Discovery of new particles?

Faculty of Science and Technology of the University of Coimbra

An international team, with the participation of five researchers from the Faculty of Science and Technology of the University of Coimbra (FCTUC), today announced to the scientific community an amazing number of events registered by XENON1T, the most sensitive system ever in the detection of matter dark.

The nature of these events, however, is not yet fully understood, therefore the discovery of dark matter is not declared. Its signature is similar to that produced by residual amounts of tritium (a hydrogen atom with two neutrons and a proton in the nucleus), but it can also be a sign of something much more important: the existence of a new type of particle called the solar axion or previously unknown properties of neutrinos.

The XENON1T was in operation between 2016 and 2018 in Italy, in the underground laboratory of Gran Sasso, under 1300 meters of rock. Designed for the extremely rare detection of dark matter, this system of very high sensitivity has already proved to be able to record other events of very difficult detection. For example, last year the direct measure achieved with this system, for the first time in history, of the rarest nuclear decay in the universe was published in the prestigious journal Nature.

The XENON1T system targets two tons of ultra-purified xenon. “In general, radiation passing through the target can generate very small signals of light and charge. The overwhelming majority of these signals (more than 99.9%) are due to radiation of known origin, which allows scientists to calculate with great precision the number of events expected. And here, 22.8% more events were observed in relation to the forecast”, says José Matias-Lopes, a researcher at the Instrumentation, Biomedical Engineering and Radiation Physics Laboratory (LIBPhys) at FCTUC and coordinator of the Portuguese team.

One possible explanation “will have to do with the presence of tritium, a radioactive isotope of hydrogen. Some tritium atoms in 10 billion trillion xenon atoms would be enough to justify the excess of recorded events, but there is still no way to measure these very small concentrations and thus confirm this hypothesis”, he explains.

Another possibility, “much more interesting, is the existence of a new type of particle. In fact, the excess of observed events has energies similar to those expected for axions produced in the sun“, says José Matias-Lopes.

Axons are theoretically predicted particles, the sun being able to be an intense source of them. Although axons are not dark matter, their sighting would be the first of a new class of particles whose existence is solidly supported by theoretical studies. This discovery would have a strong impact on the advancement of knowledge, not only from Astrophysics but also from Physics itself. Additionally, axons produced at the beginning of the universe can also explain the origin of dark matter.

The third and final advanced explanation for the observed excess comes from neutrinos, which pass billions through our bodies every second, without a trace. To confirm this hypothesis, the magnetic moment (a characteristic of all particles) of neutrinos would have to be higher than the value predicted by the theory, which would require the need to create new paradigms and physical models capable of explaining it.

Of the three explanations considered by scientists from the XENON collaboration, “the most favoured in statistical terms is that of solar axons, with a probability of around 99.98% that the signals are from this source. But even with this high degree of probability, it cannot be declared a discovery ”, reveals José Matias-Lopes.

The other two possibilities, tritium or neutrinos with the greater magnetic moment, are also highly likely, about 99.93% in both cases, to be the cause of the observed excess.

The XENON1T will be replaced by a new, even more, sensitive detection system, the XENONnT, which is expected to come on stream this summer. Scientists predict that two or three months later they will have confirmation of the origin of this signal; whether it is due to a contaminant or something truly revolutionary: a new particle or type of interaction that goes beyond what is already known.

Times of great advances and discoveries are therefore approaching, which lead to large steps forward in the knowledge of Humanity“, affirms the consortium.

The XENON consortium is made up of 163 scientists from 28 research groups from the USA, Germany, Portugal, Switzerland, France, the Netherlands, Sweden, Japan, Israel and Abu Dhabi. Portugal has been a partner in this collaboration sits inception in 2005, through the LIBPhys team at the University of Coimbra.

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