The Search for Neutrino Oscillations ¯νµ→¯νe with KARMEN

KARMEN, the KArlsruhe Rutherford Medium Energy Neutrino experiment, is located at the pulsed spallation neutron source ISIS of the Rutherford Appleton Laboratory. In the ISIS beam stop ν µ , ν e and ¯ ν µ are produced from the π + –µ + decay chain at rest with energies up to 52.8 MeV. Besides a very low ¯ ν e contamination, ISIS stands out for its unique time structure. This allows for a highly sensitive search for ¯ ν µ →¯ ν e –oscillations with the KARMEN detector, a 56 t segmented liquid scintillation calorimeter with very good time, energy and position resolution. In 1996 an additional third veto counter was installed within the 7000 t steel blockhouse that shields the detector against cosmic induced background. Covering the detector from all sides it strongly reduces the background of cosmic induced high energy neutrons by a factor of 40. Here we present the data taken after this major upgrade from Feb. 1997 until Feb. 1999. Since there is no indication for any beam excess, an upper limit for the ¯ ν µ →¯ ν e oscillation is deduced using the Unified Approach based on a maximum likelihood analysis. The result, as all the data presented before, questions the interpretation of the LSND beam excess as an indication for ¯ ν µ →¯ ν e oscillation. The search for neutrino oscillations is one of the most fascinating topics of modern particle physics. The KArlsruhe Rutherford Medium Energy Neutrino experiment KARMEN searches for neutrino oscillations in different appearance (ν µ → ν e [2] and ¯ ν µ → ¯ ν e) and disappearance modes (ν e → ν x [3]). The physics program of KARMEN also includes the investigation of ν–nucleus interactions [4] as well as the search for lepton number violating decays of pions and muons and a test of the V–A structure of the µ + decay [5]. In the following we present the result of the search for ¯ ν µ →¯ ν e oscillation on the basis of the data taken from February 1997 until February 1999 (KARMEN 2 data) after the experiment upgrade in 1996. The data taken before the upgrade from 1990-1995 (KARMEN 1 data) is not included in the analysis. Such a combined analysis would yield a much lower sensitivity due to the relatively high cosmic induced neutron background of the KARMEN 1 data. …