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Register a new userSearch for Heavy Neutrinos in $pi to mu u$ Decay
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Heavy neutrinos were sought in pion decays $pi^+ rightarrow mu^+ u$ by examining the observed muon energy spectrum for extra peaks in addition to the expected peak for a massless neutrino. No evidence for heavy neutrinos was observed. Upper limits were set on the neutrino mixing matrix $|U_{mu i}|^2$ in the neutrino mass region of 15.7--33.8 MeV/c$^2$, improving on previous results by an order of magnitude.
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A search for the D0 to pi+ pi- mu+ mu- decay, where the muon pair does not originate from a resonance, is performed using proton-proton collision data corresponding to an integrated luminosity of 1.0 fb-1 recorded by the LHCb experiment at a centre-of-mass energy of 7 TeV. No signal is observed and an upper limit on the relative branching fraction with respect to the resonant decay mode D0 to pi+ pi- phi(to mu+ mu-), under the assumption of a phase-space model, is found to be B(D0 to pi+ pi- mu+ mu-)/B(D0 to pi+ pi- phi(to mu+ mu-)) < 0.96 at the 90% confidence level. The upper limit on the absolute branching fraction is evaluated to be B(D0 to pi+ pi- mu+ mu-) < 5.5 x 10-7 at 90% confidence level. This is the most stringent to date.
A search for heavy Majorana neutrinos produced in the $B^- to pi^+mu^-mu^-$ decay mode is performed using 3 fb$^{-1}$ of integrated luminosity collected with the LHCb detector in $pp$ collisions at center-of-mass energies of 7 TeV and 8 TeV at the LHC. Neutrinos with masses in the range 250-5000 MeV and lifetimes from zero to 1000 ps are probed. In the absence of a signal, upper limits are set on the branching fraction ${cal{B}}(B^- to pi^+mu^-mu^-)$ as functions of neutrino mass and lifetime. These limits are on the order of $10^{-9}$ for short neutrino lifetimes of 1 ps or less. Limits are also set on the coupling between the muon and a possible fourth-generation neutrino.
A search for massive neutrinos has been made in the decay $pirightarrow e^+ u$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($pirightarrow e^+ u_h$). Upper limits (90 % C.L.) on the neutrino mixing matrix element $|U_{ei}|^2$ in the neutrino mass region 60--135 MeV/$c^2$ were set, which are %representing an order of magnitude improvement over previous results.
An upper limit on the branching ratio for the decay $K^+ ! rightarrow ! pi^+ u overline{ u}$ is set at $2.4 times 10^{-9}$ at the 90% C.L. using pions in the kinematic region $214~{rm MeV}/c < P_pi < 231~{rm MeV}/c$. An upper limit of $5.2 times 10^{-10}$ is found on the branching ratio for decays $K^+ ! rightarrow ! pi^+ X^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0$.
We observe a net beam-excess of $8.7 pm 6.3$ (stat) $pm 2.4$ (syst) events, above 160 MeV, resulting from the charged-current reaction of $ u_{mu}$ and/or $bar u_{mu}$ on C and H in the LSND detector. No beam related muon background is expected in this energy regime. Within an analysis framework of $pi^0 to u_{mu}bar u_{mu}$, we set a direct upper limit for this branching ratio of $Gamma(pi^0 to u_mu bar u_mu) / Gamma(pi^0 to all) < 1.6 times 10^{-6}$ at 90% confidence level.
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