Search

The three body pion decays π+→l+νX(l=e,μ), where X is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on Γ(π+→e+νX)/Γ(π+→μ+νμ) in the mass range 0

A search for massive neutrinos has been made in the decay π+→e+ν. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (π→e+νh). Upper limits (90% C.L.) on the neutrino mixing matrix element |Uei|2 in the neutrino mass region 60–135 MeV/c2 were set and are an order of magnitude improvement over previous results.

Evidence of the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\nu}$ decay was searched for using E949 (Brookhaven National Laboratory, USA) experimental data with an exposure of $1.70\ifmmode\times\else\texttimes\fi{}1{0}^{12}$ stopped kaons. The data sample is dominated by the background process ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\gamma}$. An upper limit on the decay rate $\mathrm{\ensuremath{\Gamma}}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\nu})l2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{\ensuremath{\Gamma}}({K}^{+}\ensuremath{\rightarrow}\text{all})$ at 90% confidence level was set assuming the standard model muon spectrum. The data are presented in such a way as to allow calculation of rates for any assumed ${\ensuremath{\mu}}^{+}$ spectrum.

Evidence of a heavy neutrino, ν H , in the K + → μ + ν H decays was sought using the E949 experimental data with an exposure of 1.70 × 1 0 12 stopped kaons. With the major background from the radiative K + → μ + ν μ γ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between the muon and heavy neutrinos, | U μ H | 2 , were set at the level of 1 0 - 7 to 1 0 - 9 for the heavy neutrino mass region 175 to 300 MeV / c 2 .

The positron spectrum from the decay ..pi../sup +/..-->..e/sup +/..nu../sub e/ has been examined for evidence of peaks arising from an admixture in the ..nu../sub e/ weak eigenstate of massive neutrinos. Limits on the intensity of such peaks, together with the measured ..pi -->..e..nu.. branching ratio, have been used to derive constraints on the neutrino mixing parameters over the range 4 MeV

A search for the decays ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ and ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{0}$, where ${X}^{0}$ is any weakly interacting neutral particle or system of neutral particles with $150l{M}_{{X}^{0}}l250$ MeV/${\mathit{c}}^{2}$, was performed in the pion kinematic region below the ${K}_{\ensuremath{\pi}2}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{0})$ peak. An upper limit on $B({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})$ is set at 1.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ (90% C.L.) and $B({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{0})$ is restricted to the ${10}^{\ensuremath{-}7}$ level or lower. Limits are also set for the production of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ through scalar or tensor interactions at 1.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ and 1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ (90% C.L.), respectively.

A measurement of the decay K{sup +{yields}{pi}0{mu}+{nu}}{sub {mu}{gamma}}has been performed with the E787 detector at Brookhaven National Laboratory. Forty events were observed in the signal region with the background expectation of (16.5{+-}2.7) events. The branching ratio was measured to be (1.58{+-}0.46(stat.){+-}0.08(syst.))x10{sup -5} in the kinematic region E{sub {gamma}>}30 MeV and {theta}{sub {mu}{gamma}>}20 deg., where E{sub {gamma}}is the energy of the emitted photon and {theta}{sub {mu}{gamma}}is the angle between the muon and the photon in the K{sup +} rest frame. The results were consistent with theoretical predictions.

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ in the pion momentum region $140l{P}_{\ensuremath{\pi}}l199\text{ }\text{ }\mathrm{MeV}/c$ in an exposure of $1.71\ifmmode\times\else\texttimes\fi{}{10}^{12}$ stopped kaons with an estimated total background of $0.93\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat}{)}_{\ensuremath{-}0.24}^{+0.32}(\mathrm{syst})$ events. This brings the total number of observed ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.73}_{\ensuremath{-}1.05}^{+1.15})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. An interpretation of the results for alternative models of the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}+nothing$ is also presented.

Experiment E949 at Brookhaven National Laboratory studied the rare decay K{sup +}-->pi{sup +} nu{ovr {nu}} and other processes with an exposure of 1.77 x 10{sup 12} k{sup +}'s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of 0.30 {+-} 0.03 events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio B(K{sup +}-->pi{sup +} nu{ovr {nu}}) = (1.47{sub -0.89}{sup +1.30}) x 10{sup -10}, consistent with the standard model prediction of (0.74 {+-} 0.20) x 10{sup -10}. This is a more detailed report of results previously published [V.V. Anisimovsky et al., Phys. Rev. Lett. 93, 031801 (2004)].

