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The LSND experiment

The Liquid Scintillator Neutrino Detector (LSND) has recently reported the observation of events which can be interpreted in terms of $\overline{\nu}_{\mu} - \overline{\nu}_{e}$ oscillations. The LSND experiment uses neutrinos from $\pi$ and $\mu$ decay from the beam stop of the Los Alamos Meson Physics Facility (LAMPF). i) $\pi^{+} \rightarrow \mu^{+}\nu_{\mu}$ (in flight or at rest); ii) $\mu^{+} \rightarrow e^{+}\overline{\nu}_{\mu}\nu_{e}$ (at rest); iii) $\pi^{-} \rightarrow \mu^{-}\overline{\nu}_{\mu}$ (in flight); iv) $\mu^{-} \rightarrow e^{-}\nu_{\mu}\overline{\nu}_{e}$ (at rest). The relative yield of $\overline{\nu}_{e}$ above an energy of 36 MeV is only $\sim 4\times 10^{-4}$. LSND consists of a tank containing 167 tons of liquid scintillator (doped mineral oil, CH2). It detects $\overline{\nu}_{e}$ by the reaction $\overline{\nu}_{e} + p \rightarrow e^{+}+n$ which gives a prompt e+ signal followed by a delayed 2.2 MeV $\gamma$-ray from the capture reaction np $\rightarrow$ d$\gamma$. This experiment has reported the observation of 22 events with e+ energy between 36 and 60 MeV and only $4.6 \pm 0.6$ background events. The probability that this excess results from a statistical fluctuation of the background is $4.1 \times10^{-8}$. If attributed to $\overline{\nu}_{\mu} - \overline{\mu}_{e}$ oscillations, it corresponds to an oscillation probability of $(0.31\pm^{0.11}_{0.10}\pm 0.005) \times 10^{-2}$, averaged over the neutrino energy spectrum and spatial distribution. Figure 1 shows the favoured regions of the oscillation parameter plane, together with the regions excluded by other experiments. 
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