Background free search for neutrinoless double beta decay with GERDA Phase II
Abstract
The Standard Model of particle physics cannot explain the dominance of matter over anti-matter in our Universe. In many model extensions this is a very natural consequence of neutrinos being their own anti-particles (Majorana particles) which implies that a lepton number violating radioactive decay named neutrinoless double beta (0ββ) decay should exist. The detection of this extremely rare hypothetical process requires utmost suppression of any kind of backgrounds. The GERDA collaboration searches for 0ββ decay of 76Ge (76Ge →\,76Se + 2e-) by operating bare detectors made from germanium with enriched 76Ge fraction in liquid argon. Here, we report on first data of GERDA Phase II. A background level of ≈10-3 cts/(keV·kg·yr) has been achieved which is the world-best if weighted by the narrow energy-signal region of germanium detectors. Combining Phase I and II data we find no signal and deduce a new lower limit for the half-life of 5.3·1025 yr at 90 % C.L. Our sensitivity of 4.0·1025 yr is competitive with the one of experiments with significantly larger isotope mass. GERDA is the first 0ββ experiment that will be background-free up to its design exposure. This progress relies on a novel active veto system, the superior germanium detector energy resolution and the improved background recognition of our new detectors. The unique discovery potential of an essentially background-free search for 0ββ decay motivates a larger germanium experiment with higher sensitivity.