XENON1T S2-only Data Release

Today, XENON releases data from the XENON1T S2-only / light dark matter search published in Physical Review Letters 123, 251801 (2019). You can find the data release at:

This includes observed events, background models, and response matrices to construct arbitrary signal models. Together, these allow researchers to constrain their own dark matter models using the XENON1T S2-only data. We included a jupyter notebook example to help you get started.

We look forward to seeing what models the community will think of!

Zooming in on Axions

The latest results from a search for QCD axions from the Sun as well as axion-like particles of solar and dark matter origins were presented by Dr. Michelle Galloway from the University of Zurich on June 26th at the “Zooming in on Axions in the Early Universe” workshop hosted by CERN. With an unprecedented low background of 76 ± 2 stat events/(tonne × year × keV) between 1–30 keV, XENON1T is uniquely poised to explore new parameter space for these electronic-recoil channels via the axio-electric effect. Our search revealed an excess of events in the (1 – 7) keV region, favoring these channels over background with significances of 3.5 sigma for solar axions/ALPs and 3.0 sigma global (4.0 local) for ALP dark matter with a peak at 2.3 +- 0.2 keV (68% C.L.). The talk reviewed the detection principles, cross checks of our results, discrepancy with stellar constraints, and presented a hypothesis of a new background from a previously undetected tritium component. Presentation materials can be found here.

 

Observation of Excess Events in the XENON1T Dark Matter Experiment

Press release, June 17, 2020. For immediate release. A pre-print of this publication reporting the data analysis and details of the observed excess is available on arxiv.org, and in the meantime also directly here for download. These results were first presented on June 17 in a dedicated webinar by graduate student Evan Shockley from the University of Chicago. The slides of this presentation and a recording are available.

Scientists from the international XENON collaboration announced today that data from their XENON1T, the world’s most sensitive dark matter experiment, show a surprising excess of events. The scientists do not claim to have found dark matter. Instead, they say to have observed an unexpected rate of events, the source of which is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium (a hydrogen atom with one proton and two neutrons), but could also be a sign of something more exciting—such as the existence of a new particle known as the solar axion or the indication of previously unknown properties of neutrinos.

The XENON1T detector. Visible is the bottom array of photomultiplier tubes, and the copper structure that creates the electric drift field.

XENON1T was operated deep underground at the INFN Laboratori Nazionali del Gran Sasso in Italy, from 2016 to 2018. It was primarily designed to detect dark matter, which makes up 85% of the matter in the universe. So far, scientists have only observed indirect evidence of dark matter, and a definitive, direct detection is yet to be made. So-called WIMPs (Weakly Interacting Massive Particles) are among the theoretically preferred candidates, and XENON1T has thus far set the best limit on their interaction probability over a wide range of WIMP masses. In addition to WIMP dark matter, XENON1T was also sensitive to different types of new particles and interactions that could explain other open questions in physics. Last year, using the same detector, these scientists published in Nature the observation of the rarest nuclear decay ever directly measured.

The excess observed in XENON1T in the electronic recoil background at low energies, compared to the level expected from known backgrounds indicated as the red line.

The XENON1T detector was filled with 3.2 tonnes of ultra-pure liquefied xenon, 2.0 t of which served as a target for particle interactions. When a particle crosses the target, it can generate tiny signals of light and free electrons from a xenon atom. Most of these interactions occur from particles that are known to exist. Scientists therefore carefully estimated the number of background events in XENON1T. When data of XENON1T were compared to known backgrounds, a surprising excess of 53 events over the expected 232 events was observed.

This raises the exciting question: where is this excess coming from?

One explanation could be a new, previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the XENON1T detector. Tritium, a radioactive isotope of hydrogen, spontaneously decays by emitting an electron with an energy similar to what was observed. Only a few tritium atoms for every 10 25 (10,000,000,000,000,000,000,000,000!) xenon atoms would be needed to explain the excess. Currently, there are no independent measurements that can confirm or disprove the presence of tritium at that level in the detector, so a definitive answer to this explanation is not yet possible.

More excitingly, another explanation could be the existence of a new particle. In fact, the excess observed has an energy spectrum similar to that expected from axions produced in the Sun. Axions are hypothetical particles that were proposed to preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them. While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but never observed class of new particles, with a large impact on our understanding of fundamental physics, but also on astrophysical phenomena. Moreover, axions produced in the early universe could also be the source of dark matter.

Alternatively, the excess could also be due to neutrinos, trillions of which pass through your body, unhindered, every second. One explanation could be that the magnetic moment (a property of all particles) of neutrinos is larger than its value in the Standard Model of elementary particles. This would be a strong hint to some other new physics needed to explain it.

Of the three explanations considered by the XENON collaboration, the observed excess is most consistent with a solar axion signal. In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2/10,000 chance that the observed excess is due to a random fluctuation rather than a signal. While this significance is fairly high, it is not large enough to conclude that axions exist. The significance of both the tritium and neutrino magnetic moment hypotheses corresponds to 3.2 sigma, meaning that they are also consistent with the data.

