Search for neutrinoless double- beta decay in 136Xe with a time projection chamber.

A xenon time projection chamber (TPC) with an active volume of 180 liters has been built to study neutrinoless double-β decay in 136 Xe. The experiment was performed in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62.5% in 136 Xe. The experimental details, background considerations, detector performance, and data analysis are discussed. From 6830 h of data, no evidence has been found for the 0v 0 + → 0 + transition. Half-life limits of T 1/2 0v > 3.4(6.4) × 10 23 yr in the mass mechanism mode, and T 1/2 0v > 2.6(4.9) × 10 23 yr in the right-handed currents mode, at the 90(68)% C.L., were derived, corresponding to an upper limit on the Majorana neutrino mass parameter of about 2.8 eV