Abstract

We study lepton flavor observables in the Standard Model (SM) extended with all dimension-$6$ operators which are invariant under the SM gauge group. We calculate the complete one-loop predictions to the radiative lepton decays $\mu\to e\gamma$, $\tau \to \mu \gamma$ and $\tau\to e\gamma$ as well as to the closely related anomalous magnetic moments and electric dipole moments of charged leptons, taking into account all dimension-$6$ operators which can generate lepton flavor violation. Also the 3-body flavor violating charged lepton decays $\tau^\pm \to \mu^\pm \mu^+ \mu^-$, $\tau^\pm\to e^\pm e^+ e^- $, $\tau^\pm \to e^\pm \mu^+ \mu^- $, $\tau^\pm \to \mu^\pm e^+ e^- $, $\tau^\pm \to e^\mp \mu^\pm \mu^\pm$, $\tau^\pm \to \mu^\mp e^\pm e^\pm $ and $\mu^\pm \to e^\pm e^+ e^-$ and the $Z^0$ decays $Z^0\to\ell_i^+\ell_j^-$ are considered, taking into account all tree-level contributions.

Highlights

  • In most theories of physics beyond the Standard Model (SM) that have been considered, the SM is recovered at low energies via the decoupling of the heavy particles with masses of the order of Λ ≫ MZ

  • The search for lepton flavor violation (LFV) is very promising since in the SM all flavor violating effects in the charged lepton sector are proportional to the very small neutrino masses - e.g. the decay rates of heavy charged leptons into lighter ones are suppressed by the ratio m2ν/MW2 and are by far too small to be measurable in any foreseeable experiment

  • We find that the precision of Z0 → ljlj decay width measurements limit the sizes of Cφ(1l) jj, Cφ(3l) jj and Cφjje Wilson coefficients so stringently that no sizable effects in the corresponding anomalous magnetic moments are possible for any lepton flavor

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Summary

The lepton flavor violating operators of dimension-6

The complete (but still reducible) list of independent operators of dimension-5 and dimension-6 which can be constructed out of SM fields and which are invariant under the SM gauge group fields was first derived in ref. [44]. The (φ†φ)(liejφ) operator does not contain gauge boson fields and modifies only Higgs and Goldstone boson couplings, which in principle could affect our results It gives new O(1/Λ2) contribution to the charged lepton mass matrix: mlf i. The triple coupling of the physical Higgs boson h0 to charged leptons, as well as all quadruple and quintuple vertices derived from Qfeφi 3 can still be flavor violating. Their contributions to the processes discussed below vanish or are small due to an additional suppression of ml/mh0, compared to the dominant contributions from Qφe, Q(φ1l) and Q(φ3l) operators.. In the appendix we list the Feynman rules arising from the operators given in table 5 which are necessary in order to calculate the flavor observables discussed

Observables related to the effective lepton-photon coupling
Radiative lepton decays
Anomalous magnetic moments and electric dipole moments
Lepton flavor violating Z0 decays
Numerical analysis
Conclusions
Feynman rules for 4-fermion operators li2 i Λ2
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