Scientists ponder inconclusive data from dark matter experiment

The XENON. dark matter detectors are checked out during assembly. Image: XENON Collaboration

On 17 June, scientists reviewing data from the XENON dark matter experiment in Italy reported finding “a surprising excess of events” that could be signs of solar axions, previously undetected sub-atomic particles generated by nuclear reactions in the Sun’s interior. Or, as reported by Science magazine, the signals could be “evidence that common particles called neutrinos are more magnetic than expected. Or the result of contamination within the detector. Or just a statistical fluke.”

“The scientists do not claim to have found dark matter,” the XENON collaboration said in a release. “Instead, they say (they) 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.”

Uncertainty over initial results is not unusual in the ongoing search for dark matter, the as-yet-unseen material thought to pervade the universe, providing the gravitational glue that holds galaxies together. Dark matter is believed to make up about 27 percent of the mass-energy content of the universe while “normal” matter, the material astronomers can actually see, makes up just 5 percent. Dark energy, the mysterious force accelerating the expansion of the cosmos, makes up the rest.

The international XENON collaboration is not claiming a discovery of any sort, much less detection of dark matter. But the announcement triggered widespread interest given the prospect of a new particle or process that might force theorists to sharpen their pencils.

The XENON1T dark matter experiment, the most sensitive such study to date, is located in the underground Gran Sasso National Laboratory in Italy. It is designed to look for flashes of light in a reservoir of liquid xenon that should occur when weakly interacting massive particles, or WIMPs – dark matter candidates – occasionally crash into normal nuclei.

Incoming particles, occasionally striking a xenon nuclei or its surrounding electrons triggers a detectable energy release that can be used to calculate the incoming particle’s properties. Image: XENON Collaboration

While no such flashes have yet been seen, the detector also can detect less massive particles crashing into electrons in the xenon. During the most recent observing run with upgraded equipment, 285 events were observed when only 232 were expected.

Solar axions are one possible explanation, but ghostly neutrinos generated by fusion reactions in the Sun could be responsible for the signals if they are more magnetic than predicted. It’s also possible the events were triggered by tritium, a radioactive isotope of hydrogen, contaminating the detector. Just three tritium atoms per kilogram of xenon would be enough to explain the observations.

“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,” the collaboration said in its release.

“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.”