Muonium-antimuonium conversion in models with heavy neutrinos

Abstract

We study muonium-antimuonium conversion and µ + e − → µ − e + scattering within two different lepton-flavor-violating models with heavy neutrinos: model I is a typical seesaw that violates lepton number as well as flavor; model II has a neutrino mass texture where lepton number is conserved. We look for the largest possible amplitudes of these processes that are consistent with current bounds. We find that model I has very limited chance of providing an observable signal, except if a finely tuned condition in parameter space occurs. Model II, on the other hand, requires no fine tuning and could cause larger effects. However, the maximum amplitude provided by this model is still two orders of magnitude below the sensitivity of current experiments: one predicts an effective coupling G MM up to 10 −4 GF for heavy neutrino masses near 10 TeV. We have also clarified some discrepancies in previous literature on this subject.

Impact of sterile neutrinos on nuclear-assisted cLFV processes

“…This observable has been addressed in the context of several extensions of the SM via additional sterile states, mostly in relation with type I seesaw realisations with heavy RH neutrinos [109][110][111].…”

“…There are four different diagrams which can be separated in two contributions, those which are common to both Dirac and Majorana neutrinos (figure 2, upper boxes) and two other which appear only if neutrinos are Majorana particles ( figure 2, lower boxes). The computation of the box diagrams of figure 2 (in a unitary gauge) allows to write the effective coupling G MM as [109,110]: …”

Lepton flavor violation in muonium decay and muon colliders in models with heavy neutrinos

“…Further details were given in previous works [15,16]. The main features of these models are (i) they contain heavy neutrinos, (ii) in the mass eigenstate basis, the heavy neutrinos couple with charged leptons of mixed flavor under weak interactions (just like Cabbibo-Kobayashi-Maskawa mixing in the quark sector), and (iii) the neutrinos are generally of Majorana type.…”

“…(4). In the case of no CP violation and real mass matrix M, we can choose the unitary matrix U as a real orthogonal matrix which diagonalizes the symmetric and real matrix M; then some of the λ j 's are +1 and others are −1 [16]. Nonetheless we can choose all λ j = +1, but then in general the unitary matrix U will not be real.…”

“…The explicit expressions of these functions are given in Ref. [16]. (15)- (19) involve summation over all 2-component neutrinos, where N L is the number of standard (light) ones and N H the number of extra (heavy) ones (= N L , 2N L in Models I, II, respectively).…”