Abstract
We perform a comprehensive study of charged lepton flavour violation in Randall–Sundrum (RS) models in a fully 5D quantum-field-theoretical framework. We consider the RS model with minimal field content and a “custodially protected” extension as well as three implementations of the IR-brane localized Higgs field, including the non-decoupling effect of the KK excitations of a narrow bulk Higgs. Our calculation provides the first complete result for the flavour-violating electromagnetic dipole operator in Randall–Sundrum models. It contains three contributions with different dependence on the magnitude of the anarchic 5D Yukawa matrix, which can all be important in certain parameter regions. We study the typical range for the branching fractions of μ→eγ, μ→3e, μN→eN as well as τ→μγ, τ→3μ and the electron electric dipole moment by a numerical scan in both the minimal and the custodial RS model. The combination of μ→eγ and μN→eN currently provides the most stringent constraint on the parameter space of the model. A typical lower limit on the KK scale T is around 2 TeV in the minimal model (up to 4 TeV in the bulk Higgs case with large Yukawa couplings), and around 4 TeV in the custodially protected model, which corresponds to a mass of about 10 TeV for the first KK excitations, far beyond the lower limit from the non-observation of direct production at the LHC.
Highlights
Rare lepton decays are among the promising indirect probes for physics beyond the Standard Model (SM)
Despite the fact that the RS model is to some extent protected from large flavour-changing neutral currents (FCNCs) [7], the strongest constraint on the scale of the extra dimension comes from Kaluza–Klein (KK) gluon mediated F = 2 processes—notably in the kaon system [8]
Even though we aim to study lepton flavour violation, the operators with two quark and two lepton fields have to be included in LLdiFmV-6, since they contribute to muon conversion in nuclei
Summary
Rare lepton decays are among the promising indirect probes for physics beyond the Standard Model (SM). Despite the fact that the RS model is to some extent protected from large flavour-changing neutral currents (FCNCs) [7], the strongest constraint on the scale of the extra dimension comes from Kaluza–Klein (KK) gluon mediated F = 2 processes—notably in the kaon system [8]. These bounds can, to some degree, be avoided by imposing some structure on the quark Yukawa matrices in the five-dimensional (5D) theory [9] or by extending the strong gauge group [10]. This is precisely the situation were low-energy precision observables can help considerably constraining the model
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