Evidence for neutrino oscillations from muon decay at rest

Abstract

A search for \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$\ensuremath{\rightarrow}\ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ oscillations has been conducted at the Los Alamos Meson Physics Facility using \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$ from ${\mathrm{\ensuremath{\mu}}}^{+}$ decay at rest. The \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ are detected via the reaction \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{ep}}$\ensuremath{\rightarrow}${\mathit{e}}^{+}$n, correlated with the 2.2 MeV \ensuremath{\gamma} from np\ensuremath{\rightarrow}d\ensuremath{\gamma}. The use of tight cuts to identify ${\mathit{e}}^{+}$ events with correlated \ensuremath{\gamma} rays yields 22 events with ${\mathit{e}}^{+}$ energy between 36 and 60 MeV and only 4.6\ifmmode\pm\else\textpm\fi{}0.6 background events. The probability that this excess is due entirely to a statistical fluctuation is 4.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$. A ${\mathrm{\ensuremath{\chi}}}^{2}$ fit to the entire ${\mathit{e}}^{+}$ sample results in a total excess of 51.${0}_{\mathrm{\ensuremath{-}}19.5}^{+20.2}$\ifmmode\pm\else\textpm\fi{}8.0 events with ${\mathit{e}}^{+}$ energy between 20 and 60 MeV. If attributed to \ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{\ensuremath{\mu}}}$\ensuremath{\rightarrow}\ensuremath{\nu}${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{e}}$ oscillations, this corresponds to an oscillation probability (averaged over the experimental energy and spatial acceptance) of (0.31\ifmmode\pm\else\textpm\fi{}0.12\ifmmode\pm\else\textpm\fi{}0.05)%. \textcopyright{} 1996 The American Physical Society.