Dark Matter Scientists Surprised by Observation of ‘Excess of Events’

June 17, 2020 | By Cynthia Dillon

Scientists from the international XENON collaboration shared news today (interactions.org) that data from their XENON1T—the world's most sensitive dark matter experiment—show a surprising excess of events. This doesn’t mean that they found dark matter, only that they observed an unexpected rate of events. Admittedly, they don’t fully understand what they observed, but they have some ideas.

“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 it could also be a sign of something more exciting, such as the existence of a new particle known as the axion or the indication of previously unexpected properties of neutrinos,” explained Kaixuan Ni, associate professor of physics at UC San Diego. 

To make the observations, this contingent of the international XENON research team—163 scientists from 28 institutions around the world, including UC San Diego, Columbia University, Purdue University, Rice University, Rensselaer Polytechnic Institute and the University of Chicago—filled the XENON1T detector with more than 7,000 pounds of ultra-pure liquefied xenon—a chemical element belonging to the class of noble gases. More than 4,000 pounds of it 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.

XENON1T detector. Credit: XENON Collaboration

“Therefore, we carefully estimated the number of background events in XENON1T. When data of XENON1T were compared to known backgrounds, there was a surprising surplus of 53 events, over the expected 232 events,” said Jingqiang Ye, a physics graduate student, who is a member of the Ni Group and a recipient of the Abeloe Graduate Fellowship in the Division of Physical Sciences at UC San Diego.

According to the researchers, one explanation of the origins of the excess could be a previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the XENON1T detector. A radioactive isotope of hydrogen, tritium spontaneously decays by emitting an electron with an energy similar to what was observed.

“Only a few tritium atoms for every 10 septillion (10,000,000,000,000,000,000,000,000!) xenon atoms would be needed to explain the excess,” said Ye. “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.”

Ni noted that other explanations were possible, including 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 are naturally raised in a theory proposed to preserve a time-reversal symmetry of the nuclear force, and the sun may be a strong source of them,” said Ni.

While these 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.

Another option is that the observed excess could be attributed to neutrinos, trillions of which pass through our bodies, unhindered, every second.

“One explanation could be that the ‘magnetic moment’ (a property of elementary particles) of neutrinos is larger than its value in the standard model of particle physics,” said Ni. “This would be a strong hint that some other new physics is needed to explain it. This is the beauty of our detector, despite having been designed for detecting dark matter particles its extremely low background makes it ideal for a variety of other new physics searches.”

Of the three explanations considered by the XENON collaborators, the observed excess is most consistent with an axion signal. In statistical terms, the axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2-in-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,” said Ni. “The significance of both the tritium and neutrino magnetic moment hypotheses, however, corresponds to 3.2 sigma, meaning that these two are still consistent with 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 improved sensitivity from XENONnT, the XENON collaborators say they are confident that they 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 excess might be due to small traces of tritium, but the idea that we might be sitting on something more exotic is really exciting for us,” said Luca Grandi, associate professor at the University of Chicago. “Given our estimates, we expect that XENONnT will be able to distinguish among the various hypotheses in a few months of data-taking, and this makes even more worth the big effort made early in the year to seal the new detector before the lockdown kicked in.”

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 percent 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 Weakly Interacting Massive Particles (WIMPs)) 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, XENON1T scientists published in Nature the observation of the rarest nuclear decay ever directly measured. 

“This new and exciting result from XENON1T really pays off the effort which went into realizing the first liquid xenon time-projection chamber with a massive target at the tonne scale and an unprecedented low background,” said Elena Aprile, professor of physics at Columbia University, founder and spokesperson of the project and former Margaret Burbidge Visiting Professor at UC San Diego.

XENON research is supported by the National Science Foundation, the Swiss National Science Foundation, German Ministry for Education and Research, Max Planck Gesellschaft, Deutsche Forschungsgemeinschaft, Netherlands Organisation for Scientific Research (NWO), Netherlands eScience Center (NLeSC), SURF Cooperative, Weizmann Institute of Science, Israeli Centers Of Research Excellence (I-CORE), Pazy-Vatat, Fundacao para a Ciencia e a Tecnologia, Region des Pays de la Loire, Knut and Alice Wallenberg Foundation, Kavli Foundation, JSPS Kakenhi in Japan, the Abeloe Graduate Fellowship and Istituto Nazionale di Fisica Nucleare. It was supported as well by the European Unions Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement.