Testing sterile neutrino extensions of the Standard Model at the Circular Electron Positron Collider.

Extending the Standard Model 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 the Circular Electron Positron Collider (CEPC) for testing such extensions. An interesting 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 active (SM) neutrinos. When the masses of the sterile neutrinos are well above the electroweak scale, they affect precision data via effective non-unitarity of the leptonic mixing matrix in a model independent way. The expected improvement of the electroweak precision observables from the CEPC may allow to test mixings between active and sterile neutrinos down to (using currently discussed CEPC performance parameters). For sterile neutrinos with masses around the electroweak scale, direct searches are possible. Such tests are given by the search for sterile neutrino decays at the pole, by deviations from the SM cross section for four leptons at and beyond the threshold, and by Higgs boson production and decays. The expected sensitivities at the CEPC could reach down to mixings as small as . [ABSTRACT FROM AUTHOR]