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%.

SAIP 2017: Latest results from XENON1T

At the 62nd annual conference of the South African Institute of Physics (SAIP), hosted by the University of Stellenbosch, Jacques Pienaar presented the results of our first science run with XENON1T. While a dark matter particle candidate still eludes us, we are able to demonstrate that for the first time a tonne-scale liquid Xenon dark matter detector is not only operating, but doing so very successfully.

The work done up to this point has given us a thorough understanding of the electronic and nuclear recoil response in our detector, which we can use to look for dark matter candidates. This of course is just the start. In this first result we had an exposure of only 0.1 ton.years, but our design goal is 2 ton.years. Therefore much work still lies ahead to probe for dark matter, and indeed we have more than 3 times as much data available already to push the bounds of our knowledge further. Stay tuned!

Modulation results from Xenon100 presented at PASCOS 2017 conference.

On Tuesday 20th of June, we presented our latest results on Electronic Recoil Modulations with 4 years of Xenon100 data at the PASCOS 2017 conference held in Madrid. After a short introduction, by M.L. Benabderrahmane, to the dark matter modulation as a signal, the main results have been presented, namely the test statistics of unbinned profile likelihood to search for the modulation period using three different sets of data. The first set contains the single scatter events in the energy range 2-5.8keV, the second set contains Multiple scatter events in the same energy range and the last one contains single scatters in the energy range 6-20keV. The last two samples are used as a sideband. The results of the likelihood gives a period of 431 days which is different from the one observed by the DAMA/LIBRA collaboration. Our single scatter modulation at 431 days has a global significance below 2sigma. The local test statistics for one year period gives a 1.8sigma. Similarity of the spectra between the two control samples and the signal sample disfavors the possibility for a modulation due to Dark Matter interaction.

Talk at WIN2017: First Results from the XENON1T experiment

This talk by Sara Diglio (slides) was presented at the WIN2017 conference at the University of California in  Irvine, from June 19 – 24, 2017.

ReStoX is an original cryogenic system designed for experiments that make use of high quantities of liquid xenon. It allows to store the total amount of xenon in gaseous, liquid or solid phase and to fill it into the detector vessel under high purity conditions. The system is crucial in case of emergencies that might require a fast recovering of the whole xenon present in the detector. ReStoX is currently being used by the XENON1T experiment and a future upgrade for XENONnT has already started.

On Tuesday, May 30, we presented the first XENON1T results in a seminar at LNGS, the laboratory where our experiment is hosted. The seminar was presented by Marco Selvi (INFN Bologna) in the Fermi room, the main auditorium at LNGS, and introduced by the LNGS director prof. Stefano Ragazzi in front of about 40 scientists.

After a short introduction on Dark Matter (you may guess that at LNGS they are well aware of the details of this physics puzzle! ), we described the construction and commissioning phase of the various systems crucial to run our detector.

We then focused mainly on the performances of XENON1T in the first science run,
where we reached the lowest ER background ever achieved in a dark matter experiment.

Also our sensitivity is very good, being it also the best out of the various direct search dark matter experiment, even with just 34 days of data acquisition.

With our result, XENON1T (and LNGS with) is back at the frontline of the race to finally detect dark matter particles … we look forward to analyse the already acquired >70 days of data !