KARMEN: Present Neutrino-Oscillation Limits and Perspectives after the Upgrade

The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS. It provides nu_mu's, nu_e's and nu_mu_bar's in equal intensities from the pi+ mu+ decay at rest. The oscillation channels nu_mu->nu_e and nu_mu_bar->nu_e_bar are investigated with a 56t liquid scintillation calorimeter at a mean distance of 17.6m from the nu source. No evidence for oscillations could be found with KARMEN, resulting in 90% CL exclusion limits of sin(2th) < 8.5 10^-3 (nu_mu_bar->nu_e_bar) and sin(2th) < 4.0 10^-2 (nu_mu->nu_e) for Delta m^2 > 100eV^2. In 1996, the KARMEN neutrino experiment has been upgraded by an additional veto system. Vetoing of cosmic muons passing the 7000t massive iron shielding of the detector suppresses energetic neutrons from deep inelastic scattering of muons as well as from mu-capture in iron. Up to 1996, these neutrons penetrating into the detector represented the main background for the nu_mu_bar->nu_e_bar oscillation search. With an expected reduction of the background rate by a factor of 40 the experimental sensitivity for nu_mu_bar->nu_e_bar will be significantly enhanced towards sin(2th) = 10^-3 for large Delta m^2.