Neutrino masses and lepton flavor violation in thick brane scenarios

Abstract

We address the issue of lepton flavor violation and neutrino masses in the ''fat-brane'' paradigm, where flavor changing processes are suppressed by localizing different fermion field wave functions at different positions ͑in the extra dimensions͒ in a thick brane. We study the consequences of suppressing lepton number violating charged lepton decays within this scenario for lepton masses and mixing angles. In particular, we find that charged lepton mass matrices are constrained to be quasidiagonal. We further consider whether the same paradigm can be used to naturally explain small Dirac neutrino masses by considering the existence of three right-handed neutrinos in the brane, and discuss the requirements to obtain phenomenologically viable neutrino masses and mixing angles. Finally, we examine models where neutrinos obtain a small Majorana mass by breaking lepton number in a far away brane and show that, if the fat-brane paradigm is the solution to the absence of lepton number violating charged lepton decays, such models predict, in the absence of flavor symmetries, that charged lepton flavor violation will be observed in the next round of rare muon or tau decay experiments.

Phenomenological implications of neutrinos in extra dimensions

“…In particular, it is important to note that while some unknown mechanism could be responsible for suppressing effects which lead, e.g., to proton decay, it is hard to believe that there are no extra contributions to lepton flavour violating processes, given that the neutrino data indicates that individual lepton flavours are strongly broken symmetries (see [36] for a discussion of these effects and possible ways of addressing this issue). Nonetheless, we will ignore here such contributions, which are impossible to estimate.…”

(S)fermion masses in the fat brane scenario

“…They are simpler than other configurations in the literatures [2,3,4,5,6,7] in the following points:…”

“…These small parameters are obtained by a small overlap of wave functions, even if the parameters in a fundamental theory are of order unity. This mechanism has been applied to various phenomenological issues so far, such as the fermion mass hierarchy [2,3,4,5,6,7], the doublet-triplet splitting [8,9,10], and the sfermion mass generation [3,11,6,12].…”

“…The setup of authors [2,4,5] is as follows. A Higgs field lives in the bulk and its zero mode wave function is flat.…”

Neutrino masses with a single generation in the bulk

“…Introducing n such fermions would of course solve the problem, but would be in opposition to the approach, where at least the charged family multiplication simply results from the topological structure. Fortunately, even with the simplest use of one N field, we get easily two light states (this is a funciton of the chosen charge assignements, see * Other possibilities include putting left-and right-handed neutrinos to very different places inside the thick brane [6] or putting the scalar (not the fermion) in the bulk. The latter approach can be easily implemented in the model of Ref.…”