Search for 2 nu beta beta decay of Xe-136 to the 0(1)(+) excited state of Ba-136 with the EXO-200 liquid xenon detector

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless pp decay of Xe-136 to the ground state of Ba-136. We report here on a search for the two -neutrino pp decay of Xe-136 to the first 0+ excited state, 0(1)(+), of Ba-136 based on a 100 kg yr exposure of Xe-136. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7 x 10(24) yr. We find no statistically significant evidence for the 20P decay to the excited state resulting in a lower limit of Ti-1/2(7 nu) (0(+) -> 0(1)(+)) > 6.9 x10(23) yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5 x 10(25) yr.1551sciescopu

Investigation of radioactivity-induced backgrounds in EXO-200

The search for neutrinoless double-beta decay (0 nu beta beta) requires extremely low background and a good understanding of their sources and their influence on the rate in the region of parameter space relevant to the 0 nu beta beta signal. We report on studies of various beta and gamma backgrounds in the liquid-xenon-based EXO-200 0 nu beta beta experiment. With this work we try to better understand the location and strength of specific background sources and compare the conclusions to radioassay results taken before and during detector construction. Finally, we discuss the implications of these studies for EXO-200 as well as for the next-generation, tonne-scale nEXO detector117211scopu

Deep neural networks for energy and position reconstruction in EXO-200

We apply deep neural networks (DNN) to data from the EXO-200 experiment. In the studied cases, the DNN is able to reconstruct the relevant parameters - total energy and position - directly from raw digitized waveforms, with minimal exceptions. For the first time, the developed algorithms are evaluated on real detector calibration data. The accuracy of reconstruction either reaches or exceeds what was achieved by the conventional approaches developed by EXO-200 over the course of the experiment. Most existing DNN approaches to event reconstruction and classification in particle physics are trained on Monte Carlo simulated events. Such algorithms are inherently limited by the accuracy of the simulation. We describe a unique approach that, in an experiment such as EXO-200, allows to successfully perform certain reconstruction and analysis tasks by training the network on waveforms from experimental data, either reducing or eliminating the reliance on the Monte Carlo. (c) 2018 IOP Publishing Ltd and Sissa Mediala

Search for Neutrinoless Double-beta Decay with the Complete EXO-200 Dataset

A search for neutrinoless double-beta decay (0 nu beta beta) in Xe-136 is performed with the full EXO-200 dataset using a deep neural network to discriminate between 0 nu beta beta and background events. Relative to previous analyses, the signal detection efficiency has been raised from 80.8% to 96.4 +/- 3.0%, and the energy resolution of the detector at the Q value of Xe-136 0 nu beta beta has been improved from sigma/E = 1.23% to 1.15 +/- 0.02% with the upgraded detector. Accounting for the new data, the median 90% confidence level 0 nu beta beta half-life sensitivity for this analysis is 5.0 x 10(25) yr with a total Xe-136 exposure of 234.1 kg 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 3.5 x 10(25) yr at the 90% confidence level. C,Published by the American Physical Society11Nsciescopu

Study of silicon photomultiplier performance in external electric fields

We report on the performance of silicon photomultiplier (SiPM) light sensors operating in electric field strength up to 30 kV/cm and at a temperature of 149 K, relative to their performance in the absence of an external electric field. The SiPM devices used in this study show stable gain, photon detection efficiency, and rates of correlated pulses, when exposed to external fields, within the estimated uncertainties. No visible damage to the surface of the devices was caused by the exposure. (c)2018 IOP Publishing Ltd and Sissa Mediala

VUV-Sensitive Silicon Photomultipliers for Xenon Scintillation Light Detection in nEXO

Future ton-scale liquefied noble gas detectors depend on efficient light detection in the vacuum ultraviolet (VUV) range. In the past years, silicon photomultipliers (SiPMs) have emerged as a valid alternative to standard photomultiplier tubes or large-area avalanche photodiodes. The next-generation double-beta decay experiment, nEXO, with a 5-ton liquid xenon time projection chamber will use SiPMs for detecting the 175-nm xenon scintillation light, in order to achieve an energy resolution of sigma/Q(beta beta) = 1%. This paper presents recent measurements of the VUV-HD generation SiPMs from Fondazione Bruno Kessler, Trento, Italy, in two complementary setups. It includes measurements of the photon-detection efficiency (PDE) with gaseous xenon scintillation light in a vacuum setup and dark measurements in a dry nitrogen gas setup. We report improved PDE at 175 nm compared to previous generation devices that would meet the criteria of nEXO. Furthermore, we present the projected nEXO detector light collection and energy resolution that could be achieved by using these SiPMs © 2018 IEEE