Tag Archives: neutrinoless double beta decay

More than dark matter – XENON1T at Neutrino 2018

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 $\beta$-decays. Two XENON collaboration posters at Neutrino 2018 in the beginning of June showcased the prospects for the detection of two such decays.

Chiara Capelli’s poster on neutrinoless double beta decay presented at Neutrino 2018

First, there is neutrinoless double $\beta$ -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 $\beta$ -decay. In the years to come these detectors will complement existing experiments.

Poster on double electron capture presented at Neutrino 2018 by Alexander Fieguth and Christian Wittweg

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.

 

The energy spectrum and resolution of XENON1T

The search for new physics with a large underground xenon detector is like listening to your favorite song in a quiet room with high end headphones for the first time. Even if you have listened to the song a thousand times, you will be surprised by all the small nuances that have been there all along and that you did not hear before. This is either because it was too loud around you or because your headphones were not good enough. The quiet room in this analogy is the xenon detector that has been made from materials selected for their ultra-low radioactivity and that is shielded by a water tank, a mountain and ultimately the xenon in the detector itself. The high end headphones on the other hand are the extremely sensitive photomultipliers, data acquisition system and tailor-made software to read out the signals produced by particles interacting inside the detector.

As you may have read before on this blog (we love to point this out…) XENON1T is the lowest background dark matter detector in the world. But the fact that the detector is so quiet does not mean that it does not measure anything. As a very sensitive instrument it is able to detect even the faintest signals from radioactive decays in the detector materials or the xenon itself. Over the course of one year these decays amount to a sizeable amount of data. The picture below shows what this looks like.

A preliminary energy spectrum from electronic recoil background data for the second science run of the XENON1T experiment.

The x-axis denotes the energies of particles measured with the XENON1T detector. These are mostly electrons, x-rays and higher energy $\gamma$-rays. The y-axis shows how many of these particles have been counted over the whole measurement time of the last science run of the experiment. In order to have a better comparability with similar experiments, the event count has been divided by the live time of the experiment, its mass and the step size on the energy axis (the binning) in which we count. One can see that even in the highest peaks we measure less than one event per kilogram detector material and day of measurement time in a 100 keV energy window. A quiet room, indeed. And the features in the spectrum are all those nuances that one could not see before. So what are they?

One can divide the spectrum into several regions. Only the small portion of data in the very left of the plot next to the first grey-shaded region is relevant to the standard dark matter search. The heavy and non-relativistic WIMP is expected to only deposit very little energy, so it resides here. The following grey region is blinded, which means it has deliberately been made inacessible to XENON analysers. The reason for this is that it might contain traces of a rare nuclear decay of Xe-124, the two neutrino double electron capture, that has not been observed until now, and we do not want to bias ourselves in looking for it. The large region from about 100-2300 keV contains multiple peaks. Each of these peaks belongs to a monoenergetic $\gamma$-line of a radioactive isotope contained in the detector materials or the extremely pure xenon itself. One can easily see that the peaks are sitting on an irregular continuous pedestal. This is created by $\gamma$-rays depositing only part of their total energy due to Compton scattering inside or outside the detector, $\beta$ decays of radioisotopes inside the detector, and the two neutrino double $\beta$-decay of Xe-136. The latter produces a continuous energy spectrum over the whole energy range that ends at 2458 keV. The decay is rare, but becomes relevant due to the large amount of Xe-136 in the detector and the relative smallness of other background contributions. Xe-136 is also responsible for the second gray-shaded region at high energies which might contain an experimental signature of its neutrinoless double $\beta$-decay. This hypothetical decay mode would produce a monoenergetic line centered at the end of the aforementioned spectrum at 2458 keV. The observation of this decay would be a gateway to new physics and complements the physics program of XENON1T. As their signatures have to be distinguished from other background components the energy resolution of the detector becomes crucial.

Preliminary energy resolution of the XENON1T experiment as a function of the measured particle energy.

To grasp the concept of energy resolution one can imagine the following situation in the energy spectrum. If you have two peaks next to one another, one your sought-after signal and one a pesky background, how far do they have to be apart in order to be seen as individual peaks? This of course relies on how wide they are. Thus, the energy resolution in XENON1T is characterized by the width of peaks in the energy spectrum relative to their measured energy. By fitting Gaussian functions to all the peaks in the spectrum at the top one obtains the ratio of peak width to peak center. This is what the above plot shows for several liquid xenon dark matter experiments. One can see that with an increase in particle energy the resolution improves. It is also evident that XENON1T leads the pack over a wide energy range. This is underlines that XENON1T is the astroparticle physics equivalent of high-end headphones. With these the XENON collaboration is in the position to pursue several exciting physics channels apart from weakly interacting massive particles. So stay tuned for the analyses to come.