Indirect searches for sterile neutrinos at a high-luminosity Z-factory

Abstract

A future high-luminosity Z-factory will offer the possibility to study rare Z decays, as those leading to lepton flavour violating final states. Processes such as Z → ∓ 1 ± 2 are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating Z decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one sterile fermion state, and two wellmotivated frameworks of neutrino mass generation, the Inverse Seesaw embedded into the Standard Model, and the νMSM. Our study shows that sterile neutrinos can give rise to contributions to BR(Z → ∓ 1 ± 2 ) within reach of the FCC-ee. We also discuss the complementarity between a high-luminosity Z-factory and low-energy charged lepton flavour violation facilities.

Impact of sterile neutrinos on nuclear-assisted cLFV processes

“…The ISS mass regions leading to the above signals can be further probed via complementarity studies, if one again considers cLFV from the µ − τ sector, and its potential observation at a future collider. As was already the case for the simple "3+1" toy model, the (3,3) ISS regions with a BR(µ − e − → e − e − , Al) 10 −17 would also induce BR(Z → µτ ) within reach of the FCC-ee [41].…”

“…In addition to the three light (mostly active) neutrinos -whose masses are given by a type I seesaw relation -the neutral lepton spectrum further contains three heavy states (with masses m ν 4−6 ); should the latter three play a rôle regarding DM, or the BAU, their spectrum and couplings to the active states are subject to several (strong) conditions [43][44][45]. As previously done in [41], we parametrise the physical νMSM degrees of freedom in terms of its six mass eigenvalues, encoding all physical mixing angles and CP violating phases (Dirac and Majorana) in an effective 6 × 6 unitary mixing matrix U (which allows to readily implement the already well-established bounds on the νMSM parameter space). 4 The angles θ lj , l = 1, 2, 3, j = 4, 5, 6 encode the active-sterile mixings, while the mixings between the sterile states are given by three additional angles θ 45,46,56 .…”

“…In fact, most of the points within COMET sensitivity are predicted to account for a BR(Z → µτ ) lying within FCC-ee reach ( 3 × 10 −13 ) [41]. This implies that this simple "3+1" toy model can be probed via two fully independent -and yet strongly complementarycLFV experimental approaches.…”

“…In the basis where the charged lepton mass matrix is diagonal, the leptonic mixing matrix is given by the rectangular 3 × 9 sub-matrix corresponding to the first three columns of U, with the 3 × 3 block corresponding to the (non-unitary)Ũ PMNS . In order to derive the contributions to the studied observables, a scan is carried over the 9 × 9 neutrino mass matrix (for a detailed discussion of the numerical studies, see [41]): the modulus of the entries of the M R and µ X matrices are randomly taken to lie on the intervals 0.5 GeV (M R ) i 10 6 GeV and 0.01 eV (µ X ) ij 1 MeV, with complex entries for the lepton number violating matrix µ X ; a modified Casas-Ibarra parametrisation [42] for…”

Electron electric dipole moment in Inverse Seesaw models

“…Extensions of the SM with sterile fermions, which accommodate oscillation data, may also have an impact on other observables, such as charged lepton flavour violation (cLFV) in Higgs [34][35][36], neutral Z boson [37][38][39] and meson decays [40][41][42][43][44]. Sterile fermions may also contribute to lepton flavour conserving observables such as the dipole moments of charged leptons [45][46][47].…”