Abstract:

Although not innovative, the idea to constrain the last unknown mixing angle with an optimised reactor neutrino experiment is growing in Europe, Japan, Russia, and US since several months. Indeed, such an optimised experiment could increase the sensitivity to the parameter by a factor ten or more with respect to the best constraint obtained by the CHOOZ experiment: < 20% (90% C.L). In addition, not being sensitive to matter effects, CP-delta phase, and mass-hierarchy, the reactor constraint is competitive but also complementary to the measurements to be done by the next generation of long-baseline accelerator experiments. Among several experimental sites proposed all around the world we investigated the possibility to use French reactors, and especially the well known CHOOZ site. At this location, a 10-20 tons detector could be installed at 1.1 km from the nuclear cores in the existing former CHOOZ pit. A second detector (identical), needed to monitor the neutrino flux and spectrum as well as to decrease the detection systematic uncertainties, could be installed in a new underground laboratory to be built at 100-200m from the cores. With such a configuration, a sensitivity < 3% could be achieved after three years of operation. I will present the physical context, the Double-CHOOZ site, the new detector design forseen, the backgrounds, and the strategy to reduce the systematic uncertainties at the required level for the success of the experiment. Our goal is to present a proposal early next year to go towards data taking before 2008.