XENON1T was operated deep underground at the INFN Laboratori Nazionali del Gran Sasso in Italy, from 2016 to end of 2018. It was primarily designed to detect dark matter, which makes up 85% of the matter in the universe and for which there is only indirect evidence so far. XENON1T didn’t detect dark matter, but has reached the world-leading sensitivity for the search for WIMPs (Weakly Interacting Massive Particles), which are among the theoretically preferred candidates for dark matter. In addition, XENON1T was also sensitive to different types of new particles and interactions that could explain other open questions in physics. Last year, using the same detector, the XENON collaboration published in Nature the observation of the rarest nuclear decay ever directly measured.
The XENON1T detector, optimised for the search for rarest events, was filled with 3.2 tonnes of ultra-pure, at –95°C liquefied xenon, the innermost 2.0 tonnes of which served as a target for dark matter. When a particle crosses the target, it may collide with a xenon atom generating tiny signals of light and free electrons from the hit xenon atom. Most of these interactions occur from particles that are known to exist. Thus, a number of measures was applied to reduce these disturbing background events to an unprecedentedly low level. And the scientists carefully estimated the residual number of background events. Comparing the data of XENON1T to backgrounds, they observed a surprising excess of 53 events over the expected 232 events.
Now, where is this excess coming from?
One explanation could be a new, previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the liquid xenon. Tritium, a radioactive isotope of hydrogen with two extra neutrons, spontaneously decays by emitting an antineutrino and an electron with an energy distribution similar to what was observed. Only a few tritium atoms for every 1025 xenon atoms (corresponding to about 2.2 kg of xenon) would be sufficient to explain the excess. Currently, there are no independent measurements that could confirm or disprove the presence of tritium at that level in the detector, so a definitive answer to this explanation is not yet possible.
More excitingly, another explanation could be the existence of a new particle. In fact, the excess observed has an energy spectrum similar to that expected from axions produced in the Sun. Axions are hypothetical particles that were proposed to understand a symmetry of nuclear forces observed in nature. The Sun may be a strong source of axions. While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but not yet observed class of new particles, with a large impact on our understanding of fundamental physics, but also on astrophysical phenomena. Moreover, axions produced in the early universe could also be the source of dark matter.
Alternatively, the excess could also be due to surprising properties of neutrinos, trillions of which pass through the detector, unhindered, every second. One explanation could be that the magnetic moment of neutrinos is larger than its value in the Standard Model of elementary particles. This would be a strong hint to some other “new physics”.
Of the three explanations considered by the XENON collaboration, the observed excess is most consistent with a solar axion signal. In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2/10,000 chance that the observed excess is due to a random fluctuation (which is thus not fully excluded) rather than a signal. While this significance is fairly high, it is not large enough to conclude that axions exist. The significance of both the tritium and neutrino magnetic moment hypotheses corresponds to 3.2 sigma, meaning that they are also consistent with the data.
XENON1T is now upgrading to its next phase, XENONnT, with an active xenon mass three times larger and a background that is expected to be lower. With better data from XENONnT, the XENON collaboration is confident it will soon find out whether this excess is a mere statistical fluke, a background contaminant, or something far more exciting: a new particle or interaction that goes beyond known physics.
The XENON collaboration comprises 163 scientists from 28 institutions across 11 countries. Five German institutions are significantly involved: The Max Planck Institute for Nuclear Physics in Heidelberg was responsible for the light sensors, the detection of trace amounts of radioactivity in the detector material and the liquid xenon, the University of Münster developed the cryogenic distillation system for removal of radioactive impurities from liquid xenon and a general xenon purification system, the University of Mainz was responsible for the muon system and contributed substantially to the xenon recovery and storage system, and the University of Freiburg was responsible for the detector design and development of the data acquisition electronics. All institutes as well as the Karlsruhe Institute of Technology that recently joined the collaboration are involved in data analysis.
Observation of Excess Electronic Recoil Events in XENON1T, XENON Collaboration, arXiv:2006.09721 [hep-ex]