Search for neutrino oscillations at a fission reactor

The energy spectrum of neutrinos from a fission reactor was studied with the aim of gaining information on neutrino oscillations. The well-shielded detector was set up at a fixed position of 8.76 m from the pointlike core of the Laue-Langevin reactor in an antineutrino flux of 9.8×{}10¹¹ cm^-2 s^-1. The target protons in the reaction νₑp→e⁺n were provided by liquid-scintillator counters (total volume of 377 1) which also served as positron detectors. The product neutrons moderated in the scintillator were detected by ³He wire chambers. A coincidence signature was required between the prompt positron and the delayed neutron events. The positron energy resolution was 18% full width at half maximum at 0.91 MeV. The signal-to-background ratio was better than 1: 1 between 2 and 6 MeV positron energy. At a counting rate of 1.58 counts per hour, 4890±{}180 neutrino-induced events were detected. The shape of the measured positron spectrum was analyzed in terms of the parameters Δ² and sin²2θ for two-neutrino oscillations. The experimental data are consistent with no oscillations. An upper limit of 0.15 eV² (90% C.L.) for the mass-squared differences Δ² of the neutrinos was obtained, assuming maximum mixing of the two-neutrino states. The ratio of the measured to the expected integral yield of positrons assuming no osciliations was determined to be ∫Yₑₓₚ∫Yₜₕ=0.955±0.035(statistical)±0.110(systematic).