Abstract
The KARMEN experiment at the ISIS spallation source has enhanced its sensitivity for ν μ → ν e oscillations in 1996 by the installation of an additional large area veto counter system. Consisting of 136 highly transparent plastic scintillator modules, the new veto is completely embedded inside the experiment's 7000 tonne steel blockhouse. It allows to eliminate the main background component in the search for ν μ → ν e appearance originating from high energy neutrons induced by deep inelastic scattering of cosmic ray muons in the massive shielding. First measurements after the detector upgrade show a substantial reduction of this background meeting the expected factor of 40. If no oscillation signal is seen after 3 years of measuring (1997–1999) KARMEN will be able to exclude mixing amplitudes of sin 22 Θ≃1 x10 -3. The ongoing KARMEN2 measurements will thus allow a decisive and reliable test of the entire parameter space favoured by the positive result of LSND. An even higher oscillation sensitivity can be achieved by experiments at future high intensity short spill proton accelerators such as the planned European Spallation Source ESS.
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