Initial results from the CHOOZ experiment

Initial results are presented from CHOOZ, a long-baseline reactor-neutrino vacuum-oscillation experiment. Electron antineutrinos from the reactors were detected by a liquid scintillation calorimeter located underground at a distance of about 1 km. The 300 MWE rock overburden strongly reduced backgrounds due to cosmic-ray muons, leading to a background rate of about one event per day, more than an order of magnitude smaller than the observed neutrino signal. From the statistical agreement between detected and expected neutrino event rates, we find (at 90 % C.L.) no evidence for neutrino oscillations in the νe disappearance mode for the parameter region given approximately by Δm2 > 0.9 10-3 eV2 for maximum mixing and sin22θ > 0.18 for large Δm2 (author)