Tag Archives: science

XENON1T at Lake Louise

Physics meets winter sports at the Lake Louise Winter Institute, a particle physics conference held annually in the beautiful Canadian Rockies. On February 12, 2019, Evan Shockley from University of Chicago presented at the conference on behalf of the XENON collaboration. The talk focused on the latest, world-leading WIMP results, and included a status update on XENON1T and its imminent upgrade, XENONnT. The talk is available here.

XENONnT will feature a larger detector and even lower background than XENON1T, making it ~10 times more sensitive to interactions from dark matter and other rare processes. With installation coming later this year, it’s an exciting time for the XENON collaboration and the field of dark matter research!

 

Constraining the spin-dependent WIMP-nucleon interaction with XENON1T

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.

The first limits on spin-independent WIMP-pion interactions

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. 

A WIMP scattering on a pion exchanged within the xenon nucleus

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.

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!

Search for bosonic super-WIMP interactions with the XENON100 experiment

While the microscopic nature of dark matter in the Universe is largely unknown, the simplest assumption which can explain all existing observations is that it is made of a new, as yet undiscovered particle. Leading examples are weakly interacting massive particles (WIMPs), axions or axion-like particles (ALPs), and sterile neutrinos. WIMPs with masses in the GeV range, as well as axions/ALPs are examples for cold dark matter while sterile neutrinos with masses at the keV-scale are an example for warm dark matter. Cold dark matter particles were nonrelativistic at the time of their decoupling from the rest of the particles in the early universe. In contrast, warm dark matter particles remain relativistic for longer, retain a larger velocity dispersion, and thus more easily free-stream out from small-scale perturbations. Astrophysical and cosmological observations constrain the mass of warm dark matter to be larger than about 3keV/c2, with a more recent lower limit from Lyman-alpha forest data being 5.3keV/c2. Another example for warm dark matter particles are bosonic super-WIMPs. These particles, with masses at the keV-scale, could couple electromagnetically to standard model particles via the axioelectric effect, which is an analogous process to the photoelectric effect, and thus be detected in direct detection experiments.

The limit derived from the XENON100 experiment on the coupling of SuperWIMPs.

We searched for vector and pseudo-scalar bosonic super-WIMPs with the XENON100 experiment. The super-WIMPs can be absorbed in liquid xenon and the expected signature is a monoenergetic peak at the super-WIMP’s rest mass. A profile likelihood analysis of data with an exposure of 224.6 live days × 34kg showed no evidence for a signal above the expected background. We thus obtained new upper limits in the (8 − 125) keV/c2 mass range, excluding couplings to electrons with coupling constants of gae > 3 × 10−13 for pseudo-scalar super-WIMPs and α′/α > 2 × 10−28 for vector super-WIMPs, respectively. We expect to improve upon these results with the XENON1T detector, which operates a larger mass of liquid xenon with reduced backgrounds. Our results were published in Physical Review D 96, 122002 (2017) and are of course also available at the arxiv.

Search for WIMP Inelastic Scattering Off Xenon Nuclei With XENON100

Most direct detection searches focus on elastic scattering of galactic dark matter particles off nuclei, where the keV-scale nuclear recoil energy is to be detected. In this work, the alternative process of inelastic scattering is explored, where a WIMP-nucleus scattering induces a transition to a low-lying excited nuclear state. The experimental signature is a nuclear recoil detected together with the prompt de-excitation photon. We consider the scattering of dark matter particle off 129Xe isotope, which has an abundance of 26.4\% in natural xenon, and when excited to it lowest-lying 3/2+ state above the ground state it emits a 36.9 keV photon. This electromagnetic nuclear decay has a half-life of 0.97 ns.

The WIMP inelastic scattering  is complementary to spin-dependent, elastic scattering, and dominates the integrated rates above 10 keV of deposited energy. In addition, in case of a positive signal, the observation of inelastic scattering would provide a clear indication of the spin-dependent nature of the fundamental interaction.

The search is performed using XENON100 Run-II science data, which corresponds to an exposure of 34×224.6 kg×days. No evidence of dark matter is found and a limit on dark matter inelastic interaction cross section is set. Our result, shown in the Figure, is the most stringent limit for the spin-dependent inelastic scattering to date, and set the stage for a sensitive search of inelastic WIMP-nucleus scattering in running or upcoming liquid xenon experiments such as XENON1T, XENONnT, LZ, and DARWIN.

Full details may be found in this article: Phys. Rev. D 96, 022008 and on the arxiv.

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!

The traditional approach for WIMP nucleus interaction studies in direct detection experiment is to consider just two types of interactions, the spin independent (SI) and the spin dependent (SD) ones. However, these are not the only types of interactions possible. In recent years, a non-relativistic effective field theory approach has been studied. In this framework, 14 new interaction operators arise. These operators include the SI and SD ones among others. Some of these new operators are momentum dependent and predict a non-exponential event rate as function of energy, in contrast to the traditional expected signals. Moreover, some of these operators predict energy recoils above the upper threshold of the standard analyses done in direct detection experiments. For XENON100, this threshold is 43keV (nuclear recoil).

