XENON1T at the annual meeting of the Swiss Physical Society, 2018

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.

XENON1T Result covered by CERN Ccourier

XENON1T results from a 1 ton-year dark matter exposure.

Our latest dark matter results from XENON1T, the most sensitive search for WIMPs with an unprecedented liquid xenon exposure of 1 ton-years and a world-record low background level is featured in the July/August 2018 edition of the CERN Courier, the International Journal of High-Energy Physics. Read the full article here.


XENON1T At Dark Matter Summer School 2018

Some of our young collaboration members attended a Dark Matter Summer School at the University of Albany from July 16th through the 20th.

XENON1T at DMSS 2018Featured from left to right: Kelly Odgers, Chloé Therreau, Amanda Depoian, Abigail Kopec, Dr. Ethan Brown, Arianna Rocchetti, Matthew Bernstein, and Leaf Swordy.

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/.

Time stability of signals in XENON1T

One important requirement for every experiment running over long periods of time is the detector’s stability. In the XENON1T experiment we can monitor the time evolution of the response to interactions happening inside the TPC over a wide range of energies and for both gamma and alpha particles of known energies. For this purpose we exploit mono-energetic lines of different nature:

83mKr (9, 32 and 41 keV gamma lines)
The lowest energy calibration lines are from decays of the metastable krypton isotope 83mKr, which is regularly (approximately every 2 weeks) injected in the LXe target to calibrate the spatial dependency of detector’s response. 83mKr decays to stable state by emitting two gammas of 32.2 and 9.4 keV. In some cases the two gammas can be emitted so close in time that cannot be resolved (the half-life of the second transition is 157 ns) resulting thus in a combined gamma line of 41 keV.

131mXe and 129mXe (164 and 236 keV gamma lines)
Metastable isotopes of xenon are activated during calibrations of the detector’s response to low energetic nuclear recoils, which are performed using external neutron sources such as a DD fusion neutron generator or a 241AmBe source. The activated xenon isotopes 131mXe and 129mXe decay to stable state emitting a 164 and 236 keV gamma respectively. Their half-life is in the order of 10 days and such gamma lines are therefore present only for few weeks after any neutron calibration.

High energy gamma lines (1.8, 2.2, 2.6 MeV)
We can also monitor the signals of gamma lines in the MeV energy range which are originated from the (very low) radioactivity of detector’s materials. In particular, we study the gammas from 214Bi (1.8 and 2.2 MeV) and 208Tl (2.6 MeV). Even if the statistics of such energy lines is not very large, they allow a monthly monitoring of signals stability throughout the science run.

222Rn (5.5 MeV alphas)
The radioactive 222Rn is a noble gas that permanently emanates from the detector components into the LXe reservoir. The activity of its alpha decay to 218Po in our detector is of ~13 µBq/kg producing a clear energy line at 5.5 MeV with enough statistics to monitor the detector’s response at this energy on a daily basis.

Time evolution of the light yield for various energy lines over the long science run of XENON1T. Shaded regions highlight calibration runs: Rn220 (for low energy electronic recoils), Kr83m (for spatial corrections), AmBe and neutron generator (for low energy nuclear recoils).

The prompt scintillation light signal (called S1) detected after energy depositions of known energy gives us a measure of the light yield, in units of detected photoelectrons (PE) per keV.  Therefore, we measure the light yield for all the energy lines mentioned above at different times during the science run. The big XENON1T detector proofs its remarkable stability as the S1 signals stay flat over time for all the different sources.

Relative variation over time of the light yield for different energy lines.

If we look at the relative deviation from the average light yield of the various energy lines, we find that their time trends are in pretty good agreement. The stability level is observed at 0.2% for both the 41 keV gamma and the 5.5 MeV alpha, spanning over almost 1 year time range. This is an excellent demonstration of how smoothly the XENON1T detector operated during the dark matter search data acquisition. Stay tuned!



XENON1T Talk at NDM 2018

On July 2nd, 2018, Kaixuan Ni of UC San Diego presented the recent results and the current status of the XENON1T experiment at the 6th Symposium on Neutrinos and Dark Matter in Nuclear Physics (NDM 2018) in Daejeon, Korea. The recent released results from XENON1T set the world’s most stringent limits on the WIMP-nucleon spin-independent interactions for all WIMP masses above 6 GeV. The detector currently keeps taking dark matter search data with improved detector performance and with reduced background. An upgrade of the detector to XENONnT is scheduled in 2019. With a total of 8-tonnes of liquid xenon, XENONnT will boost the dark matter search sensitivity by a factor of 10 and is targeting at dark matter candidates from the supersymmetric models.

Latest XENON1T results at ICHEP2018 in Seoul

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.

Spatial distribution of unblinded and de-salted data.

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.

Radon mitigation for XENONnT

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.

More than dark matter – XENON1T at Neutrino 2018

XENON1T may have been designed to search for dark matter, but it turns out that we can do a lot more with it. As the amount of xenon increases and backgrounds go down, the experiment starts to check all the boxes for a neutrino detector and becomes sensitive to rare physics processes, such as double $\beta$-decays. Two XENON collaboration posters at Neutrino 2018 in the beginning of June showcased the prospects for the detection of two such decays.

