XENON1T presented at the german physics society spring meeting

The spring meeting of the german physics society took place from 19th to 23rd March in Würzburg, a very historic city with its baroque Residence from 1744 that belongs to the UNESCO world heritage. The meeting is a yearly get-together of physicists working in german institutions and provides the opportunity to exchange and learn about new projects and results within the particle physics community. The conference program can be found here.

During my presentation of the XENON1T experiment, I tried to share my excitement about the upcoming results from the new data set of our second science run (SR1) that was acquired during the course of last year within 247 live days. Here is one slide showing the collected data in the S2 vs. S1 space on the right:

For comparison, the data from the first science run (SR0) that was ended by an earthquake is shown in the left figure. Already with SR0 which was a factor of 8 shorter than SR1 we could set the most stringent limit on spin-independent WIMP-nucleon cross-sections and prove a detector background level that makes XENON1T the most sensitive experiment worldwide. Hence, we are eager to unblind the signal region (marked by the blue band) in the new data set after some final checks of the analysis and find out if we actually measured a few WIMP events. We would be able to see a 3 sigma excess of a signal with a cross section just below the upper limit of SR0 with more than 50% probability. So maybe the discovery of dark matter is just around the corner?

 

XENON1T Calibrations Talk at APS April Meeting

At the 2018 April Meeting of APS last weekend, I presented a brief summary of how and why we calibrate the XENON1T detector. The April Meeting is one of the largest American physics conferences and covers a broad range of research, from nuclear and particle physics to gravitation and cosmology. Below you can see one of the slides that I presented:

This shows how we use data from calibrations to understand every piece of physics in our detector, from a particle entering and hitting a xenon atom to the measurement of the light and charge produced by this interaction. Combining the many different calibrations we do, we develop a complete model of XENON1T which is then used in a statistics framework to determine whether the background data we’ve taken contains WIMPs. Stay tuned as it won’t be too long before we can release those results as well!

XENON1T presented at Rencontres de Moriond Electroweak

Last week I had the opportunity to present the XENON1T experiment at the Recontres de Moriond electroweak conference in La Thuile Italy in the beautiful Aosta Valley. This meeting is one of the most important meetings for LHC physics, but has slowly expanded to encapsulate a variety of topics, including the hunt for dark matter. The conference program and slides are available on indico. The XENON1T presentation focused on our dark matter search results from last spring as well as the upcoming result using about a factor of 10 more exposure, which is under intense preparation for release. The whole presentation is available from the indico page but here is one slide from it:

Here we discuss how we were able to increase the amount of liquid xenon we use for our dark matter search from ~1000kg to ~1300kg. The top left plot shows an example larger search volume (red) compared to the smaller volume used for the first result. But it’s not so simple as just adding volume. While our inner detector is completely free of WIMP-like background, the outer radii contain background components that can mimic WIMPs. This is illustrated in the bottom right plot where the background-free inner volume (right) is contrasted with the full search volume containing the outer radial sections (left). The full volume has a contribution from PTFE (Teflon) surface background (green contour and points) that is absent as soon as we consider only the inner volume.

Our statistical interpretation has been updated so it is smart enough to take this into account. We parameterize our entire search region in both radial and spatial dimensions with expected signal and background distributions described at each location. This allows us to fully exploit the sensitivity of our innermost background-free volumes while also gaining a modest improvement from the outermost ones.

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.

Poster at UCLA DM 2018: Position Correction Improvement for XENON1T

A poster by Jingqiang Ye was presented at the UCLA Dark Matter 2018 Conference, Feb. 21 – 23, 2018.

The figures show the low-energy background events distributed in our detector after application of an algorithm to correct their positions. Background events can be seen to cluster mostly at the surfaces of the detector, at high radii and at the cathode near the bottom. (The color scale is logarithmic)

Event interaction position is important for background rejection, likelihood analysis etc. Our 3D position reconstruction is based on event drift time and PMT hit patterns. However, as the drift field is not perfectly vertical, the reconstructed position at the gate does not exactly correspond to the interaction position. To get to a corrected position, a data-driven method based on the radioactive isotope Krypton-83m is developed. The idea is to utilize the radial uniformity of Krypton-83m events.  Regular Krypton-83m calibrations throughout the whole science run can guarantee that we have sufficient statistics to properly correct positions for different radius, angle, depth and time. Thanks to this new position algorithm, we were able to increase the useful exposure by around 30%.

XMASS Members join XENON

XENON1T is the largest and most sensitive WIMP dark matter detector to date, recording scientific data in the Italian Laboratori Nazionali del Gran Sasso (LNGS). Our collaboration recently grew larger again and now has more than 160 members from 27 institutions. As of December 1st, 2017, key members of the Japanese XMASS collaboration have officially joined XENON and will contribute to the realization of the upcoming XENONnT.

Participants of our collaboration meeting early 2018 in Florence, including our newest colleagues from the Japanese XMASS collaboration.

