Our latest XENON1T paper on details of our analysis was presented at the Low Radioactivity Techniques, a conference focused on low background experiments. In the talk (that you can find here), the response model of the detector, the challenges of background modeling, as well as the used techniques were described. In a low background experiment is often hard to asses the expected distribution of events due to lack of statistics and to many subtle effects. In the talk a novel technique was described to introduce a well-motivated systematic uncertainty to the background model based on a calibration sample, which can be relevant to other low background experiments.
The universe is almost 14 billion years old. An inconceivable length of time by human standards – yet compared to some physical processes, it is but a moment. There are radioactive nuclei that wdecay on much longer time scales. Using our XENON1T detector at the INFN Gran Sasso National Laboratory, we were able to observe the decay of Xenon-124 atomic nuclei for the first time.
The half-life of a process is the time after which half of the radioactive nuclei present in a sample have decayed away. The half-life measured for Xenon-124 is about one trillion times longer than the age of the universe. This makes the observed radioactive decay, the so-called double electron capture of Xenon-124, the rarest process ever seen happening in a detector. “The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is – also for signals which are not from dark matter,” says Prof. Christian Weinheimer from the University of Münster (Germany) whose group lead the study. In addition, the new result provides information for further investigations on neutrinos, the lightest of all elementary particles whose nature is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the US and the Middle East. The results were published in the science journal “Nature”.
A sensitive dark matter detector
The Gran Sasso Laboratory of the National Institute for Nuclear Physics (INFN) in Italy, where scientists are currently searching for dark matter particles is located about 1,400 meters beneath the Gran Sasso massif, well protected from cosmic rays which can produce false signals. Theoretical considerations predict that dark matter should very rarely “collide” with the atoms of the detector. This assumption is fundamental to the working principle of the XENON1T detector: its central part consists of a cylindrical tank of about one meter in length filled with 3,200 kilograms of liquid xenon at a temperature of –95° C. When a dark matter particle interacts with a xenon atom, it transfers energy to the atomic nucleus which subsequently excites other xenon atoms. This leads to the emission of faint signals of ultraviolet light which are detected by means of sensitive light sensors located in the upper and lower parts of the cylinder. The same sensors also detect a minute amount of electrical charge which is released by the collision process.
The new study shows that the XENON1T detector is also able to measure other rare physical phenomena, such as double electron capture. To understand this process, one should know that an atomic nucleus normally consists of positively charged protons and neutral neutrons, which are surrounded by several atomic shells occupied by negatively charged electrons. Xenon-124, for example, has 54 protons and 70 neutrons. In double electron capture, two protons in the nucleus simultaneously “catch” two electrons from the innermost atomic shell, transform into two neutrons, and emit two neutrinos. The other atomic electrons reorganize themselves to fill in the two holes in the innermost shell. The energy released in this process is carried away by X-rays and so-called Auger electrons. However, these signals are very hard to detect, as double electron capture is a very rare process which is hidden by signals from the omnipresent natural radioactivity.
This is how the XENON collaboration succeeded with this measurement: The X-rays from the double electron capture in the liquid xenon produced an initial light signal as well as free electrons. The electrons were moved towards the gas-filled upper part of the detector where they generated a second light signal. The time difference between the two signals corresponds to the time it takes the electrons to reach the top of the detector. Scientists used this interval and the information provided by the sensors measuring the signals to reconstruct the position of the double electron capture. The energy released in the decay was derived from the strength of the two signals. All signals from the detector were recorded over a period of more than one year, however, without looking at them at all as the experiment was conducted in a “blind” fashion. This means that the scientists could not access the data in the energy region of interest until the analysis was finalized to ensure that personal expectations did not skew the outcome of the study. Thanks to the detailed understanding of all relevant sources of background signals it became clear that 126 observed events in the data were indeed caused by the double electron capture of Xenon-124.
Using this first-ever measurement, the physicists calculated the enormously long half-life of 1.8×1022 years for the process. This is the slowest process ever measured directly. It is known that the atom Tellurium-128 decays with an even longer half-life, however, its decay was never observed directly and the half-life was inferred indirectly from another process. The new results show how well the XENON1T detector can detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also search for the so-called neutrino-less double electron capture which could shed light on important questions regarding the nature of neutrinos.
Status and outlook
XENON1T acquired data from 2016 until December 2018 when it was switched off. The scientists are currently upgrading the experiment for the new “XENONnT” phase which will feature a three times larger active detector mass. Together with a reduced background level this will boost the detector’s sensitivity by an order of magnitude.