XENON1T is now upgrading to its next phase–XENONnT–with an active xenon mass three times larger and a background that is expected to be lower than that of XENON1T. With better data from XENONnT, the XENON collaboration is confident it will soon find out whether this excess is a mere statistical fluke, a background contaminant, or something far more exciting: a new particle or interaction that goes beyond known physics.

What is Dark Matter made of?

Part of the mystery of Dark Matter is that we know it is there– just not what it is.
In Discover magazine you can read about some of the top candidates:
https://www.discovermagazine.com/the-sciences/what-is-dark-matter-made-of-these-are-the-top-candidates. The primary candidate that XENON searches for is a Weakly Interacting Massive Particle (WIMP): Including our main spin-independent search, as well as further WIMP  interaction models. In addition, recent XENON analyses have constrained a number of alternative models, including Axion-like particles and dark photons.

Distillation campaign for XENONnT finished

The up-coming XENONnT experiment utilizes a total of 8.3 tonnes of xenon to search for the ever elusive dark matter particles. In addition to the existing 3.3 tonnes of ultra-pure xenon from XENON1T, another 5 tonnes of xenon were purchased by the XENON collaboration. 

Before the new gas can be used for XENONnT, it needs to be purified. Besides oxygen, nitrogen and water that potentially absorb the light and charge signals in the detector, the radioactive noble gas Kr-85 within the xenon needs to be removed. Kr-85 is a man-made isotope created in nuclear bomb testing and nuclear fuel reprocessing. It makes up a fraction of 10-11 of the natural krypton (Kr-nat) abundance.

The commercially available xenon arrives with a Kr-nat in xenon concentration on the order of 10-6 (ppm, parts per million) to 10-9  (ppb, parts per billion) and needs to be purified down to a concentration of 0.1 x 10-12 (ppt, parts per trillion). To put this in relation: When purchased, an Olympic swimming pool filled with liquid xenon contains a 10 liter bucket of krypton. After purification, 200 Olympic swimming pools filled with liquid xenon contain together just one single droplet of krypton.

The purest xenon on Earth can be produced with the help of our Krypton Distillation Column located underground in the service building of XENON1T/nT as seen in the picture below. The purification method is based on the separation due to the different boiling points of xenon and krypton. While xenon is in its liquid form at -100°C, krypton, as the lighter atom, prefers to stay in its gaseous form. Like that, krypton is enriched at the top of our distillation tower from where it is removed and stored in a bottle as so-called “offgas”. The purified xenon can exit the distillation system at the bottom.

The picture shows the service building of XENON1T/nT. Bottles with new xenon, containing tiny trace amounts of the radioactive noble gas Kr-85, were connected to the “Bottle rack” (Blue-red-dashed line). The xenon is guided into the “Distillation column” to separate krypton from xenon. At the top, krypton-enriched xenon is extracted as “offgas” (red line), while at the bottom, the purified xenon is taken out (Blue line). Purified xenon is either stored inside “ReStoX-I” (Left side, blue line), the storage system of XENON1T, or in “ReStoX-II” (Right side, blue line), a newly installed storage system for XENONnT.

In total, over 100 bottles of freshly delivered xenon were installed in two bottle batches at the “Bottle rack”. Here, xenon samples from each batch were measured with a connected residual gas analyzer (RGA) system. Xenon from one of the bottle batches was continuously filled to the distillation system. Purified xenon was stored  either to the Recovery and Storage for XENON1T (ReStoX-I) (left side of the picture) or to the ReStoX-II system (right side of picture), a newly installed subsystem for XENONnT. ReStoX-II is a system designed to rapidly recover and safely store up to 10 tonnes of xenon, that will serve as an fast recovery system during operation of the XENONnT experiment as well as xenon storage previous to the start of the experiment.

The full distillation campaign was split into three phases starting from April 2019 and was finished in July 2019. Xenon samples were extracted to measure the purified xenon purity at MPIK Heidelberg with a rare gas mass spectrometer

As always in our collaboration, this operation too was an interplay between different groups: The bottle rack was installed by MPIK Heidelberg, the Distillation Column was operated by WWU Münster, and the ReStoX-I and -II systems were built and monitored by Columbia University in New York and Subatech-CNRS. The existing slowcontrol system was updated for the distillation campaigns by the Weizmann Institute of Science. Furthermore, local support was given by the group of INFN. Finally, to exchange bottles and to monitor the system 24/7, shifters from all over the collaboration supported the core distillation team.

XENON at the 2019 Swiss-Austrian Physical Society Meeting

Five members of the University of Zurich group participated at the 2019 Swiss-Austrian Physical Society Meeting in Zurich, Switzerland.

