Searching for neutrinoless double beta decay with GERDA

TL;DR

GERDA uses advanced germanium detectors and shielding to search for neutrinoless double beta decay, achieving a background-free regime that enhances sensitivity and sets new lower limits on decay half-life.

Contribution

GERDA introduces a novel experimental design with ultra-low background, enabling background-free operation for neutrinoless double beta decay searches in Ge76.

Findings

No evidence of decay found; lower half-life limit is 8.0e25 years.

Background-free operation allows sensitivity to grow linearly with exposure.

Expected to surpass 1e26 years in half-life sensitivity within 2018.

Abstract

The GERmanium Detector Array (GERDA) experiment located at the INFN Gran Sasso Laboratory (Italy), is looking for the neutrinoless double beta decay of Ge76, by using high-purity germanium detectors made from isotopically enriched material. The combination of the novel experimental design, the careful material selection for radio-purity and the active/passive shielding techniques result in a very low residual background at the Q-value of the decay, about 1e-3 counts/(keV kg yr). This makes GERDA the first experiment in the field to be background-free for the complete design exposure of 100 kg yr. A search for neutrinoless double beta decay was performed with a total exposure of 47.7 kg yr: 23.2 kg yr come from the second phase (Phase II) of the experiment, in which the background is reduced by about a factor of ten with respect to the previous phase. The analysis presented in this paper includes 12.4 kg yr of new Phase II data. No evidence for a possible signal is found: the lower limit for the half-life of Ge76 is 8.0e25 yr at 90% CL. The experimental median sensitivity is 5.8e25 yr. The experiment is currently taking data. As it is running in a background-free regime, its sensitivity grows linearly with exposure and it is expected to surpass 1e26 yr within 2018.