The KARMEN experiment  Searches for Neutrino Oscillations  Searches for Neutrino Oscillations

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.