Observation of Excess Events in the XENON1T Dark Matter Experiment


Are they a sign of an infinitesimal impurity or a need to rewrite the physics books?

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Scientists from the international XENON collaboration announced today 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; rather, they have observed an unexpected number 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 a hydrogen isotope, tritium, but it could also be a sign of something more exciting, for example, the existence of a new particle called a solar axion, or of previously unknown properties of neutrinos.

Dr. Ran Budnik of the Weizmann Institute of Science’s Particle Physics and Astrophysics Department is a member of the team operating the XENON1T deep underground in the INFN Laboratori Nazionali del Gran Sasso in Italy. The XENON collaboration comprises 163 scientists from 28 institutions across 11 countries. That experiment, which ran from 2016 to 2018, was primarily designed to detect dark matter, thought to make up 85% of the matter in the universe. So far, XENON1T has set the best limit on the interaction probability over a wide range of theoretical masses for one possible type dark matter. XENON1T was also sensitive to different types of new particles and interactions. Last year, using the same detector, these scientists reported in Nature their observation of the rarest nuclear decay ever directly measured.

The XENON1T detector was filled with 3.2 tons of ultra-pure liquefied xenon, 2.0 t of which served as a target for particle interactions. A handful of particles that crossed this target hit a xenon atom and generated tiny signals of light and free electrons from the atom. Most of these interactions resulted from particles that are known to exist, so the scientists in first carefully estimated the number of “background events” projected to occur over the two-year period. When data of XENON1T were compared to known backgrounds, an excess of 53 events over the expected 232 events was observed.

One explanation for the excess could be a previously unconsidered source of background events caused by the presence of tiny amounts of tritium (a hydrogen atom with one proton and two neutrons) in the XENON1T detector. 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.

Their detection would mark the first observation of a new class of new particle

But another explanation could be the existence of a new particle. The excess observed has an energy spectrum similar to that expected from axions produced in the Sun. Axions are hypothetical particles that have been proposed to preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them. While solar axions are not dark matter candidates, their detection would mark the first observation of a new class of new particle; this would have a large impact on our understanding of fundamental physics, as well as of astrophysical phenomena. And axions produced in the early universe could, according to one theory, also be the source of dark matter.

Alternatively, the excess could be due to neutrinos, which rarely interact with matter. The magnetic moment (a property of all particles) of neutrinos could be larger than the value assigned them in the Standard Model of elementary particles. This would be a strong hint that some new physics might be needed to explain the discrepancy.

Of the three explanations considered by the XENON collaboration, the observed excess is most consistent with a solar axion signal, though the other two cannot be ruled out, at this stage.

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.

Dr. Ran Budnik's research is supported by the Weizmann Institute "la Caixa" Foundation Postdoctoral Fellowships. Dr. Budnik is the incumbent of the Aryeh and Ido Dissentshik Career Development Chair.