A sample of kinematically identified K{sup +}{yields}{pi}{sup +}{pi}{sup 0} decays obtained with the E949 detector was used to search for the helicity-suppressed decay {pi}{sup 0}{yields}{nu}{nu} resulting in an upper limit of 2.7x10{sup -7} at 90% confidence level. The upper limit is also applicable to {pi}{sup 0} decays into unknown weakly-interacting particles.

We report the results of a search for the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ in the kinematic region with ${\ensuremath{\pi}}^{+}$ momentum $140lPl195\mathrm{MeV}/c$ using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of $1.22\ifmmode\pm\else\textpm\fi{}0.24$ events results in a 90% C.L. upper limit of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ on the branching ratio of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}.$ We also report improved limits on the rates of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{0}$ and ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{1}{X}^{2}$ where ${X}^{0}{,X}^{1}{,X}^{2}$ are hypothetical, massless, long-lived neutral particles.

We have performed a search for the angular-momentum forbidden decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\gamma}$ with the E787 detector at BNL. No events were observed in the ${\ensuremath{\pi}}^{+}$ kinematic region around $227 \mathrm{MeV}/c.$ An upper limit on the branching ratio for the decay is determined to be $3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ at $90%$ confidence level.

Evidence of massive neutrinos was sought in the ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}\ensuremath{\nu}$ decay spectrum with the background from the $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\rightarrow}e$ decay chain highly suppressed. Upper limits (90% C.L.) were set on the neutrino mixing parameter ${|{U}_{\mathrm{ei}}|}^{2}l{10}^{\ensuremath{-}7}$ in the mass region $50l{m}_{\ensuremath{\nu}}l130$ MeV/${\mathit{c}}^{2}$.

An upper limit on the branching ratio for the decay K{sup +}{r_arrow}e{sup +}{nu}{mu}{sup +}{mu}{sup {minus}} is set at 5.0{times}10{sup {minus}7} at a 90{percent} confidence level, consistent with predictions from chiral perturbation theory. {copyright} {ital 1998} {ital The American Physical Society}

Evidence of massive neutrinos was sought in the {pi}{sup +}{implies}{mu}{sup +}{ital x} decay spectrum in the mass region {ital M}{sub {ital x}}=30{endash}33.91 MeV/{ital c}{sup 2}. Upper limits on the branching ratio {ital R}{sub {mu}}{sub {ital x}}{le}(4{endash}6){times}10{sup -}{sup 5} (90{percent} C.L.) were set for this region, which correspond to the neutrino mixing parameter {vert_bar}{ital U}{sub {mu}}{sub {ital x}}{vert_bar}{sup 2}{much_gt}10{sup -}{sup 3}{endash}10{sup -}{sup 4}. {copyright} {ital 1995 The American Physical Society.}

Limits on the lepton-flavor-violating reactions ${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$+Z\ensuremath{\rightarrow}${e}^{\mathrm{\ensuremath{-}}}$+Z and ${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$+Z\ensuremath{\rightarrow}${e}^{+}$+(Z-2), muon-electron and muon-positron conversion, have been obtained from a search performed at TRIUMF using a time-projection chamber. Upper limits (90% C.L.) for the branching ratios compared to ordinary muon capture for a titanium target are R---(Ti)=\ensuremath{\Gamma}(${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$Ti\ensuremath{\rightarrow}${\mathit{e}}^{\mathrm{\ensuremath{-}}}$Ti)/\ensuremath{\Gamma}(${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$Ti capture)4.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ and ${\mathit{R}}_{+}$(Ti)=\ensuremath{\Gamma}(${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$Ti\ensuremath{\rightarrow}${\mathit{e}}^{+}$Ca*)/\ensuremath{\Gamma}(${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$Ti capture).1.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ A smaller data set obtained using a lead target yielded ${R}_{\mathrm{\ensuremath{-}}}$(Pb)4.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$. The implications of these results for extensions of the standard model which allow lepton-flavor violation are discussed.

A measurement of the ..pi -->..e..nu.. branching ratio using a NaI(Tl) spectrometer yields a value GAMMA((..pi -->..e..nu..)+(..pi -->..e..nu gamma..))/ GAMMA((..pi --> mu nu..)+(..pi --> mu nu gamma..)) = (1. 218 +- 0.014) x 10/sup -4/. The result is consistent with expectations of the standard electroweak theory incorporating electron-muon universality.

Data from a new study of the decay ..pi -->..e..nu.. are used to place bounds on Majoron-neutrino couplings and to obtain inclusive branching-ratio limits for ..pi -->..e..nu..M, where M is a new particle with a mass in the range 0 to 125 MeV. A limit for the process ..pi../sup +/..-->..e/sup +/..nu nu..nu-bar is also obtained.