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Observation of Excess Events in the XENON1T Dark Matter Experiment

Scientists from the international XENON collaboration announced on June 17th that data from their XENON1T, the world’s most sensitive dark matter experiment, show a surprising excess of events. The scientists do not claim to have found dark matter. Instead, they say to have observed an unexpected rate of events, the source of which is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium, but could also be a sign of something more exciting – such as the existence of a new particle known as the solar axion or the indication of previously unknown properties of neutrinos.
“Our result could be the onset of new physics” says Prof. Manfred Lindner (MPIK), Co-Spokesperson of the XENON Collaboration and APPEC-SAC member. “We emphasize very carefully conventional explanations of the observed excess, but about 90 theoretical publications within one month show that it could point to exciting new physics”.

The central part (the TPC) of the XENON1T detector. Credits: XENON Collaboration

XENON1T was operated deep underground at the INFN Laboratori Nazionali del Gran Sasso in Italy, from 2016 to 2018. It was primarily designed to detect dark matter, which makes up 85% of the matter in the universe. So far, scientists have only observed indirect evidence of dark matter, and a definitive, direct detection is yet to be made. So-called WIMPs (Weakly Interacting Massive Particles) are among the theoretically preferred candidates, and XENON1T has thus far set the best limit on their interaction probability over a wide range of WIMP masses. In addition to WIMP dark matter, 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, these scientists published in Nature the observation of the rarest nuclear decay ever directly measured.
“While our detector was mainly designed to detect dark matter particles, its low energy threshold coupled to an extremely low background allows us to search for other rare interactions and particles beyond the standard model of particle physics,” says Prof. Laura Baudis (UZH), member of the APPEC SAC, and one of the leading members of the project.

The XENON1T detector was filled with 3.2 tonnes of ultra-pure liquefied xenon, 2.0 t of which served as a target for particle interactions. When a particle crosses the target, it can generate tiny signals of light and free electrons from a xenon atom. Most of these interactions occur from particles that are known to exist. Scientists therefore carefully estimated the number of background events in XENON1T. When data of XENON1T were compared to known backgrounds, a surprising excess of 53 events over the expected 232 events was observed. This raises the exciting question: 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 XENON1T detector. Tritium, a radioactive isotope of hydrogen, spontaneously decays by emitting an electron with an energy similar to what was observed. Only a few tritium atoms for every 1025 xenon atoms would be needed to explain the excess. Currently, there are no independent measurements that can confirm or disprove the presence of tritium at that level in the detector, so a definitive answer to this explanation is not yet possible.

View into the water tank, lined with reflecting foil, and the XENON1T detector. Sensitive sensors identify light signals induced in the water by cosmic radiation. Credits: XENON Collaboration

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 preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them. While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but never observed class of new particles.
“This would have a large impact on our understanding of fundamental physics, and of astrophysical phenomena. Moreover, axions produced in the early universe could also be the source of dark matter,” says Baudis.

Alternatively, the excess could also be due to neutrinos, trillions of which pass through your body, 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 needed to explain it.

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 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.
“We are very excited about this new result from our tonne-scale liquid xenon detector with an incredibly low background that hasn’t been reached by any other experiment in the field” says Baudis.
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 than that of XENON1T. 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.
“XENON1T was primarily built to search for WIMPs”, says Lindner. “This are top candidates for the Dark Matter in the Universe, but the record exposure and the very low threshold allows us to look for other exciting new physics. Maybe we stepped on some other exciting new physics which we can further explore with XENONnT which was already assembled and should become operational in the next months”.

The XENON collaboration comprises 163 scientists from 28 institutions across 11 countries. The European participation in XENON is very strong, with the following groups being involved: INFN Gran Sasso, Bologna, Napoli and Torino, and L’Aquila University in Italy, MPIK Heidelberg, the Universities of Freiburg, Mainz, Münster and KIT Karlsruhe in Germany, the University of Zurich in Switzerland, Subatech, LAL, LPNHE in France, Nikhef in the Netherlands, Stockholm University in Sweden and the University of Coimbra in Portugal.
Together with the other international collaborators, these groups are responsible for many of the crucial systems in XENON: the TPC and the photosensors and their readout, the data acquisition and light calibration systems, the xenon storage and distillation systems for radon and krypton, the material screening and radon emanation measurements, along with other background mitigation techniques, the neutron and muon veto systems.

XENON Collaboration

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