Searches for Sterile Neutrinos at Future Electron-Proton Colliders

Sterile neutrinos are an attractive extension of the Standard Model of elementary particles towards including a mechanism for generating the observed light neutrino masses. We discuss that when an approximate protective "lepton number"-like symmetry is present, the sterile neutrinos can have masses around the electroweak scale and potentially large neutrino Yukawa couplings, which makes them well testable at planned future particle colliders. We systematically discuss the production and decay channels for sterile neutrinos at electron-proton colliders and give a complete list of the leading order signatures for sterile neutrino searches. We highlight several novel search channels and present a first look at the possible sensitivities for the active-sterile mixing parameters and the heavy neutrino masses. We also compare the performance of electron-proton colliders with the ones of proton-proton and electron-positron colliders, and discuss the complementarity of the different collider types.Comment: Conference proceedings for the DIS 2017 in Birmingham, 13 pages, 8 figures, 2 table

Testing sterile neutrino extensions of the Standard Model at future lepton colliders

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).Comment: Version 2: typos fixed, references added, matches published versio

Sterile neutrino searches via displaced vertices at LHCb

We explore the sensitivity of displaced vertex searches at LHCb for testing sterile neutrino extensions of the Standard Model towards explaining the observed neutrino masses. We derive estimates for the constraints on sterile neutrino parameters from a recently published displaced vertex search at LHCb based on run 1 data. They yield the currently most stringent limit on active-sterile neutrino mixing in the sterile neutrino mass range between 4.5 GeV and 10 GeV. Furthermore, we present forecasts for the sensitivities that could be obtained from the run 2 data and also for the high-luminosity phase of the LHC.Comment: 5 pages, 6 Figures, 1 Table, version to appear in PL

Sterile neutrino searches at future e−e+e^-e^+, pppp, and e−pe^-p colliders

Sterile neutrinos are among the most attractive extensions of the SM to generate the light neutrino masses observed in neutrino oscillation experiments. When the sterile neutrinos are subject to a protective symmetry, they can have masses around the electroweak scale and potentially large neutrino Yukawa couplings, which makes them testable at planned future particle colliders. We systematically discuss the production and decay channels at electron-positron, proton-proton and electron-proton colliders and provide a complete list of the leading order signatures for sterile neutrino searches. Among other things, we discuss several novel search channels, and present a first look at the possible sensitivities for the active-sterile mixings and the heavy neutrino masses. We compare the performance of the different collider types and discuss their complementarity.Comment: 20 pages + appendix + references, 21 figures, 10 Tables; added references; extended discussion of displaced vertex searches at pp an ep collider

Displaced vertex searches for sterile neutrinos at future lepton colliders

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 $10^{-11}$.Comment: 13 pages plus references, 8 figures, 1 table, matches published versio

Higgs production from sterile neutrinos at future lepton colliders

In scenarios with sterile (right-handed) neutrinos that are subject to an approximate "lepton-number-like" symmetry, the heavy neutrinos (i.e. the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings while accounting for the smallness of the light neutrinos' masses. In these scenarios, the on-shell production of heavy neutrinos and their subsequent decays into a light neutrino and a Higgs boson constitutes a hitherto unstudied resonant contribution to the Higgs production mechanism. We investigate the relevance of this resonant mono-Higgs production mechanism in leptonic collisions, including the present experimental constraints on the neutrino Yukawa couplings, and we determine the sensitivity of future lepton colliders to the heavy neutrinos. With Monte Carlo event sampling and a simulation of the detector response we find that, at future lepton colliders, neutrino Yukawa couplings below the percent level can lead to observable deviations from the SM and, furthermore, the sensitivity improves with higher center-of-mass energies (for identical integrated luminosities).Comment: 26 pages, 9 figures, 7 tables; v2: references adde

Prospects for Heavy Scalar Searches at the LHeC

In this article we study the prospects of the proposed Large Hadron electron Collider (LHeC) in the search for heavy neutral scalar particles. We consider a minimal model with one additional complex scalar singlet that interacts with the Standard Model (SM) via mixing with the Higgs doublet, giving rise to a SM-like Higgs boson $h_1$ and a heavy scalar particle $h_2$. Both scalar particles are produced via vector boson fusion and can be tested via their decays into pairs of SM particles, analogously to the SM Higgs boson. Using multivariate techniques we show that the LHeC is sensitive to $h_2$ with masses between 200 and 800 GeV down to scalar mixing of $\sin^2 \alpha \sim 10^{-3}$