Abstract
We investigate the sensitivity of future lepton colliders to displaced vertices from the decays of long-lived heavy (almost sterile) neutrinos with electroweak scale masses and detectable time of flight. As future lepton colliders we consider the FCC-ee, the CEPC, and the ILC, searching at the Z-pole and at the center-of-mass energies of 240, 350 and 500 GeV. For a realistic discussion of the detector response to the displaced vertex signal and the Standard Model background we consider the ILC's Silicon Detector (SiD) as benchmark for the future lepton collider detectors. We find that displaced vertices constitute a powerful search channel for sterile neutrinos, sensitive to squared active-sterile mixing angles as small as $10^{-11}$.
Highlights
We have investigated the sensitivity to sterile neutrinos with electroweak-scale Majorana masses from the search for displaced vertices at future lepton colliders
We deepened and extended previous work on displaced vertex searches for sterile neutrinos at future lepton colliders in various ways: we considered an explicit low scale seesaw benchmark model, the Symmetry Protected Seesaw Scenario (SPSS), and calculated the heavy-neutrino-production cross section with WHIZARD, including initial state radiation and initial state polarisation
We considered the Future Circular electron positron Collider (FCC-ee), Circular Electron Positron Collider (CEPC), and the International Linear Collider (ILC) and included the different center-of-mass energies planned for the respective physics programs, i.e. the Z pole run, the Higgs run at 240 or 250 GeV, the top threshold scan at 350 GeV and, for the ILC, 500 GeV
Summary
Sterile (or right-handed) neutrinos can have Majorana masses around the electroweak (EW) scale and unsuppressed active-sterile mixings, when they are subject to a “lepton-numberlike” symmetry. We note that the SPSS allows for additional sterile neutrinos, provided their mixings with the other neutrinos are negligible, or their masses are very large, such that their effects decouple. This is a minimal framework that can explain the two observed mass squared differences of the light neutrinos and features four independent parameters relevant for collider experiments, namely the three yνα and M. Due to the mixing between the active and sterile neutrinos, the light and heavy neutrino mass eigenstates interact with the weak gauge bosons. Further observable features of models with right-handed neutrinos have been investigated with respect to collider phenomenology in [3, 26,27,28,29, 31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]
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