS and in the DY process were calculated directly in the
respective channels. We also present some new results on the dispersive and
absorptive contributions of the triangle anomaly graph to the DIS process.Comment: 23 pages, 5 figures, typos corrected. Version to appear in Phys. Rev.
Hadronic light-by-light scattering contribution to the muon g-2: an effective field theory approach
The hadronic light-by-light contribution to a_{mu}, the anomalous magnetic
moment of the muon, is discussed from the point of view of an effective
low-energy theory. As an application, the coefficient of the leading logarithm
arising from the two-loop graphs involving two anomalous vertices is computed,
and found to be positive. This corresponds to a positive sign for the pion-pole
contribution to the hadronic light-by-light correction to a_{mu}, and to a
sizeable reduction of the discrepancy between the present experimental value of
a_{mu} and its theoretical counterpart in the standard model.Comment: 4 pages, 1 figure. v2: published versio
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change
Virtual Compton scattering off the nucleon at low energies
We investigate the low-energy behavior of the four-point Green's function
describing virtual Compton scattering off the nucleon.
Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive
the leading terms of an expansion of the operator in the four-momenta and
of the initial and final photon, respectively.
The model-independent result is expressed in terms of the electromagnetic
form factors of the free nucleon, i.e., on-shell information which one obtains
from electron-nucleon scattering experiments.
Model-dependent terms appear in the operator at ,
whereas the orders and are
contained in the low-energy theorem for , i.e., no new
parameters appear.
We discuss the leading terms of the matrix element and comment on the use of
on-shell equivalent electromagnetic vertices in the calculation of ``Born
terms'' for virtual Compton scattering.Comment: 26 pages, RevTex, to appear in Phys. Rev.
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