Author Archives: Christian Wittweg

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.