Since we don’t know how dark matter interacts with more familiar particles, we have to break up our search for weakly interacting massive particles (WIMPs) in terms of their possible interactions with xenon nuclei. While many complex interactions are possible, we generally start with two simple cases: WIMP-nucleus interactions that don’t depend on the nuclear spin, and those that do. XENON1T set a world-leading constraint on the former, “spin-independent” interaction in 2018. Today, we released our first results constraining the latter, “spin-dependent” interaction. The results are shown in the following figure:
The spin-dependent WIMP-nucleon interaction contains a range of possible cases, so experiments typically consider two extreme ones: the case where WIMPs only scatter off protons, and the case where they only scatter off neutrons. Most of the spin in xenon is carried by neutrons, so xenon experiments are better at constraining the neutron-only case. These results set the most stringent limit on this case, using the same data and procedure as the spin-independent result. We also tried out a new method of combining our constraints with complementary searches at particle accelerators, following the example of PICO-60. An open-access pre-print version of the paper is available on the arXiv.
Some of our young collaboration members attended a Dark Matter Summer School at the University of Albany from July 16th through the 20th.
They obtained a broader understanding of the current state of Dark Matter research; especially cosmological and astrophysical evidence for Dark Matter; the best-motivated theoretical dark matter particle models; and various detection techniques. It was also an opportunity for them to meet and connect with their colleagues in the field and hone presentation skills. More information and uploaded talk slides can be found at https://indico.cern.ch/event/704938/.
The XXXIX International Conference on High Energy Physics (ICHEP2018) was taking place from July 4 – 11, 2018 in Seoul, Korea. After a warm welcome in this modern and traditional metropolis with over 10 million citizens, I was invited to present the recent results from XENON1T in a Dark Matter parallel session.
Here is one slide of my talk visualizing the spatial distribution of the unblinded and de-salted events.
The left plot shows the X- and Y- distribution, while the right plot indicates the radius R versus depth Z for the same set of data. The enlarged fiducial volume of 1.3 tons with respect to the first result, is highlighted by the pink line. For the analysis, a core volume (green line) was defined to distinguish WIMP-like events over neutron-like background events. The different events are visualized by pie charts, where the color code resembles the relative probability from each background component assigned by the best-fit. The larger a pie is, the more “WIMPy” it is. As you can see, only a few “WIMPy” events were found that are comparable to the background model expectations. From this, we derived the most stringent limits on spin-independent WIMP-nucleon cross sections.
At the end of my talk, I also reported on the status of XENONnT, which will feature a 10x higher sensitivity than XENON1T. One main task is radon mitigation, one of the dominant backgrounds, which is visualized in this slide.
In a first step, a careful material selection needs to be made to avoid radon emanation from the start. Then, a new high throughput radon distillation column is under development to further reduce the radon contribution. Additionally, a new custom-made radon-free magnetically-coupled piston pump was built and installed at XENON1T in June 2018. With that, the radon budget in XENON1T was reduced by almost half (45%), which is an important step for the future XENONnT experiment.
The full talk is publicly available here.
Results from XENON1T, the world’s largest and most sensitive detector dedicated to a direct search for Dark Matter in the form of Weakly Interacting Massive Particles (WIMPs), are reported today (Monday, 28th May) by the spokesperson, Prof. Elena Aprile of Columbia University, in a seminar at the hosting laboratory, the INFN Laboratori Nazionali del Gran Sasso (LNGS), in Italy. The international collaboration of more than 165 researchers from 27 institutions, has successfully operated XENON1T, collecting an unprecedentedly large exposure of about 1 tonne x year with a 3D imaging liquid xenon time projection chamber. The data are consistent with the expectation from background, and place the most stringent limit on spin-independent interactions of WIMPs with ordinary matter for a WIMP mass higher than 6 GeV/c². The sensitivity achieved with XENON1T is almost four orders of magnitude better than that of XENON10, the first detector of the XENON Dark Matter project, which has been hosted at LNGS since 2005. Steadily increasing the fiducial target mass from the initial 5 kg to the current 1300 kg, while simultaneously decreasing the background rate by a factor 5000, the XENON collaboration has continued to be at the forefront of Dark Matter direct detection, probing deeper into the WIMP parameter space.
WIMPs are a class of Dark Matter candidates which are being frantically searched with experiments at the Large Hadron Collider, in space, and on Earth. Even though about a billion WIMPs are expected to cross a surface of one square meter per second on Earth, they are extremely difficult to detect. Results from XENON1T show that WIMPs, if they indeed comprise the Dark Matter in our galaxy, will result in a rare signal, so rare that even the largest detector built so far can not see it directly. XENON1T is a cylindrical detector of approximately one meter height and diameter, filled with liquid xenon at -95°C, with a density three times that of water. In XENON1T, the signature of a WIMP interaction with xenon atoms is a tiny flash of scintillation light and a handful of ionization electrons, which themselves are turned into flashes of light. Both light signals are simultaneously recorded with ultra-sensitive photodetectors, giving the energy and 3D spatial information on an event-by-event basis.
In developing this unique type of detector to search for a rare WIMP signal, many challenges had to be overcome; first and foremost the reduction of the overwhelmingly large background from many sources, from radioactivity to cosmic rays. Today, XENON1T is the largest Dark Matter experiment with the lowest background ever measured, counting a mere 630 events in one year and one tonne of xenon in the energy region of interest for a WIMP search. The search results, submitted to Physical Review Letters, are based on 1300 kg out of the total 2000 kg active xenon target and 279 days of data, making it the first WIMP search with a noble liquid target exposure of 1.0 tonne x year. Only two background events were expected in the innermost, cleanest region of the detector, but none were detected, setting the most stringent limit on WIMPs with masses above 6 GeV/c² to date. XENON1T continues to acquire high quality data and the search will continue until it will be upgraded with a larger mass detector, being developed by the collaboration. With another factor of four increase in fiducial target mass, and ten times less background rate, XENONnT will be ready in 2019 for a new exploration of particle Dark Matter at a level of sensitivity nobody imagined when the project started in 2002.
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?
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 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.
The x-axis denotes the energies of particles measured with the XENON1T detector. These are mostly electrons, x-rays and higher energy -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 -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 -rays depositing only part of their total energy due to Compton scattering inside or outside the detector, decays of radioisotopes inside the detector, and the two neutrino double -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 -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.
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.
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.
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.
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.