INITIAL RESULTS FROM THE CHOOZ LONG-BASE-LINE REACTOR NEUTRINO OSCILLATION EXPERIMENT

Initial results are presented from CHOOZ(1), a long-baseline reactor-n eutrino vacuum-oscillation experiment. The data reported here were tak en during the period March to October 1997, when the two reactors ran at combined power levels varying from zero to values approaching their full rated power of 8.5 GW (thermal). Electron antineutrinos from the reactors were detected by a liquid scintillation calorimeter located at a distance of about 1 km. The detector was constructed in a tunnel protected from cosmic rays by a 300 MWE rock overburden. This massive shielding strongly reduced potentially troublesome 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 neutrin o signal. From the statistical agreement between detected and expected neutrino event rates, we find (at 90% confidence level) no evidence f or neutrino oscillations in the <(nu)over bar>(e) disappearance mode f or the parameter region given approximately by Delta m(2) > 0.9 10(-3) eV(2) for maximum mixing and Delta m(2). (C) 1998 Elsevier Science B. V.