Search for Neutrinoless Double-Beta Decay with the Upgraded EXO-200 Detector

Results from a search for neutrinoless double-beta decay ($0\nu \beta \beta$) of ${}^{136}\mathrm{Xe}$ are presented using the first year of data taken with the upgraded EXO-200 detector. Relative to previous searches by EXO-200, the energy resolution of the detector has been improved to $\sigma /E=1.23\%$, the electric field in the drift region has been raised by 50%, and a system to suppress radon in the volume between the cryostat and lead shielding has been implemented. In addition, analysis techniques that improve topological discrimination between $0\nu \beta \beta$ and background events have been developed. Incorporating these hardware and analysis improvements, the median 90% confidence level $0\nu \beta \beta$ half-life sensitivity after combining with the full data set acquired before the upgrade has increased twofold to $3.7\times {10}^{25}\mathrm{yr}$. No statistically significant evidence for $0\nu \beta \beta$ is observed, leading to a lower limit on the $0\nu \beta \beta$ half-life of $1.8\times {10}^{25}\mathrm{yr}$ at the 90% confidence level.

Figure 1

Comparison between SS events in Phase II data (open markers) and MC simulation (lines) for calibrations using ${}^{60}\mathrm{Co}$ (green), ${}^{226}\mathrm{Ra}$ (blue), and ${}^{228}\mathrm{Th}$ (red) sources positioned near the cathode. The bottom shows the ratio between data and MC. The inset compares the corresponding SS fraction, $\mathrm{SS}/(\mathrm{SS}+\mathrm{MS})$, for the calibration data and MC.

Figure 2

Measured energy resolution $\sigma /E$ for the 2615 keV ${}^{208}\mathrm{Tl}$ $\gamma$ line in calibration data taken at the cathode position throughout Phase I and Phase II. The measured resolution before (blue) and after (red) applying the software denoising algorithm in Phase I is shown. The data acquired between restart of operations and the start of Phase II, when $V_C$ was raised to $-12\mathrm{kV}$, were not used in the current analysis.

Figure 3

Comparison between data (dots) and MC simulations (solid and dashed lines) for the three individual variables used in the BDT: (a) number of channels; (b) rise time of the charge pulse; and (c) standoff distance; as well as for the overall discriminator distribution (d). Both source calibration data using the ${}^{226}\mathrm{Ra}$ source at the cathode (blue dashed) and the background-subtracted $2\nu \beta \beta$ spectrum from low-background data (black solid) are shown. Only SS events are depicted in the plots. Statistical error bars on the data points are included, but are typically smaller than the marker size. The expected BDT discriminator distribution for a $0\nu \beta \beta$ signal from MC simulation is indicated by the red filled region. All distributions are normalized by the area, and the edge bins account for overflow.

Figure 4

Best fit to the low-background data SS energy spectrum for Phase I (top left) and Phase II (bottom left). The energy bins are 15 and 30 keV below and above 2800 keV, respectively. The inset shows a zoomed-in view around the best-fit value for $B{Q}_{\beta \beta }$. Top right: Projection of events within $B{Q}_{\beta \beta }\pm 2\sigma$ on the BDT fit dimension. Bottom right: MS energy spectra above the ${}^{40}\mathrm{K}$ $\gamma$ line.