On May 31st 2018, XENON1T released the result of a search for dark matter interacting with xenon atoms using an exposure of 1 tonne-year. Papers presenting the scientific results are written to be brief, and communicate the most important information to the scientific community. Therefore, many details of the instrument, reconstruction of events and analysis work by the entire collaboration must be left out of the science papers. XENON1T has previously published a paper focusing on the operation of the detector itself. A new paper by XENON1T now goes into the details of the analysis of the XENON1T data, and another one, on the event reconstruction and calibration, is being prepared.
XENON1T detects the scintillation light and ionization electrons that energy depositions in the two tonne liquid xenon target produce. In addition to WIMPs, different background sources can produce an S1+S2 signal. The expected S1,S2 distribution may change depending on whether the energy deposition happens by a recoil on an electron of the xenon atom or the nucleus. This is one of the main methods XENON uses to discriminate against backgrounds, since WIMPs, which scatter on the xenon nucleus, have a mean S2 lower than 99.7% of the dominant background component, which is made up of scatters on electrons.
Modelling how an electronic or nuclear recoil will look like in the detector is crucial both to know the shape of a WIMP signal, and to model the backgrounds well. XENON1T uses a comprehensive fit to multiple calibration sources to constrain the distributions of backgrounds and signals in the analysis space; S1, S2 and the radius from the center axis of the detector.
Some background components are harder to model directly, and are estimated by using sidebands or other data samples. In the XENON1T analysis, coincidences between unrelated, lone S1 and S2 events were modeled this way, in addition to the surface background– events occurring close to or at the detector wall.
The models of each background and the signal, for two separate science runs, are put together in a likelihood, which is a mathematical function of the WIMP signal strength as well as nuisance parameters. These are unknowns that could change the analysis, such as the true expectation value for each background component. The likelihood also contains multiple terms representing measurements of nuisance parameter, which constrain them when the likelihood is fitted to the data collected by XENON1T.
The value of the likelihood evaluated at a specific signal strength has a random distribution which is estimated using simulated realizations of the experimental outcome. The final statistical limits are computed by comparing the likelihood computed on the actual data with the distributions found from the simulations:
The models and tools used in the XENON1T spin-independent analysis are also used to explore alternative models of dark matter, such as spin-independent interactions and scatterings between WIMPs and pions, with more to come!
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.
XENON1T was built to observe the recoil of xenon-atoms, which may be caused by the interaction of a Weakly Interacting Massive Particle (WIMP) as it passes through the detector. A recoiling xenon atom produces scintillation light and ionization that XENON1T detects as an S1 and S2 signal, which carry information of the recoil type, energy and position in the detector. The first results of the XENON1T were published on the spin-independent WIMP-nucleon interaction, which is expected to dominate the WIMP scattering rate. However, models of WIMPs exist where this contribution would be suppressed or vanish. XENON has therefore performed searches for alternative WIMP-recoil spectra, such as the one expected if the scattering depends on the nucleon spins.
A careful accounting of all the possible WIMP-nucleon interactions showed that WIMPs can also interact with pions— subatomic particles that contribute to the strong force that binds atoms together. The figure illustrates a WIMP (χ) scattering via a mediator line on a pion (π) exchanged between a proton and a neutron in the xenon nucleus. The xenon atom recoils from the interaction, which can be observed with our detector. Similarly to the spin-independent recoil, the wimp-pion interaction happens in a way where the WIMP scatters coherently, off the entire xenon atom together.
The analysis was performed with the same tools as the main XENON1T spin-independent WIMP search, and 1 tonne-years of data. No significant evidence for a signal was observed, so we set the first limits on the spin-independent WIMP-pion interaction strength. An open access pre-print of the paper can be found on the arxiv.
Two members of the University of Zurich group gave talks on XENON1T at the annual meeting of the Swiss Physical Society in Lausanne, Switzerland. Chiara Capelli presented the latest news from the experiment and in particular the recently presented limit on spin-independent WIMP-nucleon cross-section, while Adam Brown spoke about the ongoing work searching for the inelastic scattering of WIMPs.
One of the key slides from Chiara’s talk is below. In the top-right you can see the WIMP-search data pre-unblinding, and in the bottom-right the efficiency for detecting nuclear recoils which happen in our fiducial volume. In the full talk, which is available here, she also presented the final limit and then gave a update on the preparations for the detector upgrade to XENONnT which are ongoing at the University of Zurich.
Adam’s talk focussed instead on an alternative possibility of searching for WIMPs via their inelastic scattering off xenon nuclei. During the interaction the nucleus is excited, and so the usual nuclear recoil signal would be observed in coincidence with the 39.6 keV gamma ray from the de-excitation of the nucleus. One of the attractions of this search channel, which is however less sensitive than elastic scattering, is that it distinguishes between spin-dependent and spin-independent WIMP interactions: a spin-dependent interaction is needed to change the nuclear spin state during its excitation. Again, the full talk is available online here.
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.