Search for Neutrinoless Double Beta Decay with the NEMO-3 Detector:. First Results

The NEMO-3 detector, which has been operating in the Fréjus Underground Laboratory since February 2003, is devoted to searching for neutrinoless double beta decay (ββ0v). The expected performance of the detector has been successfully achieved. Half-lives of the two neutrinos double beta decay (ββ2v) have been measured for ¹⁰⁰Mo, ⁸²Se, ⁹⁶Zr, ¹¹⁶Cd and ¹⁵⁰Nd. After 265 days of data collection from February 2003 until March 2004, no evidence for neutrinoless double beta decay (ββ0v) was found from ~7 kg of ¹⁰⁰Mo and ~1 kg of ⁸²Se. The corresponding lower limits for the half-lives are 3.5 × 10²³ years at 90% C.L for ¹⁰⁰Mo and 1.9 × 10²³ years for ⁸²Se. Limits for the effective Majorana neutrino mass are <m_v> < 0.7 - 1.2 eV for ¹⁰⁰Mo and <m_v> < 1.3 - 3.2 eV for ⁸²Se. Radon is the dominant background today and a radon trapping factory will be in operation by the end of September 2004. The NEMO-3 expected sensitivity after 5 years of data is 0.2 eV.