Results of a search for neutrinoless double-$\beta$ decay using the COBRA demonstrator

Neutrinoless double-$\beta$ decay ($0\nu \beta \beta$ decay) is a hypothetical process that can occur if the neutrino is its own antiparticle. The COBRA Collaboration operates a demonstrator to search for these decays at the Laboratori Nazionali del Gran Sasso in Italy using CdZnTe semiconductor detectors. The exposure of 234.7 kg d considered in this analysis was collected between September 2011 and February 2015. The analysis focuses on the decay of the nuclides ${}^{114}\mathrm{Cd},{}^{128}\mathrm{Te},{}^{70}\mathrm{Zn},{}^{130}\mathrm{Te}$, and ${}^{116}\mathrm{Cd}$. A Bayesian analysis is performed to estimate the signal strength of $0\nu \beta \beta$ decay. No signal is observed for any of these nuclides. Therefore, the following half-life limits at 90% credibility are set: $T_{1/2}^{0\nu }>1.6\times {10}^{21}\mathrm{yr} \left({}^{114}\mathrm{Cd}\right),T_{1/2}^{0\nu }>1.9\times {10}^{21}\mathrm{yr} \left({}^{128}\mathrm{Te}\right),T_{1/2}^{0\nu }>6.8\times {10}^{18}\mathrm{yr} \left({}^{70}\mathrm{Zn}\right),T_{1/2}^{0\nu }>6.1\times {10}^{21}\mathrm{yr} \left({}^{130}\mathrm{Te}\right)$, and $T_{1/2}^{0\nu }>1.1\times {10}^{21}\mathrm{yr} \left({}^{116}\mathrm{Cd}\right)$.

Figure 1

The effective FWHM as a function of the energy determined with calibration measurements (markers). The line energies used are 238 keV $\left({}^{212}\mathrm{Pb}\right)$, 511 keV (annihilation $\gamma$ radiation), 1275 keV $\left({}^{22}\mathrm{Na}\right)$, and 2614 keV $\left({}^{208}\mathrm{Tl}\right)$. Also shown is the parametrization (red line) including the corresponding uncertainty (yellow band).

Figure 2

Difference between the estimated and the true energy for several $\gamma$ lines (markers) as well as the systematic uncertainty associated with the energy calibration (red dashed line).

Figure 3

The energy spectrum of all detectors after the full event selection. Three $\gamma$ lines are identified: the annihilation line at 511-keV, the 1275-keV line of ${}^{22}\mathrm{Na}$, and the 1460-keV line of ${}^{40}\mathrm{K}$. The line at 3.5 MeV is most likely caused by $\alpha$ radiation on the cathode side.

Figure 4

Energy spectra of all isotopes. Also shown are the best-fit results (red) and the corresponding smallest 68% uncertainty bands of the fits (yellow). The solid black lines corresponding to signal contributions equal the 90% upper limits. The black dashed lines indicate the expected statistical fluctuations of the number of counts in each bin.