Adam Brown contributed with a poster on the XENONnT upgrades and status and Ricardo Peres on the software for the supernova early warning system:

Giovanni Volta, Michelle Galloway and Chiara Capelli contributed with talks on the general XENON1T results, the ongoing search for dark absorption and the analysis on high energy events respectively. The full talks are linked. Below a key slide from each talk is shown: the spin-independent elastic WIMP-nucleon scattering limit at 90% CL still are the most sensitive limits on WIMP dark matter. The motivation for light dark matter searches is becoming more and more pressing. And our reconstruction of single-site and multiple-site interactions for the neutrinoless double beta decay search significantly improves our capability to contribute to this exciting science channel.

The XENON1T Data Acquisition System

Featuring several kilometers of cables, dozens of analog electronics modules, crates of purpose-built specialty computers, and backed by a small server farm, the XENON1T data acquisition system (DAQ) was designed to put our data onto disks. The XENON Collaboration recently published a technical paper on our DAQ in JINST, of course also available on arXiv.

The XENON1T detector measures light, which creates analog electrical signals in 248 independent photo-sensors. The DAQ is responsible for converting these analog signals to a digital, storage-ready format, deciding what types of aggregate signal indicate the presence of a physical interaction in the detector, and recording all the interesting data onto disk for later storage and analysis.

A photo of the XENON1T DAQ room, deep underground at the Gran Sasso lab. Pictured left to right: the DAQ server rack, (red) digitizers (amplifiers facing backwards), cathode high voltage supply, muon veto DAQ, slow control server rack.

There are a couple novel aspects of this system. The first is that the data is streamed constantly from the readout electronics onto short-term storage, recording all signals above a single photo-electron with high (>93%) efficiency. This is different from a conventional data acquisition system, which usually would require certain hardware conditions to be met to induce acquisition,  also called a trigger. We defer our trigger to the software stage, giving us a very low energy threshold.

The software trigger itself was implemented as a database query, which is another novel aspect of the system. Pre-trigger data was stored in a MongoDB NoSQL database and the trigger logic scanned the database looking for signals consistent with S1’s (light) and S2’s (charge). If the algorithm found a matching signal, it would retrieve all the nearby data from the database and write it to storage. Because of the speed of NoSQL databases, this worked the same in both dark matter search mode, where we record just a few counts per second, and calibration modes, where we could record hundreds of counts per second.

To complete the high-tech upgrade of our system, we also ran the user interface as a web service. This means the system could be controlled from laptops, smartphones, or tablets anywhere with a 4G connection, contributing to the high uptime of the detector.

The DAQ is currently being updated to double its capacity to read out the XENONnT detector, so stay tuned.

Search for light dark matter interactions enhanced by the Migdal effect in XENON1T

When a particle elastically scatters off a xenon nucleus, it has been assumed that electron clouds immediately follow the motion of the nucleus, but in reality it takes some time for the atomic electrons to catch up, resulting in ionization and excitation of the atom. This effect is called the Migdal effect, which was predicted by A. B. Migdal and recently reformulated in the context of Dark Matter searches by Ibe. et alWhile the elastic scattering of WIMPs produces nuclear recoils, the Migdal effect predicts secondary electronic recoils that can accompany a nuclear recoil. Unlike nuclear recoils, electronic recoils lose negligible energy as heat, because electrons have small masses compared with xenon nuclei. This results in a lower energy threshold for electronic recoil signals – in XENON1T, down to about 1 keV. Therefore, searching for the electronic recoil signals induced by the Migdal effect enables a significant boost of XENON1T’s sensitivity to low-mass dark matter, based on this lowered threshold. In this search, we adopted an approach that utilizes the ionization signal only (so-called S2-only analysis), as well as both scintillation and ionization signals (S1-S2 analysis), which enables to lower the detection threshold. We interpreted the results in different cases: spin-(in)dependent (SI/SD) WIMP-nucleon interaction and the scenario where the interaction is mediated by a scalar force mediator (light mediator). The results for the spin-(in)dependent WIMP-nucleon interaction are shown in the following figure:
 
We set the most stringent upper limits on the SI and SD WIMP-nucleon interaction cross-sections for masses below 1.8 GeV and 2 GeV, respectively. Together with the standard nuclear recoil search, XENON1T results have thus reached unprecedented sensitivities to both low-mass (sub-GeV) and high-mass (GeV – TeV) WIMPs. An open access pre-print of the paper can of course be found on the arxiv.

Talk at Lepton Photon 2019

A talk on the measurement of the double electron capture half-life of xenon-124 with the XENON1T experiment was given at the Lepton Photon 2019 conference in Toronto in August 2019. Ethan Brown from Rensselaer Polytechnic Institute presented this exciting result, demonstrating the power of the ultra-low background in XENON1T. This yielded the measurement of the longest process ever directly observed at 1.8×10^22 years, a trillion times longer than the age of the Universe.

Some of the dark matter search results were also presented in this talk, advertising the incredible success of the XENON program and the science reach of the XENON1T experiment in rare event detection.