M. Perrottet

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 sizable reduction of the discrepancy between the present experimental value of a(mu) and its theoretical counterpart in the standard model.

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 JP quantum numbers 0−, 1−, 0+ and 1+ to the lightest state and to treat 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 (p4) in the chiral expansion, with the phenomenologically known couplings is also made.

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 Greens function.

Two loop electroweak corrections to the muon g-2: A New class of hadronic contributions

Abstract We discuss, within the framework of the Standard Model, the calculation of the two-loop electroweak contributions to the anomalous magnetic moment of the muon involving triangle fermionic loops of leptons and quarks. Because of the large ratios of masses involved, these contributions are rather large. The result we obtain differs from a previous estimate reported in the literature. The discrepancy originates in the cancellation of anomalies in SU (3) c × SU (2) L × U (1) y , a cancellation that requires the consideration of both leptons and quarks within each generation and that had been previously overlooked.

Decay of pseudoscalars into lepton pairs and large N(c) 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.

New nonrenormalization theorems for anomalous three point functions

Nonrenormalization theorems involving the transverse, i.e. non anomalous, part of the VVA correlator in perturbative QCD are proven. Some of the consequences and questions they raise are discussed.

Higher order hadronic corrections to the anomalous magnetic moment of the muon

Abstract The contributions to the muon g -factor anomally, a μ ≡ 1 2 (g-2) from higher order virtual hadronic effects have been estimated. Many of these contributions can be still expressed as a convolution integral over the total e + e − → hadrons cross section σ H ( t ) and are thus reliably estimated. The rest of the contributions, which involve the light by light hadronic amplitude, have been calculated within a SU(3) color ×SU(4) flavor scheme. For the total hadronic contribution to α mu , which also includes the dominant hadronic corrections as well as an estimate, using asymptotic freedom, of the induced contribution from the asymptotic behaviour of σ H(√>7.4 GeV) we obtain αμ (hadrons)=(66.4±10.2)×10 −9 .

Possible bounds on compositeness from inclusive one jet production in large hadron colliders

Abstract By analyzing deviations from QCD for inclusive jet production, we derive bounds that could be obtained at LHC and SSC on the compositeness scale Λ q (Λ g ) which governs a typical four-quark (two-quark-two-gluon) contact interaction. We also study similar bounds for a two-quark-two-lepton contact term from dilepton production.

More structure in the string tension

Abstract Monte Carlo data for the string tension of mixed action SU(2) gauge theory are confronted with strong coupling expansions and weak coupling universality constraints. It turns out that either the Wilson action is unique in that it scales precociously as is usually accepted, or that there is additional structure in the Wilson string tension (and a larger value of the Λ-parameter). After finishing this work we received a preprint by Bhanot and Dashen, whose string tension data are shown to fit the strong coupling expansion. This explains their anomaly in the renormalisation group scale.

Anomalous ward-identities, gauge-variance and appearance of ghosts in Higgs-Kibble type theories

Abstract It is shown how anomalies of the Adler type obliterate unitarity of otherwise renormalizable theories of the Higgs-Kibble type.