XENON1T may have been designed to search for dark matter, but it turns out that we can do a lot more with it. As the amount of xenon increases and backgrounds go down, the experiment starts to check all the boxes for a neutrino detector and becomes sensitive to rare physics processes, such as double -decays. Two XENON collaboration posters at Neutrino 2018 in the beginning of June showcased the prospects for the detection of two such decays.
First, there is neutrinoless double -decay of the xenon isotope Xe-136. Here, two neutrons in the atomic nucleus are simultaneously converted into two protons. In order to conserve the total charge that increased by +2 with the protons two electrons with the charge -2 have to be emitted. In the standard model one would also need two anti-electron neutrinos to conserve lepton number. But this process goes beyond the standard model of particle physics. Its detection would imply that neutrinos are their own anti-particles and the violation of lepton number could be the key to understanding why the universe is dominated by matter compared to anti-matter today. Chiara Capelli, a PhD student from the Zürich XENON group, presented a poster where she checked the sensitivity of current and future xenon detectors for neutrinoless double -decay. In the years to come these detectors will complement existing experiments.
A second poster by Alexander Fieguth and Christian Wittweg from the Münster group outlined an ongoing search for the double electron capture of Xe-124. This decay is the other way round: Two neutrons are made from protons at the same time. The necessary electrons for charge conservation are taken right from the electronic shell of the xenon atom itself. Two electron neutrinos are emitted to conserve lepton number. Although the neutrinoless case is also thinkable, the standard model decay with two neutrinos is exciting in itself. It is predicted but has not been detected so far. It t would be the longest-lived nuclear decay process ever observed directly. As XENON1T has the largest mass of Xe-124 in an experiment to date – about 1.5 kg due to the rarity of Xe-124 in natural xenon – it will be the most sensitive detector to search for this double electron capture process.
All in all, the future looks bright for large xenon detectors in neutrino physics and there are a bunch of exciting publications to look forward to.