The Search for Neutrino Oscillations ν̄μ→ ν̄e with KARMEN

The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS. It provides νμ’s, νe’s and ν̄μ’s in equal intensities from the π–μ–decay at rest (DAR). The oscillation channel ν̄μ→ ν̄e is investigated in the appearance mode with a 56 t liquid scintillation calorimeter at a mean distance of 17.7m from the ν–source looking for p ( ν̄e , e + ) n reactions. The cosmic induced background for this oscillation search could be reduced by a factor of 40 due to an additional veto counter installed in 1996. In the data collected through 1997 and 1998 no potential oscillation event was observed. Using a unified approach to small signals this leads to an upper limit for the mixing angle of sin(2Θ) < 1.3 · 10 (90%CL) at large ∆m . The excluded area in (sin(2Θ),∆m) covers almost entirely the favored region defined by the LSND ν̄μ→ ν̄e evidence.