Conceptual Study of an ``Anti-Tagged'' Experiment Searching for muon-neutrino --> electron-neutrino Oscillation

We study the conceptual feasibility of a high energy, ``short baseline'', zero background experiment to search for muon-neutrino --> electron-neutrino oscillations and fully covering the area where the LSND experiment claims evidence. The natural electron-neutrino background of the muon-neutrino beam, from K and mu decays in the decay tunnel, is suppressed by a hadron blind detector that vetoes, by time coincidence, a possible electron-neutrino signal in the neutrino detector (anti-tagging technique). We discuss this new idea and we study a possible implementation in the old neutrino line of the PS accelerator, which at CERN offers the ideal $L/E$ ratio. In the anti-tagged muon-neutrino beam, the electron-neutrino contamination can be reduced by more than two orders of magnitude over conventional beams, down to 5*10^{-5}. In an ideal appearance experiment using a 300 t detector one would expect after two years 112 events according to the LSND result, with a background of 1.1-2.4 events. In case of a negative search, the 90% C.L. upper limit in the mixing angle would be sin^2 < 1.8*10^{-4}$ for large Delta(m^2) and Delta(m^2) <3.3*10^{-2} eV^2 for maximal mixing.