Search

In this article we investigate the prospects of searching for sterile neutrinos in lowscale seesaw scenarios via the lepton flavour violating (but lepton number conserving) dilepton dijet signature. In our study, we focus on the final state $e^\pm \mu^\mp jj$ at the HL-LHC and the FCC-hh (or the SppC). We perform a multivariate analysis at the detector level including the dominant SM backgrounds from di-top, di-boson, and tri-boson. Under the assumption of the active-sterile neutrino mixings $|V_{ l N}|^2=|\theta_e|^2=|\theta_\mu|^2$ and $|V_{ \tau N}|^2 = |\theta_\tau|^2=0$, the sensitivities on the signal production cross section times branching ratio $\sigma(p p \to l^\pm N)\times {\rm BR} (N \to l^{ \mp} jj)$ and on $|V_{ l N}|^2$ for sterile neutrino mass $M_N$ between 200 and 1000 GeV are derived. For the benchmark $M_N=500$ GeV, when ignoring systematic uncertainties at the HL-LHC (FCC-hh/SppC) with 3 (20) ${\rm ab}^{-1}$ luminosity, the resulting 2-$\sigma$ limits on $|V_{ l N}|^2$ are $4.9\times 10^{-3}$ ($7.0\times 10^{-5}$), while the 2 -$\sigma$ limit on $\sigma \times {\rm BR}$ are $4.4\times10^{-2}$ ($1.6\times10^{-2}$) fb, respectively. The effect of the systematic uncertainty is also studied and found to be important for sterile neutrinos with smaller masses. We also comment on searches with $\tau^\pm \mu^\mp jj$ and $\tau^\pm e^\mp jj$ final states., Comment: 20 pages, 9 figures

We investigate the potential of the International Linear Collider (ILC) to probe the mechanisms of neutrino mass generation and leptogenesis within the minimal seesaw model. Our results can also be used as an estimate for the potential of a Compact Linear Collider (CLIC). We find that heavy sterile neutrinos that simultaneously explain both, the observed light neutrino oscillations and the baryon asymmetry of the universe, can be found in displaced vertex searches at ILC. We further study the precision at which the flavour-dependent active-sterile mixing angles can be measured. The measurement of the ratios of these mixing angles, and potentially also of the heavy neutrino mass splitting, can test whether minimal type I seesaw models are the origin of the light neutrino masses, and it can be a first step towards probing leptogenesis as the mechanism of baryogenesis. Our results show that the ILC can be used as a discovery machine for New Physics in feebly coupled sectors that can address fundamental questions in particle physics and cosmology., Comment: Talk presented at the International Workshop on Future Linear Colliders (LCWS2017), Strasbourg, France, 23-27 October 2017. C17-10-23.2. 11 pages, 8 figures

We investigate the question whether leptogenesis, as a mechanism for explaining the baryon asymmetry of the universe, can be tested at future colliders. Focusing on the minimal scenario of two right-handed neutrinos, we identify the allowed parameter space for successful leptogenesis in the heavy neutrino mass range between $5$ and $50$ GeV. Our calculation includes the lepton flavour violating contribution from heavy neutrino oscillations as well as the lepton number violating contribution from Higgs decays to the baryon asymmetry of the universe. We confront this parameter space region with the discovery potential for heavy neutrinos at future lepton colliders, which can be very sensitive in this mass range via displaced vertex searches. Beyond the discovery of heavy neutrinos, we study the precision at which the flavour-dependent active-sterile mixing angles can be measured. The measurement of these mixing angles at future colliders can test whether a minimal type I seesaw mechanism is the origin of the light neutrino masses, and it can be a first step towards probing leptogenesis as the mechanism of baryogenesis. We discuss how a stronger test could be achieved with an additional measurement of the heavy neutrino mass difference., Comment: 30 pages plus appendix, 13 figures, references added, discussion extended, two figures added, matches journal version

Heavy neutrino-antineutrino oscillations can naturally appear in mechanisms of low scale neutrino mass generation, where pairs of heavy neutrinos have almost degenerate masses. We discuss the case where the heavy neutrinos are sufficiently long-lived to decay displaced from the primary vertex, such that the oscillations of the heavy neutrinos into antineutrinos can potentially be observed at the (high-luminosity) LHC and at currently planned future collider experiments. The observation of these oscillations would have far-reaching consequences: it would, for instance, prove the existence of lepton number violation and the Majorana nature of neutrino masses, and it would allow a deep insight into the nature of the neutrino mass generation mechanism., Comment: 6 pages + references, 3 figures, matches published version

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 PLB

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 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 version

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 added

The renormalization group (RG) running of the neutrino mass operator is required for comparing the predictions of neutrino models at high energy with the experimental data at low energies. In the type I seesaw scenario with n_G right-handed neutrinos, the RG running is also performed in the effective theories above and between the thresholds given by the masses of the right-handed neutrinos. At these thresholds, the effective theories are matched. When calculating the two-loop RG running, the matching has to be performed at the one-loop level. In this work, we calculate the one-loop matching formulae in the MSSM extended by n_G right-handed neutrinos using supergraph techniques. Moreover we present a general formula for one-loop matching of superpotential operators which can readily be applied to any supersymmetric theory where chiral superfields are integrated out., Comment: 15 pages, 4 figures; version to appear in JHEP