The search for neutrino oscillations with KARMEN

Abstract The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS. It provides ν μ 's, ν e 's and ν μ 's in equal intensities from the π + − μ + -decay at rest (DAR). The oscillation channel ν μ → ν e 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 ( ν e , 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 2 (2Θ) −3 (90% CL) at large Δm 2 . The excluded area in (sin 2 (2Θ),Δm 2 ) covers almost entirely the favored region defined by the LSND ν μ → ν 3 evidence.