Abstract
Extending the Standard Model (SM) with sterile ('right-handed') neutrinos is one of the best motivated ways to account for the observed neutrino masses. We discuss the expected sensitivity of future lepton collider experiments for probing such extensions. An interesting testable scenario is given by 'symmetry protected seesaw models', which theoretically allow for sterile neutrino masses around the electroweak scale with up to order one mixings with the light (SM) neutrinos. In addition to indirect tests, e.g. via electroweak precision observables, sterile neutrinos with masses around the electroweak scale can also be probed by direct searches, e.g. via sterile neutrino decays at the Z pole, deviations from the SM cross section for four lepton final states at and beyond the WW threshold and via Higgs boson decays. We study the present bounds on sterile neutrino properties from LEP and LHC as well as the expected sensitivities of possible future lepton colliders such as ILC, CEPC and FCC-ee (TLEP).
Highlights
We study the present bounds on sterile neutrino properties from LEP and LHC as well as the expected sensitivities of possible future lepton colliders such as International Linear Collider (ILC), Circular Electron Positron Collider (CEPC) and FCC-ee (TLEP)
A third sterile neutrinos may exist in addition, but we assume that it has zero charge under the “lepton-number-like” symmetry such that in the symmetry limit they decouple from the other particles and will be ignored
We summarize the present constraints and possible future sensitivities on sterile neutrino parameters in the minimal symmetry protected type-I seesaw scenario in figures 12 and 13
Summary
As described in the introduction, we investigate sterile neutrinos with masses around the EW scale. Such sterile neutrino masses can be realized in a “natural” way together with large (even O(1)) Yukawa couplings to the lepton doublets and the Higgs doublet if there is a “lepton-number-like” symmetry which controls the size of the light neutrinos’ masses, i.e. protects them from getting too large.. Neutrino masses in such scenarios are small when this protective symmetry is only slightly broken, in contrast to the usual seesaw mechanism where the smallness of the light neutrinos’ masses comes from the heaviness of the sterile states. In our analysis we will focus on a minimal version of the symmetry protected scenario, where the experimentally observable effects stem from one pair of sterile neutrinos (having opposite charges under the protective symmetry)
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