Scientists from the international XENON collaboration announced on June 17 that data from their XENON1T detector, the world’s most sensitive dark matter experiment, shows a surprising excess of events. These events may suggest the existence of a new particle known as the solar axion or indicate previously unknown properties of neutrinos.
While the scientists do not claim to have found dark matter, the source of the observed unexpected rate of events is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium—a hydrogen atom with one proton and two neutrons—but could also be a sign of something more exciting: the solar axion.
XENON1T was operated deep underground at the Istituto Nazionale di Fisica Nucleare (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 the observation of the rarest nuclear decay ever directly measured in nature.
The XENON1T detector was filled with 3.2 tons of ultrapure liquefied xenon, 2 tons 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 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 an exciting question: from where is this excess coming?
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 the levels that were observed. Only a few tritium atoms for every 1025 (10,000,000,000,000,000,000,000,000!) 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 explanation is not yet possible.
More exciting, 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, 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 neutrinos, trillions of which pass through the Earth, unhindered, every second. One explanation could be that the magnetic moment—a property of all particles—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.
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.
The XENONnT experiment utilizes a total of 8.3 tons of xenon to search for the elusive dark matter particles. In addition to the existing 3.3 tons of ultrapure xenon from XENON1T, another 5 tons of xenon were purchased by the XENON collaboration.
Before the new gas can be used for XENONnT, it needs to be purified. Besides oxygen, nitrogen and water that potentially absorb the light and charge signals in the detector, the radioactive noble gas Kr-85 within the xenon needs to be removed. Kr-85 is a man-made isotope created in nuclear bomb testing and nuclear fuel reprocessing. It makes up a fraction of 10-11 of the natural krypton (Kr-nat) abundance.
The purest xenon on Earth can be produced with the help of the collaboration’s krypton distillation column located underground in the service building of XENON1T/nT as seen in the picture above. The purification method is based on the separation due to the different boiling points of xenon and krypton. While xenon is in its liquid form at -100 °C, krypton, as the lighter atom, prefers to stay in its gaseous form. Like that, krypton is enriched at the top of the distillation tower from where it is removed and stored in a bottle as so-called “offgas.” The purified xenon can exit the distillation system at the bottom.
In total, over 100 bottles of freshly delivered xenon were installed in two bottle batches at the “bottle rack.” Here, xenon samples from each batch were measured with a connected residual gas analyzer (RGA) system. Xenon from one of the bottle batches was continuously filled to the distillation system. Purified xenon was stored either in the Recovery and Storage for XENON1T (ReStoX-I) or to the ReStoX-II system, a newly installed subsystem for XENONnT. ReStoX-II is a system designed to rapidly recover and safely store up to 10 tons of xenon, that will serve as a fast recovery system during operation of the XENONnT experiment as well as xenon storage previous to the start of the experiment.