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Register a new userSearch for heavy neutral leptons in $W^+tomu^{+}mu^{pm}text{jet}$ decays
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A search is performed for heavy neutrinos in the decay of a $W$ boson into two muons and a jet. The data set corresponds to an integrated luminosity of approximately $3.0 text{ fb}^{-1}$ of proton-proton collision data at centre-of-mass energies of 7 and $8 text{ TeV}$ collected with the LHCb experiment. Both same-sign and opposite-sign muons in the final state are considered. Data are found to be consistent with the expected background. Upper limits on the coupling of a heavy neutrino with the Standard Model neutrino are set at $95%$ confidence level in the heavy-neutrino mass range from 5 to $50 text{ GeV}/c^2$. These are of the order of $10^{-3}$ for lepton-number-conserving decays and of the order of $10^{-4}$ for lepton-number-violating heavy-neutrino decays.
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Evidence of a heavy neutrino, $ u_H$, in the $K^+tomu^+ u_H$ decays was sought using the E949 experimental data with an exposure of $1.70times 10^{12}$ stopped kaons. With the major background from the radiative $K^+tomu^+ u_mugamma$ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between muon and heavy neutrino, $|U_{mu H}|^2$, were set at the level of $10^{-7}$ to $10^{-9}$ for the heavy neutrino mass region 175 to 300 MeV/$c^2$.
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.
We present upper limits on the production of heavy neutral leptons (HNLs) decaying to $mu pi$ pairs using data collected with the MicroBooNE liquid-argon time projection chamber (TPC) operating at Fermilab. This search is the first of its kind performed in a liquid-argon TPC. We use data collected in 2017 and 2018 corresponding to an exposure of $2.0 times 10^{20}$ protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of $approx 800$ MeV. HNLs with higher mass are expected to have a longer time-of-flight to the liquid-argon TPC than Standard Model neutrinos. The data are therefore recorded with a dedicated trigger configured to detect HNL decays that occur after the neutrino spill reaches the detector. We set upper limits at the $90%$ confidence level on the element $lvert U_{mu4}rvert^2$ of the extended PMNS mixing matrix in the range $lvert U_{mu4}rvert^2<(6.6$-$0.9)times 10^{-7}$ for Dirac HNLs and $lvert U_{mu4}rvert^2<(4.7$-$0.7)times 10^{-7}$ for Majorana HNLs, assuming HNL masses between $260$ and $385$ MeV and $lvert U_{e 4}rvert^2 = lvert U_{tau 4}rvert^2 = 0$.
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.
A search for $B^{0}_{s}totau^{pm}mu^{mp}$ and $B^{0}totau^{pm}mu^{mp}$ decays is performed using data corresponding to an integrated luminosity of 3 fb$^{-1}$ of proton-proton collisions, recorded with the LHCb detector in 2011 and 2012. For this search, the $tau$ lepton is reconstructed in the $tau^{-}topi^{-}pi^{+}pi^{-} u_{tau}$ channel. No significant signal is observed. Assuming no contribution from $B^{0}totau^{pm}mu^{mp}$ decays, an upper limit is set on the $B^{0}_{s}totau^{pm}mu^{mp}$ branching fraction of $mathcal{B}left( B^{0}_{s}totau^{pm}mu^{mp}right) < 4.2times 10^{-5}$ at $95%$ confidence level. If instead no contribution from $B^{0}_{s}totau^{pm}mu^{mp}$ decays is assumed, a limit of $mathcal{B}left( B^{0}totau^{pm}mu^{mp}right) < 1.4times 10^{-5}$ is obtained at $95%$ confidence level. These are the first limit on $mathcal{B}left( B^{0}_{s}totau^{pm}mu^{mp}right)$ and the worlds best limit on $mathcal{B}left( B^{0}totau^{pm}mu^{mp}right)$.
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