XMASS is a single-phase liquid xenon experiment in the Kamioka mine, the Japanese underground laboratory hosting the Nobel-prize winning SuperKamiokande experiment. Researchers come from the University of Tokyo (groups of Prof. Shigetaka Moriyama and Prof. Kai Martens), Nagoya University (group of Prof. Yoshitaka Itow) and Kobe University (group of Prof. Kentaro Miuchi). XMASS will continue to record data until the end of this year, in line with the planned start of XENONnT.

XENONnT is an upgrade phase to the currently running XENON1T experiment. With a target mass three times larger than XENON1T, and a considerably reduced background, XENONnT will explore WIMP-nucleon interactions with a ten-fold higher sensitivity than XENON1T. The Japanese groups bring expertise in LXe detector technologies and low background experiments relevant to the XENON Dark Matter program. We are excited about our newest collaborators from Japan as we continue to move forward with the XENON program at LNGS.

Search for bosonic super-WIMP interactions with the XENON100 experiment

While the microscopic nature of dark matter in the Universe is largely unknown, the simplest assumption which can explain all existing observations is that it is made of a new, as yet undiscovered particle. Leading examples are weakly interacting massive particles (WIMPs), axions or axion-like particles (ALPs), and sterile neutrinos. WIMPs with masses in the GeV range, as well as axions/ALPs are examples for cold dark matter while sterile neutrinos with masses at the keV-scale are an example for warm dark matter. Cold dark matter particles were nonrelativistic at the time of their decoupling from the rest of the particles in the early universe. In contrast, warm dark matter particles remain relativistic for longer, retain a larger velocity dispersion, and thus more easily free-stream out from small-scale perturbations. Astrophysical and cosmological observations constrain the mass of warm dark matter to be larger than about 3keV/c2, with a more recent lower limit from Lyman-alpha forest data being 5.3keV/c2. Another example for warm dark matter particles are bosonic super-WIMPs. These particles, with masses at the keV-scale, could couple electromagnetically to standard model particles via the axioelectric effect, which is an analogous process to the photoelectric effect, and thus be detected in direct detection experiments.

The limit derived from the XENON100 experiment on the coupling of SuperWIMPs.

We searched for vector and pseudo-scalar bosonic super-WIMPs with the XENON100 experiment. The super-WIMPs can be absorbed in liquid xenon and the expected signature is a monoenergetic peak at the super-WIMP’s rest mass. A profile likelihood analysis of data with an exposure of 224.6 live days × 34kg showed no evidence for a signal above the expected background. We thus obtained new upper limits in the (8 − 125) keV/c2 mass range, excluding couplings to electrons with coupling constants of gae > 3 × 10−13 for pseudo-scalar super-WIMPs and α′/α > 2 × 10−28 for vector super-WIMPs, respectively. We expect to improve upon these results with the XENON1T detector, which operates a larger mass of liquid xenon with reduced backgrounds. Our results were published in Physical Review D 96, 122002 (2017) and are of course also available at the arxiv.

Intrinsic backgrounds from Rn and Kr in the XENON100 experiment

XENON1T is currently the largest liquid xenon detector in the search for dark matter. To fully exploit the capabilities of the ton-scale target mass, a thorough understanding of radioactive background sources is required. In this paper we use the full data of the main science runs of the XENON100 experiment that were taken over a period of about 4 years to asses the target-intrinsic background sources radon (Rn-222), thoron (Rn-220) and krypton (Kr-85). We derive distributions of the individual radionuclides inside the detector (see Figure below) and quantify their abundances during the main three science runs. We find good agreement with external measurements of radon emanation and krypton concentrations, and report an observed reduction in concentrations of radon daughters that we attribute to the plating-out of charged ions on the negatively biased cathode.

The preprint of the full study is available on arXiv:1708.03617.

Figure: Spatial distributions of the various radon populations identified in XENON100.

Search for WIMP Inelastic Scattering Off Xenon Nuclei With XENON100

Most direct detection searches focus on elastic scattering of galactic dark matter particles off nuclei, where the keV-scale nuclear recoil energy is to be detected. In this work, the alternative process of inelastic scattering is explored, where a WIMP-nucleus scattering induces a transition to a low-lying excited nuclear state. The experimental signature is a nuclear recoil detected together with the prompt de-excitation photon. We consider the scattering of dark matter particle off 129Xe isotope, which has an abundance of 26.4\% in natural xenon, and when excited to it lowest-lying 3/2+ state above the ground state it emits a 36.9 keV photon. This electromagnetic nuclear decay has a half-life of 0.97 ns.

The WIMP inelastic scattering  is complementary to spin-dependent, elastic scattering, and dominates the integrated rates above 10 keV of deposited energy. In addition, in case of a positive signal, the observation of inelastic scattering would provide a clear indication of the spin-dependent nature of the fundamental interaction.

The search is performed using XENON100 Run-II science data, which corresponds to an exposure of 34×224.6 kg×days. No evidence of dark matter is found and a limit on dark matter inelastic interaction cross section is set. Our result, shown in the Figure, is the most stringent limit for the spin-dependent inelastic scattering to date, and set the stage for a sensitive search of inelastic WIMP-nucleus scattering in running or upcoming liquid xenon experiments such as XENON1T, XENONnT, LZ, and DARWIN.

Full details may be found in this article: Phys. Rev. D 96, 022008 and on the arxiv.