Lepton flavour violation in the presence of a fourth generation of quarks and leptons

Abstract

We calculate the rates for the charged lepton flavour violating (LFV) decays i → j γ, τ → π, τ → η ( ) , µ − → e − e + e − , the six three-body leptonic decays τ − → − i + j − k and the rate for µ − e conversion in nuclei in the Standard Model (SM3) extended by a fourth generation of quarks and leptons (SM4), assuming that neutrinos are Dirac particles. We also calculate branching ratios for K L → µe, K L → π 0 µe, B d,s → µe, B d,s → τ e and B d,s → τ µ. We find that the pattern of the LFV branching ratios in the SM4 differs significantly from the one encountered in the MSSM, allowing to distinguish these two models with the help of LFV processes in a transparent manner. Also differences with respect to the Littlest Higgs model with T-parity are found. Most importantly the branching ratioscan all still be as large as the present experimental upper bounds but not necessarily simultaneously. Also the rate for µ − e conversion in nuclei can reach the corresponding upper bound.

Muon conversion to electron in nuclei in type-I seesaw models

“…6 The analytic expressions used in Ref. [19] are the same ones that are valid for the case of a fourth generation of quarks and leptons [54] (that is, with an active extra neutrino instead of a singlet right-handed neutrino, which cannot decouple as the remaining low-energy theory would not be renormalizable). 7 For MN ≈ 100 GeV, we get R T i µ→e ≈ 6.7 · Br(µ → eγ) and Br(µ → eee) ≈ 0.033 · Br(µ → eγ), which differs with Eq.…”

Lepton flavor violation in the Simplest Little Higgs model

“…Thus, in the basis where the former is diagonal, the latter mixes different light lepton flavors. Denoting the eigenvalues of λ ℓ as y ℓ i , the light lepton masses are given by 37) whereas the left-handed components of the light physical fields are obtained by the replacement:…”

“…A general discussion of different scenarios, including the LHT model as well as supersymmetric and extra dimensional models, can be found in [35,36]. The comparison of muon and τ branching fractions, when the latter are known with a better precision, should eventually help to discriminate between specific models [37]. If one just relies on processes involving only the first two families, models can be essentially classified depending on the order where µ → eee appears, i.e.…”

The NA62 experiment at CERN

“…In evaluating the rates for the FCNC kaon decays, the different terms of the operator product expansion are differently sensitive to modifications from a given new-physics scenario. If BR(K L → π 0 νν) and BR(K + → π + νν) are ultimately both measured, and one or both BRs is found to differ from its SM value, it may be possible to characterize the physical mechanism involved [3], e.g., a mechanism with minimal flavor violation [4], manifestations of supersymmetry [5], a fourth generation of fermions [6], Higgs compositeness as in the littlest Higgs model [7], or an extra-dimensional mechanism such as in the Randall-Sundrum model [8]. The most precise experimental result for the BR(K + → π + νν) has been obtained by the dedicated experiments E787 and E949 at the Brookhaven National Laboratory [9] and is based on a total of 7 events using a decay-at-rest technique; the combined measurement is BR(K + → π + νν) = (17.3 ± 11.5) × 10 −11 .…”