61 research outputs found
Dynamical quark loop light-by-light contribution to muon g-2 within the nonlocal chiral quark model
The hadronic corrections to the muon anomalous magnetic moment a_mu, due to
the gauge-invariant set of diagrams with dynamical quark loop light-by-light
scattering insertions, are calculated in the framework of the nonlocal chiral
quark model. These results complete calculations of all hadronic light-by-light
scattering contributions to a_mu in the leading order in the 1/Nc expansion.
The result for the quark loop contribution is
a_mu^{HLbL,Loop}=(11.0+-0.9)*10^(-10), and the total result is
a_mu^{HLbL,NxQM}=(16.8+-1.2)*10^(-10).Comment: 11 pages, 5 figures, 1 tabl
Lepton Phenomenology of Stueckelberg Portal to Dark Sector
We propose an extension of the Standard Model (SM) with a gauge
invariant Dark Sector connected to the SM via a new portal arising in the
framework of dark photon mass generation via Stueckelberg mechanism. This
mechanism implies the existence of a scalar field , which is
shift-transformed under this group and resembles an axion-like particle (ALP)
widely addressed in the literature in different contexts. The effective dim=5
operators constructed of the covariant derivative of the field
generate flavor non-diagonal renormalizable couplings of both and
to the SM fermions . Contrary to the conventional kinetic mixing portal,
in our scenario flavor diagonal - couplings are not proportional to
the fermion charges. These features drastically change the phenomenology of
dark photon relaxing or avoiding some previously established experimental
constraints. We focus on the phenomenology of the described scenario of the
Stueckelberg portal in the lepton sector and analyze the contribution of the
dark sector fields and to the anomalous magnetic moment of muon
, Lepton Flavor Violating decays and
conversion in nuclei. We obtain limits on the model parameters from the
existing experimental data on the corresponding observables.Comment: 15 pages, 8 figure
The muon g-2: retrospective and future
Soon, new experiments at FNAL and J-PARC will measure the muon anoma-lous magnetic moments with better accuracy than before. From theoretical side, the un-certainty of the standard model prediction is dominated by the hadronic contributions. Current status of the experimental data and theoretical calculations are briefly discussed
The light-by-light contribution to the muon anomalous magnetic moment from the axial-vector mesons exchanges within the nonlocal quark model
The contribution of axial-vector mesons to the muon's anomalous magnetic
moment through a light-by-light process is considered within a nonlocal quark
model. The model is based on a four-quark interaction with scalar--pseudoscalar
and vector--axial-vector sectors. While the transverse component of the
axial-vector corresponds to a spin-1 particle, the unphysical longitudinal
component is mixed with a pseudoscalar meson. The model parameters are
re-fitted to the pion properties in the presence of pi-a_1 mixing. The obtained
estimation for the light-by-light contribution of a_1+f_1 mesons is
(3.6+-1.8)*10^{-11}.Comment: 18 pages, 10 figures, final version accepted for publication in
Physical Review
Current status of the muon g-2
The current status of the muon g-2 problem is briefly discussed. We briefly discuss the latest results on the muon g-2 measured in experiment and obtained theoretically within the standard model. Special attention is for the hadronic corrections and in particular the corrections due to the light by light scattering mechanism. For latter we present the results found in the leading in 1=N c approximation with the nonlocal chiral quark model
Dynamical quark loop light-by-light contribution to muon g-2 within the nonlocal chiral quark model
The anomalous magnetic moment of the muon in the Standard Model
194 pages, 103 figures, bib files for the citation references are available from: https://muon-gm2-theory.illinois.eduWe review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including with negligible numerical uncertainty. The electroweak contribution is suppressed by and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at and is due to hadronic vacuum polarization, whereas at the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads and is smaller than the Brookhaven measurement by 3.7. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future-which are also discussed here-make this quantity one of the most promising places to look for evidence of new physics
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