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Register a new userDark Scalars and Heavy Neutral Leptons at DarkQuest
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The proposed DarkQuest beam dump experiment, a modest upgrade to the existing SeaQuest/SpinQuest experiment, has great potential for uncovering new physics within a dark sector. We explore both the near-term and long-term prospects for observing two distinct, highly-motivated hidden sector benchmark models: heavy neutral leptons and Higgs-mixed scalars. We comprehensively examine the particle production and detector acceptance at DarkQuest, including an updated treatment of meson production, and light scalar production through both bremsstrahlung and gluon-gluon fusion. In both benchmark models, DarkQuest will provide an opportunity to probe previously inaccessible interesting regions of parameter space on a fairly short timescale when compared to other proposed experiments.
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New Physics models in which the Standard Model particle content is enlarged via the addition of sterile fermions remain among the most minimal and yet most appealing constructions, particularly since these states are present as building blocks of numerous mechanisms of neutrino mass generation. Should the new sterile states have non-negligible mixings to the active (light) neutrinos, and if they are not excessively heavy, one expects important contributions to numerous high-intensity observables, among them charged lepton flavour violating muon decays and transitions, and lepton electric dipole moments. We briefly review the prospects of these minimal SM extensions to several of the latter observables, considering both simple extensions and complete models of neutrino mass generation. We emphasise the existing synergy between different observables at the Intensity Frontier, which will be crucial in unveiling the new model at work.
We constrain the lifetime of thermally produced Heavy Neutral Leptons (HNLs) from primordial nucleosynthesis. We show that even a small fraction of mesons present in the primordial plasma leads to the over-production of the primordial helium. This puts an upper bound on the lifetime of HNLs $tau_{N}<0.02$ s for masses $m_{N}>m_{pi}$ (as compared to 0.1 s reported previously). In combination with accelerator searches, this allows us to put a new lower bound on the HNLs masses and defining the bottom line for HNL searches at the future Intensity Frontier experiments.
We present the first study of the production of a Standard Model Higgs boson at a lepton collider in association with a pair of W bosons, $e^+_{} e^-_{} to W^+_{} W^-_{} H$, in the inverse seesaw model. Taking into account all relevant experimental and theoretical constraints, we find sizable effects due to the additional heavy neutrinos up to -38% on the total cross-section at a center-of-mass energy of 3 TeV, and even up to -66% with suitable cuts. This motivates a detailed sensitivity analysis of the process $e^+_{} e^-_{} to W^+_{} W^-_{} H$ as it could provide a new, very competitive experimental probe of low-scale neutrino mass models.
Heavy neutral leptons are predicted in many extensions of the Standard Model with massive neutrinos. If kinematically accessible, they can be copiously produced from kaon and pion decays in atmospheric showers, and subsequently decay inside large neutrino detectors. We perform a search for these long-lived particles using Super-Kamiokande multi-GeV neutrino data and derive stringent limits on the mixing with electron, muon and tau neutrinos as a function of the long-lived particle mass. We also present the limits on the branching ratio versus lifetime plane, which are helpful in determining the constraints in non-minimal models where the heavy neutral leptons have new interactions with the Standard Model.
We calculate the signal rate of hypothetical heavy neutral leptons (HNL or sterile neutrinos) from kaon decays expected in the framework of the SHiP experiment. The kaons are produced in the hadronic shower initiated in the beam-dump mode by 400 GeV protons from CERN SPS. For a sufficiently light HNL (when the decays are kinematically allowed) we find kaon decays to be a noticeably richer source of HNL as compared to $D$-meson decays adopted in previous studies of the HNL phenomenology at SHiP. In particular, SHiP is capable of fully exploring the central part of the kinematically allowed region of the HNL mass and mixing with electron and muon neutrinos down to the lower cosmological bound. The latter is associated with HNL decays in the early Universe to energetic products rescattering off and thus destroying light nuclei produced at the primordial nucleosynthesis. A consistency of the HNL model with smaller mixing would require either a hierarchy -- much larger mixing of all the HNL with tau neutrino -- or non-standard cosmology and new ingredients in the HNL sector, closing the room for the minimal non-seesaw type I model with sterile neutrinos lighter than kaons.
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