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A High Statistics Search for muon-neutrino(anti-muon-neutrino) ---> electron-neutrino(anti-electron-neutrino) Oscillations in the Small Mixing Angle Regime

Limits on $ν_μ(\overlineν_μ) \to ν_e (\overlineν_e)$ oscillations based on a statistical separation of $ν_e N$ charged current interactions in the CCFR detector at Fermilab are presented. $ν_e$ interactions are identified by the difference in the longitudinal shower energy deposition pattern of $ν_e N \rightarrow eX$ versus $ν_μN \to ν_μX$ interactions. Neutrino energies range from 30 to 600 GeV with a mean of 140 GeV, and $ν_μ$ flight lengths vary from 0.9 km to 1.4 km. The lowest 90% confidence upper limit in $sin^2 2α$ of $1.1 \times 10^{-3}$ is obtained at $Δm^2 \sim 300 eV^2$. For $sin^2 2α= 1$, $Δm^2 > 1.6 eV^2$ is excluded, and for $Δm^2 \gg 1000 eV^2$, $sin^2 2α> 1.8 \times 10^{-3}$ is excluded. This result is the most stringent limit to date for $Δm^2 > 25 eV^2$ and it excludes the high $Δm^2$ oscillation region favoured by the LSND experiment. The $ν_μ$-to-$ν_e$ cross-section ratio was measured as a test of $ν_μ(\barν_μ) \leftrightarrow ν_e (\barν_e)$ universality to be $1.026 \pm 0.055$.