Abstract
The upcoming Mu3e experiment aims to search for the lepton flavour violating decay \boldsymbol{\muposeeemath} with an unprecedented final sensitivity of one signal decay in \boldsymbol{\num{e16}} observed muon decays by making use of an innovative experimental design based on novel ultra-thin silicon pixel sensors. In a first phase, the experiment is operated at an existing muon beam line with rates of up to \boldsymbol{\num{e8}} muons per second. Detailed simulation studies confirm the feasibility of background-free operation and project single event sensitivities in the order of \boldsymbol{\num{e-15}} for signal decays modelled in an effective field theory approach. The precise tracking of the decay electrons and large geometric and momentum acceptance of Mu3e enable searches for physics beyond the Standard Model in further signatures. Examples of which are searches for lepton flavour violating two-body decays of the muon into an electron and an undetected boson as well as for electron-positron resonances in \boldsymbol{\muposeeenunumath} which could result for instance from a dark photon decay. The Mu3e experiment is expected to be competitive in all of these channels already in phase I.
Highlights
The flavour of leptons is conserved in the Standard Model but – as demonstrated by the observation of neutrino oscillations – it is not conserved in nature
In a Standard Model extended to include neutrino mixing, it can be mediated in loop diagrams but it is suppressed to branching fractions below 10−54 and far beyond what experiments can observe
Any observation of μ → eee would be a clear sign for physics beyond the Standard Model
Summary
The flavour of leptons is conserved in the Standard Model but – as demonstrated by the observation of neutrino oscillations – it is not conserved in nature. The violation of the flavour of charged leptons has eluded observation so far. One example for charged lepton flavour violation is the decay μ+ → e+e−e+. In a Standard Model extended to include neutrino mixing, it can be mediated in loop diagrams (see figure 1a) but it is suppressed to branching fractions below 10−54 and far beyond what experiments can observe. Many extensions of the Standard Model predict enhanced rates for μ → eee, for example via loop diagrams with supersymmetric particles (see figure 1b) or at tree-level via a Z (see figure 1c).
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