Heavy Neutral Leptons from low-scale seesaws at the DUNE Near Detector

Abstract

Heavy nearly-sterile neutrinos are a common ingredient in extensions of the Standard Model which aim to explain neutrino masses, like for instance in Type I seesaw models, or one of its variants. If the scale of the new Heavy Neutral Leptons (HNLs) is sufficiently low, observable signatures can arise in a range of current and upcoming experiments, from the LHC to neutrino experiments. In this article, we discuss the phenomenology of sterile neutrinos in the MeV to GeV mass range, focusing on their decays. We embed our discussion in a realistic mass model and consider the resulting implications. We focus in particular on the impact on the signal of the strong polarisation effects in the beam for Majorana and (pseudo-)Dirac states, providing formulae to incorporate these in both production and decay. We study how the Near Detector of the upcoming Deep Underground Neutrino Experiment can constrain HNL states by searching for their decay products inside the detector. We conduct a Monte Carlo background analysis for the most promising signatures, incorporating the detector's particle identification capabilities, and estimate the experimental sensitivity of DUNE to these particles. We also present an estimate of the ν τ -derived HNL flux at DUNE, currently missing in the literature, which allows us to discuss searches for HNLs at higher masses.The evidence for three neutrino flavour oscillation is well established [1,2] and can be accounted for only if neutrino mass splittings are non zero [3]. This implies that neutrinos are massive and mix, forcing to consider extensions of the Standard Model (SM) to explain their origin. A simple means of doing so is to introduce the right-handed counterpart of SM neutrinos, which are singlet with respect to all SM gauge symmetries. The Lagrangian includes a Yukawa coupling between these sterile states, the Higgs boson and the leptonic doublet, which generates Dirac mass-terms below the scale of Electroweak Symmetry Breaking (EWSB), and Majorana mass terms for the new singlet states. On diagonalisation of the resulting neutrino mass matrix, the heavy neutrino states, commonly known as nearlysterile neutrinos or Heavy Neutral Leptons (HNLs) in experimental contexts, remain mainly in the sterile neutrino direction and have sub-weak interactions suppressed by elements of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix.These states have been connected to a vast range of phenomenological behaviours and even to cosmological implications (for a recent review on sterile neutrinos, see e.g. ref.[4]). For instance, nearly-sterile neutrinos in the keV region are viable warm Dark Matter candidates (see e.g. ref.[5]), whereas heavier HNLs could play a role in leptogenesis [6][7][8][9][10][11][12][13][14][15][16][17][18]. So far, some possible hints in favour of heavy neutrinos have emerged in neutrino appearance oscillation experiments, specifically LSND [19] and MiniBooNE [20][21][22] but are disfavoured by disappearance experiments [23][24][25], unless non-standard effects ar...

Dirac vs. Majorana HNLs (and their oscillations) at SHiP

“…where we have omitted the ± for brevity if they can be inferred from context, Θ α,β are the mixing angles, and v = |φ| ≈ 246 GeV is the vacuum expectation value of the Higgs field. These results are consistent with the polarized decay rates from [57], but generalize to the case where the primary decay produces a superposition of HNL helicity eigenstates. The above two expressions differ in the trace, therefore we generically expect them to produce different momentum distributions for LNC and LNV processes.…”

Searches for atmospheric long-lived particles

“…The total width is computed adding the partial widths into two-body decay channels (that is, into charged mesons and charged leptons, or neutral mesons and neutrinos), the three-body decay channels into charged leptons and neutrinos, and the three-body invisible decay into three light neutrinos. The partial widths for these decays can be found in several references [13,39,46,48] although they do not all agree. We have re-computed the two-body decay widths into mesons and leptons in the effective Lagrangian at low energies, and found good agreement with the results from the recent review in Ref.…”

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