Evidence for $\barν_μ\to\barν_e$ Oscillations from the LSND Experiment at LAMPF

Abstract

A search for $\barν_μ\to \barν_{e}$ oscillations has been conducted at the Los Alamos Meson Physics Facility by using $\barν_μ$ from $μ^+$ decay at rest. The $\barν_e$ are detected via the reaction $\barν_e\,p \rightarrow e^{+}\,n$, correlated with a $γ$ from $np\rightarrow dγ$ ($2.2\,{\rm MeV}$). The use of tight cuts to identify $e^+$ events with correlated $γ$ rays yields 22 events with $e^+$ energy between 36 and $60\,{\rm MeV}$ and only $4.6 \pm 0.6$ background events. A fit to the $e^+$ events between 20 and $60\,{\rm MeV}$ yields a total excess of $51.8^{+18.7}_{-16.9} \pm 8.0$ events. If attributed to $\bar ν_μ\rightarrow \bar ν_e$ oscillations, this corresponds to an oscillation probability of ($0.31^{+0.11}_{-0.10} \pm 0.05$)\%.

Figures (3)

• Figure 1: The energy distribution for events which pass selection B with (a) $R \ge 0$ and (b) $R>30$. Shown in the figure are the beam-excess data, estimated neutrino background (dashed), and expected distribution for neutrino oscillations at large $\Delta m^2$ plus estimated neutrino background (solid).
• Figure 2: The R distribution, beam on minus beam-off excess, for events that satisfy selection B and that have energies in the range $20<E_e<60$ MeV. The solid curve is the best fit to the data, the dashed curve is the uncorrelated $\gamma$ component of the fit, and the dotted curve is the correlated $\gamma$ component.
• Figure 3: Plot of the LSND $\Delta m^{2}~$$\sl vs$$\sin^2 2 \theta~$ favored regions. The shaded regions are the 90% or 99% likelihood regions as defined in the text, not confidence regions. Also shown are $90\%$ C.L. limits from KARMEN at ISIS (dashed curve), E776 at BNL (dotted curve), and the Bugey reactor experiment (dot-dashed curve).