Search for Neutrinoless Double-Beta Decay of ${}^{130}\mathrm{Te}$ with CUORE-0

We report the results of a search for neutrinoless double-beta decay in a 9.8 kg yr exposure of ${}^{130}\mathrm{Te}$ using a bolometric detector array, CUORE-0. The characteristic detector energy resolution and background level in the region of interest are $5.1\pm 0.3\mathrm{keV}$ FWHM and $0.058\pm 0.004(\text{stat})\pm 0.002(\text{syst})\mathrm{counts}/(\mathrm{keV}\mathrm{kg}\mathrm{yr})$, respectively. The median 90% C.L. lower-limit half-life sensitivity of the experiment is $2.9\times {10}^{24}\mathrm{yr}$ and surpasses the sensitivity of previous searches. We find no evidence for neutrinoless double-beta decay of ${}^{130}\mathrm{Te}$ and place a Bayesian lower bound on the decay half-life, $T_{1/2}^{0\nu }>2.7\times {10}^{24}\mathrm{yr}$ at 90% C.L. Combining CUORE-0 data with the 19.75 kg yr exposure of ${}^{130}\mathrm{Te}$ from the Cuoricino experiment we obtain $T_{1/2}^{0\nu }>4.0\times {10}^{24}\mathrm{yr}$ at 90% C.L. (Bayesian), the most stringent limit to date on this half-life. Using a range of nuclear matrix element estimates we interpret this as a limit on the effective Majorana neutrino mass, $m_{\beta \beta }<270–760\mathrm{meV}$.

Figure 1

Bottom: Energy spectra of physics (blue) and calibration (red) data; the latter is normalized relative to the former at 2615 keV. The peaks are identified as (1) $e^{+}{e}^{-}$ annihilation, (2) ${}^{214}\mathrm{Bi}$, (3) ${}^{40}\mathrm{K}$, (4) ${}^{208}\mathrm{Tl}$, (5) ${}^{60}\mathrm{Co}$, and (6) ${}^{228}\mathrm{Ac}$. Top: Difference of best-fit reconstructed peak energy and expected peak-energy for physics (blue points) and calibration (red) data. The blue line is the best-fit function to the physics peak residuals; the shaded band is its $1\sigma$ uncertainty.

Figure 2

Bottom: Calibration data near the 2615 keV ${}^{208}\mathrm{Tl}$ $\gamma$-ray line, integrated over all bolometer-datasets. The solid blue line is the projection of the UEML fit described in the main text. In addition to the double-Gaussian line shape for each bolometer-dataset, the fit function includes terms to model a multiscatter Compton continuum, a $\sim 30\mathrm{keV}$ Te x-ray escape peak, and a continuum background; these components, summed over all bolometer-datasets, are indicated by the blue dashed lines (a), (b), (c), and (d), respectively. Top: Normalized residuals of the data and the best-fit model.

Figure 3

Bottom: The best-fit model from the UEML fit (solid blue line) overlaid on the spectrum of $0\nu \beta \beta$ decay candidates in CUORE-0 (data points); the data are shown with Gaussian error bars. The peak at $\sim 2507\mathrm{keV}$ is attributed to ${}^{60}\mathrm{Co}$; the dotted black line shows the continuum background component of the best-fit model. Top: The normalized residuals of the best-fit model and the binned data. The vertical dot-dashed black line indicates the position of $Q_{\beta \beta }$.

Figure 4

Profile negative log-likelihood (NLL) curves for CUORE-0, Cuoricino [15, 16, 17], and their combination.

Figure 5

Constraints on $m_{\beta \beta }$ vs lightest neutrino mass ($m_{\text{lightest}}$). For the inverted (IH, green) and normal (NH, red) hierarchies the central dark band is derived from the best-fit neutrino oscillation parameters, the lighter outer band includes their $3\sigma$ uncertainties [38]. The horizontal bands delineated by the long-dashed black lines (a), the dashed beige lines (b), and the dot-dashed blue lines (c) are the range of 90% C.L. upper limits on $m_{\beta \beta }$ coming from (a) ${}^{130}\mathrm{Te}$ (CUORE-0 combined with Cuoricino), (b) ${}^{136}\mathrm{Xe}$ (EXO-200 [39], KamLAND-Zen [40] independently), and (c) ${}^{76}\mathrm{Ge}$ (combined limit from Gerda, IGEX, HDM [41]). The vertical arrows aim to emphasize the range currently probed with each isotope. The horizontal, hashed grey band indicates the range of limits on $m_{\beta \beta }$ expected from CUORE assuming its target 90% C.L. lower limit half-life sensitivity of $9.5\times {10}^{25}\mathrm{yr}$ is attained.