Search for two-neutrino double electron capture on 124Xe with the XMASS-I detector

Abstract

Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. Measurement of its two-neutrino mode would provide a new reference for the calculation of nuclear matrix elements whereas observation of its neutrinoless mode would demonstrate lepton number violation. A search for two-neutrino double electron capture on 124 Xe is performed using 165.9 days of data collected with the XMASS-I liquid xenon detector. No significant excess above background was observed and we set a lower limit on the half-life as 4.7 × 10 21 years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the 126 Xe two-neutrino double electron capture half-life of 4.3 × 10 21 years at 90% confidence level as well.

Search for two-neutrino double electron capture of Xe124 with XENON100

“…where N obs = N bkg is the expected number of counts in the ±3σ region around the 2ν2K peak, and N sig is the [23]. The blue shaded area shows the range of predicted halflives [19].…”

“…Since the XENON1T detector was also designed to measure higher energy signals more accurately than XENON100 it offers the possibility to study neutrinoless double electron capture as well as electron capture with positron emission or double positron decay where the main part of the observable energy is above 1 MeV [42]. Moreover, future multi-ton target experiments such as XMASS-II [23], LZ [22], XENONnT [41] and DARWIN [43] will have the sensitivity to investigate the parameter space even further.…”

“…There is a wide spread in the predicted half-lives of 2ν2EC for 124 Xe, from ∼ 10 20 yr to 10 24 yr due to different nuclear matrix element calculations [15][16][17][18][19][20]. Previous searches for 2ν2K of 124 Xe have been carried out using a low-background proportional counter with enriched xenon [14,21] and large-scale liquid xenon detectors [22,23]. The current best experimental limit on the half-life, T 1/2 > 4.7 × 10 21 yr (90% confidence level), is set by the XMASS experiment [23].…”

“…Previous searches for 2ν2K of 124 Xe have been carried out using a low-background proportional counter with enriched xenon [14,21] and large-scale liquid xenon detectors [22,23]. The current best experimental limit on the half-life, T 1/2 > 4.7 × 10 21 yr (90% confidence level), is set by the XMASS experiment [23].…”

Future XMASS project

“…It is designed to achieve a range of physics goals as the program develops: dark matter, neutrinoless double beta decay and 7 Be/pp solar neutrinos. The present XMASS-I detector is the first step in the project and targets direct dark matter detection with 832 kg of liquid xenon in its sensitive volume.…”