Multisite event discrimination for the majorana demonstrator

The Majorana Demonstrator is searching for neutrinoless double-beta decay ($0\nu \beta \beta$) in ${}^{76}\mathrm{Ge}$ using arrays of point-contact germanium detectors operating at the Sanford Underground Research Facility. Background results in the $0\nu \beta \beta$ region of interest from data taken during construction, commissioning, and the start of full operations have been recently published. A pulse shape analysis cut applied to achieve this result, named $AvsE$, is described in this paper. This cut is developed to remove events whose waveforms are typical of multisite energy deposits while retaining $(90\pm 3.5)\%$ of single-site events. This pulse shape discrimination is based on the relationship between the maximum current and energy, and tuned using ${}^{228}\mathrm{Th}$ calibration source data. The efficiency uncertainty accounts for variation across detectors, energy, and time, as well as for the position distribution difference between calibration and $0\nu \beta \beta$ events, established using simulations.

Figure 1

Weighting potential for the point contact (bottom center) in a PPC detector. White lines are isochrones of equal drift time for holes to reach the point contact spaced by 200 ns.

Figure 2

Charge (black, solid) and current (red, dashed) signals formed by SSE (top) and MSE (bottom) in a PPC detector. Both events have near $Q_{\beta \beta }$ energy and are from experimental data.

Figure 3

The distribution of the calibrated maximum amplitude of the current pulse ($AE/E_{\mathrm{unc}}$) vs energy ($E$) for a detector. The red dots are the mode of $AE/E_{\mathrm{unc}}$ at the evaluated energies and the red line is the quadratic polynomial fit $AE/E_{\mathrm{unc}}=-3.904\times {10}^{-2}+5.908\times {10}^{-3}\times E-2.665\times {10}^{-8}\times E^2$.

Figure 4

(a) The corrected $A$ value, also known as $AvsE$, for $100<E<3000$ keV with the excluded MSE shaded in red. (b) $AvsE$ vs energy with excluded MSE below the red line.

Figure 5

Comparison of the $A/E$ and the $AvsE$ cuts applied to ${}^{228}\mathrm{Th}$ calibration data.

Figure 6

$AvsE$ PSA performance of the operating detectors in DS5. The single-site ${}^{208}\mathrm{Tl}$ DEP events are fixed to 90% (black), the multisite SEP events (blue) are reduced to 6% by the cut.

Figure 7

$AvsE$ stability over the yearlong DS6 data set. The stability uncertainty uses the weighted standard deviation ($\sigma$), which is significantly larger than the uncertainty on the average.

Figure 8

DS0-6 energy spectrum corresponding to 26 kg yr exposure before and after the $AvsE$ cut.