XENON1T recently released a preprint with new world-leading constraints on light dark matter particles.
The challenge of light dark matter
The XENON1T detector aims find the signals of dark matter bouncing off xenon atoms.
If such a collision happens, it produces two signals: a small light flash (S1), and a cloud of free electrons that can be drifted up and extracted out of the detector (S2).
Figure: How dark matter would make S1 and S2 signals in the XENON1T detector.
However, dark matter lighter than about six times the proton mass (6 GeV/c^2) cannot push the heavy xenon atoms (131 GeV/c^2) enough to make efficiently detectable S1s. XENON1T needs both S1 and S2 to accurately reconstruct where in the detector the event happened. The time between the S1 and S2 signals reveals the depth of the event. Events at the top and bottom edge of the detector are common due to radioactive backgrounds. If we cannot reject these events, dark matter searches will not be efficient. Thus, most strong constraints on light dark matter have, until now, come from different detectors, mostly using ultra-low temperature crystals made of Germanium, Silicone, or Calcium Tungstate.
The S2-only technique
XENON1T’s new preprints use an “S2-only analysis”, where events without S1s are still considered. Advances in detector construction and analysis techniques led to a thousand times lower background level than previously achieved in S2-only searches.
For example, the S2 electron cloud becomes broader as it drifts upward, like a drop of ink spreading out in water. The deeper the event, the broader the cloud, and the longer the S2 signal lasts. Thus XENON1T could reject most of the events at the top and bottom, even without the S1, by rejecting very short and very long S2 signals.
Most theorists predict that dark matter would collide with the heavy xenon nuclei and produce “nuclear recoils”. For these, the S2-only technique is sensitive to 2-3x lower energies than traditional analyses. Thus, we get improved constraints on light dark matter:
Figure: New XENON1T limits (black lines) on light dark matter. The colored lines show previous results, including other results from XENON1T in blue.
In some models, dark matter collides with electrons around the nucleus, and produces “electronic recoils”. These make much larger S2 signals than nuclear recoils of the same S1 size. S2-only searches thus improve the energy threshold for these models by as much as a factor of ten. Combined with the lower background, XENON1T’s S2-only results thus improve the constraints on such models by several orders of magnitude:
Figure: New XENON1T limits on scattering of dark matter on electrons. (The dashed line is the same analysis repeated with more conservative assumptions.)
For more information, please see our arXiv preprint at https://arxiv.org/abs/1907.11485.
XENON was present at the ALPS conference in Austria. Chiara Capelli from University of Zurich gave a talk on behalf of the XENON collaboration. The talk focused on the latest XENON1T results on spin-independent and spin-dependent WIMPs, and on the newest results on two-neutrinos double electron capture, with a final status on the XENONnT upgrade. The talk is available here.
The universe is almost 14 billion years old. An inconceivable length of time by human standards – yet compared to some physical processes, it is but a moment. There are radioactive nuclei that wdecay on much longer time scales. Using our XENON1T detector at the INFN Gran Sasso National Laboratory, we were able to observe the decay of Xenon-124 atomic nuclei for the first time.
The half-life of a process is the time after which half of the radioactive nuclei present in a sample have decayed away. The half-life measured for Xenon-124 is about one trillion times longer than the age of the universe. This makes the observed radioactive decay, the so-called double electron capture of Xenon-124, the rarest process ever seen happening in a detector. “The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is – also for signals which are not from dark matter,” says Prof. Christian Weinheimer from the University of Münster (Germany) whose group lead the study. In addition, the new result provides information for further investigations on neutrinos, the lightest of all elementary particles whose nature is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the US and the Middle East. The results were published in the science journal “Nature”.
A sensitive dark matter detector
The Gran Sasso Laboratory of the National Institute for Nuclear Physics (INFN) in Italy, where scientists are currently searching for dark matter particles is located about 1,400 meters beneath the Gran Sasso massif, well protected from cosmic rays which can produce false signals. Theoretical considerations predict that dark matter should very rarely “collide” with the atoms of the detector. This assumption is fundamental to the working principle of the XENON1T detector: its central part consists of a cylindrical tank of about one meter in length filled with 3,200 kilograms of liquid xenon at a temperature of –95° C. When a dark matter particle interacts with a xenon atom, it transfers energy to the atomic nucleus which subsequently excites other xenon atoms. This leads to the emission of faint signals of ultraviolet light which are detected by means of sensitive light sensors located in the upper and lower parts of the cylinder. The same sensors also detect a minute amount of electrical charge which is released by the collision process.
The new study shows that the XENON1T detector is also able to measure other rare physical phenomena, such as double electron capture. To understand this process, one should know that an atomic nucleus normally consists of positively charged protons and neutral neutrons, which are surrounded by several atomic shells occupied by negatively charged electrons. Xenon-124, for example, has 54 protons and 70 neutrons. In double electron capture, two protons in the nucleus simultaneously “catch” two electrons from the innermost atomic shell, transform into two neutrons, and emit two neutrinos. The other atomic electrons reorganize themselves to fill in the two holes in the innermost shell. The energy released in this process is carried away by X-rays and so-called Auger electrons. However, these signals are very hard to detect, as double electron capture is a very rare process which is hidden by signals from the omnipresent natural radioactivity.
This is how the XENON collaboration succeeded with this measurement: The X-rays from the double electron capture in the liquid xenon produced an initial light signal as well as free electrons. The electrons were moved towards the gas-filled upper part of the detector where they generated a second light signal. The time difference between the two signals corresponds to the time it takes the electrons to reach the top of the detector. Scientists used this interval and the information provided by the sensors measuring the signals to reconstruct the position of the double electron capture. The energy released in the decay was derived from the strength of the two signals. All signals from the detector were recorded over a period of more than one year, however, without looking at them at all as the experiment was conducted in a “blind” fashion. This means that the scientists could not access the data in the energy region of interest until the analysis was finalized to ensure that personal expectations did not skew the outcome of the study. Thanks to the detailed understanding of all relevant sources of background signals it became clear that 126 observed events in the data were indeed caused by the double electron capture of Xenon-124.
Using this first-ever measurement, the physicists calculated the enormously long half-life of 1.8×1022 years for the process. This is the slowest process ever measured directly. It is known that the atom Tellurium-128 decays with an even longer half-life, however, its decay was never observed directly and the half-life was inferred indirectly from another process. The new results show how well the XENON1T detector can detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also search for the so-called neutrino-less double electron capture which could shed light on important questions regarding the nature of neutrinos.
Status and outlook
XENON1T acquired data from 2016 until December 2018 when it was switched off. The scientists are currently upgrading the experiment for the new “XENONnT” phase which will feature a three times larger active detector mass. Together with a reduced background level this will boost the detector’s sensitivity by an order of magnitude.
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!
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.
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.
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.
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!
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.
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.
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.