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Register a new userNew constraints on tau-coupled Heavy Neutral Leptons with masses $m_N = 280-970$ MeV
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A search for Heavy Neutral Leptons has been performed with the ArgoNeuT detector exposed to the NuMI neutrino beam at Fermilab. We search for the decay signature $N to u mu^+ mu^-$, considering decays occurring both inside ArgoNeuT and in the upstream cavern. In the data, corresponding to an exposure to $1.25 times 10^{20}$ POT, zero passing events are observed consistent with the expected background. This measurement leads to a new constraint at 90% confidence level on the mixing angle $leftvert U_{tau N}rightrvert^2$ of tau-coupled Dirac Heavy Neutral Leptons with masses $m_N =$ 280 - 970 MeV, assuming $leftvert U_{eN}rightrvert^2 = leftvert U_{mu N}rightrvert^2 = 0$.
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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 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 re-analyze the results of the searches for Heavy Neutral Leptons (HNLs) by the CHARM experiment. We study HNL decay channel $Nto e^{+}e^{-} u/mu^{+}mu^{-} u$ and show that, in addition to the constraints on the HNLs mixings with $ u_e$ or $ u_{mu}$, the same data also implies limits on the HNLs that mix only with $ u_{tau}$ and have masses in the range $380 text{ MeV}<m_{N}lesssim 1.6text{ GeV}$ - the region in the parameter space that was considered in the literature as a target for HNLs searches.
We study the effects induced by excited leptons on the leptonic tau decay at one loop level. Using a general effective lagrangian approach to describe the couplings of the excited leptons, we compute their contributions to the leptonic decays and use the current experimental values of the branching ratios to put limits on the mass of excited states and the substructure scale.
A search for neutral heavy leptons (NHLs) has been performed using an instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The decay channel was composed of helium bags interspersed with drift chambers, and was used in conjunction with the NuTeV neutrino detector to search for NHL decays. The data were examined for NHLs decaying into muonic final states (mu mu nu, mu e nu, mu pi, and mu rho); no evidence has been found for NHLs in the 0.25 - 2.0 GeV mass range. This analysis places limits on the mixing of NHLs with standard light neutrinos at a level up to an order of magnitude more restrictive than previous search limits in this mass range.
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