In this analysis, we extend the upper energy threshold up to ~240 keV. This value is dictated by low statistics in calibration data above it. We divide our signal region into two regimes, low recoil energy, on which we perform the same “standard” analysis done for the SI and SD cases, and high recoil energy, which is the main focus of this work.

Summary of regions of interest, backgrounds, and observed data. ER calibration data, namely 60Co and 232Th data, is shown as light cyan dots. NR calibration data (241AmBe) is shown as light red dots. Dark matter search data is shown as black dots. The red line is the threshold between the low and high energy channels. The lines in blue are the bands. For the low energy channel these are operator and mass dependent, but are shown here for a 50 GeV/c^2 WIMP using the O1 operator. For the high-energy region, the nine analysis bins are presented also in blue lines.

We find that our data is compatible with background expectations. Using a binned profile likelihood, we thus produce 90% CL exclusion limits for both elastic scattering and inelastic WIMP scattering for each operator. Find the preprint of this study on the arxiv.

The XENON100 limits (90% CLS) on isoscalar dimensionless coupling for all elastic scattering EFT operators. The
limits are indicated in solid black. The expected sensitivity is shown in green and yellow (1σ and 2σ respectively). Limits from CDMS-II Si, CDMS-II Ge, and SuperCDMS [30] are presented as blue asterisks, green triangles, and orange rectangles, respectively.

XENON1T, the most sensitive detector on Earth searching for WIMP dark matter, releases its first result

[Press Release May 2017 – for immediate release. Preprint is on the arxiv]

The best result on dark matter so far! … and we just got started!”.

This is how scientists behind XENON1T, now the most sensitive dark matter experiment world-wide, hosted in the INFN Laboratori Nazionali del Gran Sasso, Italy, commented on their first result from a short 30-day run presented today to the scientific community.

XENON1T at LNGS

XENON1T installation in the underground hall of Laboratori Nazionali del Gran Sasso. The three story building on the right houses various auxiliary systems. The cryostat containing the LXeTPC is located inside the large water tank on th left, next to the building. (Photo by Roberto Corrieri and Patrick De Perio)

Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a world-wide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties. However, these interactions are so feeble that they have escaped direct detection up to this point, forcing scientists to build detectors that are more and more sensitive. The XENON Collaboration, that with the XENON100 detector led the field for years in the past, is now back on the frontline with the XENON1T experiment. The result from a first short 30-day run shows that this detector has a new record low radioactivity level, many orders of magnitude below surrounding materials on Earth. With a total mass of about 3200kg, XENON1T is at the same time the largest detector of this type ever built. The combination of significantly increased size with much lower background implies an excellent dark matter discovery potential in the years to come.

The XENON1T TPC

Scientists assembling the XENON1T time projection chamber. (Photo by Enrico Sacchetti)

The XENON Collaboration consists of 135 researchers from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates. The latest detector of the XENON family has been in science operation at the LNGS underground laboratory since autumn 2016. The only things you see when visiting the underground experimental site now are a gigantic cylindrical metal tank, filled with ultra-pure water to shield the detector at his center, and a three-story-tall, transparent building crowded with equipment to keep the detector running, with physicists from all over the world. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), is not visible. It sits within a cryostat in the middle of the water tank, fully submersed, in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat allows keeping the xenon at a temperature of -95°C without freezing the surrounding water. The mountain above the laboratory further shields the detector, preventing it to be perturbed by cosmic rays. But shielding from the outer world is not enough since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content. Laura Baudis, professor at the University of Zürich and professor Manfred Lindner from the Max-Planck-Institute for Nuclear Physics in Heidelberg emphasize that this allowed XENON1T to achieve record “silence”, which is necessary to listen with a larger detector much better for the very weak voice of dark matter.

XENON1T first results limit

The spin-independent WIMP-nucleon cross section
limits as a function of WIMP mass at 90% confidence
level (black) for this run of XENON1T. In green and yellow
are the 1- and 2σ sensitivity bands. Results from LUX
(red), PandaX-II (brown), and XENON100 (gray)
are shown for reference.

A particle interaction in liquid xenon leads to tiny flashes of light. This is what the XENON scientists are recording and studying to infer the position and the energy of the interacting particle and whether it might be dark matter or not. The spatial information allows to select interactions occurring in the central 1 ton core of the detector. The surrounding xenon further shields the core xenon target from all materials which already have tiny surviving radioactive contaminants. Despite the shortness of the 30-day science run the sensitivity of XENON1T has already overcome that of any other experiment in the field, probing un-explored dark matter territory.  “WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon!” says Elena Aprile, Professor at Columbia University and spokesperson of the project. “The best news is that the experiment continues to accumulate excellent data which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before. A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just began with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”

As always, feel free to contact the XENON collaboration at contact@xenon1t.org.