New experimental limits on νe→ντ oscillations in 2-ν and 3-ν mixing schemes

Abstract

The 56 ton high resolution liquid scintillation calorimeter KARMEN at the beam stop neutrino source ISIS has been used to search for neutrino oscillations in the disappearance channel ${\ensuremath{\nu}}_{e}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{x}.$ The ${\ensuremath{\nu}}_{e}$ emitted in ${\ensuremath{\mu}}^{+}$ decay at rest are detected with spectroscopic quality via the exclusive charged current reaction ${}^{12}\mathrm{C}({\ensuremath{\nu}}_{e}{,e}^{\ensuremath{-}}{)}^{12}{\mathrm{N}}_{\mathrm{g}.\mathrm{s}.}$ almost free of background. Analysis of the spectral shape of ${e}^{\ensuremath{-}}$ from the ${\ensuremath{\nu}}_{e}$-induced reaction as well as a measurement of the absolute ${\ensuremath{\nu}}_{e}$ flux allows one to investigate oscillations of the type ${\ensuremath{\nu}}_{e}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\nu}}_{e}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{\ensuremath{\mu}}.$ The flux-independent ratio ${R}_{\mathrm{C}\mathrm{C}/\mathrm{N}\mathrm{C}}$ of charged current events ${}^{12}\mathrm{C}({\ensuremath{\nu}}_{e}{,e}^{\ensuremath{-}}{)}^{12}{\mathrm{N}}_{\mathrm{g}.\mathrm{s}.}$ to neutral current events ${}^{12}{\mathrm{C}(\mathrm{\ensuremath{\nu}},\mathrm{\ensuremath{\nu}}}^{\ensuremath{'}}{)}^{12}{\mathrm{C}}^{*}$ provides additional information in the oscillation channel ${\ensuremath{\nu}}_{e}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{x}.$ All three analysis methods show no evidence for oscillations. For the ${\ensuremath{\nu}}_{e}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ channel 90% confidence limits of ${\mathrm{sin}}^{2}(2\ensuremath{\Theta})<0.338$ for $\ensuremath{\Delta}{m}^{2}>~100{\mathrm{eV}}^{2}{/c}^{4}$ and $\ensuremath{\Delta}{m}^{2}<0.77{\mathrm{eV}}^{2}{/c}^{4}$ for maximal mixing in a simple two-flavor oscillation formalism are derived. A complete three-flavor analysis of the experimental data from 5 years of measurement with respect to ${\ensuremath{\nu}}_{e}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\nu}}_{e}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{\ensuremath{\mu}}$ mixing is presented.