Chiara Capelli’s poster on neutrinoless double beta decay presented at Neutrino 2018

First, there is neutrinoless double $\beta$ -decay of the xenon isotope Xe-136. Here, two neutrons in the atomic nucleus are simultaneously converted into two protons. In order to conserve the total charge that increased by +2 with the protons two electrons with the charge -2 have to be emitted. In the standard model one would also need two anti-electron neutrinos to conserve lepton number. But this process goes beyond the standard model of particle physics. Its detection would imply that neutrinos are their own anti-particles and the violation of lepton number could be the key to understanding why the universe is dominated by matter compared to anti-matter today. Chiara Capelli, a PhD student from the Zürich XENON group, presented a poster where she checked the sensitivity of current and future xenon detectors for neutrinoless double $\beta$ -decay. In the years to come these detectors will complement existing experiments.

Poster on double electron capture presented at Neutrino 2018 by Alexander Fieguth and Christian Wittweg

A second poster by Alexander Fieguth and Christian Wittweg from the Münster group outlined an ongoing search for the double electron capture of Xe-124. This decay is the other way round: Two neutrons are made from protons at the same time. The necessary electrons for charge conservation are taken right from the electronic shell of the xenon atom itself. Two electron neutrinos are emitted to conserve lepton number. Although the neutrinoless case is also thinkable, the standard model decay with two neutrinos is exciting in itself. It is predicted but has not been detected so far. It t would be the longest-lived nuclear decay process ever observed directly. As XENON1T has the largest mass of Xe-124 in an experiment to date – about 1.5 kg due to the rarity of Xe-124 in natural xenon – it will be the most sensitive detector to search for this double electron capture process.

All in all, the future looks bright for large xenon detectors in neutrino physics and there are a bunch of exciting publications to look forward to.


XENON1T probes deeper into Dark Matter WIMPs, with 1300 kg of cold Xe atoms

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.

Shown are the limits on WIMP interactions, derived from one year of XENON1T data. The inset compares our limit and sensitivity with the limit and sensitivities of previous experiments.

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.

XENON1T at the first Rucio Community Workshop at CERN

Everything scales up! Even the amount of acquired raw data in XENON1T. To handle data transfers easily, the XENON collaboration decided to let the Rucio Scientific Data Managment software do all the work. Rucio is developed at CERN and meant to manage scientific data. Data transfers, book keeping, easy data access and safety against data loss are its big advantage.

XENON1T is taking about one Terabyte of raw data per day. The detector is located at the Laboratori Nazionali del Gran Sass (LNGS) in Italy and the data need to be shipped out to dedicated computing centers for data reduction and analysis.

Individual Rucio clients access dedicated GRID disk space on world wide distributed computer facilities. Everything is controlled by a Rucio server which keeps track on storage locations, data sizes and transfers within the computer infrastructure. Rucio is developed in Python and its distribution becomes very simple.

The First Rucio Community Workshop was held at CERN on 1st and 2nd of March. Since Rucio was developed for the ATLAS collaboration, other experiments like XENON and AMS started to use Rucio a while ago. Nowadays, more collaborations such as EISCAT 3D, LIGO or NA62 (just to mention a few) became interested. The workshop allowed to meet all each other: developers and users discussed several use cases and how to improve Rucio for individual collaborations.

The XENON1T data distribution from https://indico.cern.ch/event/676472/contributions/2905755/

The XENON1T data distribution framework

We presented our integration of Rucio in the existing data handling framework. XENON1T raw data are distributed to five computing centers in Europe and the US. Each one is connected to the European Grid Interface (EGI) or the Open Science Grid (OSG) for data reduction (“processing”). Raw data are processed on the GRID and the reduced data sets are provided for the analysts on Research Computing Center (RCC) in Chicago. Beyond this, the XENON collaboration will continue to use Rucio for the upcoming XENONnT upgrade.

Towards a Neutron Veto System for the XENONnT Upgrade

Once a year the Spring-Meeting of the German Physics Society (DPG “Deutsche Physikalische Gesellschafft”) takes place. This year I had the opportunity to talk about the planned neutron Veto for XENONnT in Würzburg (19.-23.3.2018).

In order to maximize the fiducial volume, we want to veto nuclear recoils. Therefore we are working on a neutron veto system based on Gadolinium loaded liquid scintillator. The plan is to use acrylic boxes, which can be filled with liquid scintillator before being placed around the TPC cryostat.

Building on the experience of DOUBLE CHOOZ, we developed a first LAB-based liquid scintillator doped with 0.1 % Gadolinium and two wavelength shifters. Three measurements were performed to test the optical properties, transmission, emission and relative light yield. From the transmission measurement, we learned that the attenuation length of the scintillator at a wavelength of 430nm is 7.1m. The emission measurement shows the shifting due to the wavelength shifters to the visible wavelength. And with the relative light yield measurement we can compare an unloaded scintillator sample with the Gd-loaded sample. The light yield for the loaded scintillator decreases to 74% of the unloaded sample.

Our next steps will be to find a supplier who can provide us with a large and pure amount of the Gadolinium-Complex and to build a setup for neutron tagging measurements.