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/.

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 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.

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.

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%.