An experiment designed to hunt for ever elusive dark matter has returned some strange and exciting signals. The anomalies are probably not dark matter itself, but could be an indication that we’re on the right track to finding it. Out of the three possible explanations, one is unwanted interference, while the other two would herald new physics.
Running from 2016 to 2018, the XENON1T experiment was designed to search for particles of dark matter, the enigmatic substance that’s believed to outnumber regular matter five-to-one. Since dark matter should be everywhere, XENON1T watches for the rare occasions that its particles interact with regular matter.
To do so, it stares at a huge tank filled with several tons of liquid xenon. When some outside particle streaks through the tank, it excites the xenon atoms and creates a flash of light and free electrons, which XENON1T can detect.
It’s not just dark matter that can do this though – similar signals can be triggered by known particles. To filter these out, the science team calculates how many background events would be expected, and then checks if there were more signals than that.
And sure enough, the team is now reporting a “surprising excess of events.” The expected background is 232 events in that period, but an extra 53 were detected on top of that. That’s a huge amount, and shows that something strange is definitely going on. But what, exactly?
The researchers say that there are three possible explanations. Let’s get the boring one out of the way first: it could just be an unrecognized source of background interference. The signal is consistent with tritium impurities in the tank, and it would only take a few tritium atoms in 10 septillion xenon atoms to create the excess seen. Frustratingly, no instrument is sensitive enough to detect such tiny levels of tritium in the tank, so this can’t be ruled out.
Thankfully, the other two ideas are far more exciting. The team says that the best fit for the data is a hypothetical elementary particle called an axion, specifically one produced by the Sun. These particles were first proposed in the 1970s to resolve what’s known as the strong CP problem. Later on it was determined that if they had a certain mass, axions produced billions of years ago could account for the weirdness we ascribe to dark matter.
Although these specific solar axions wouldn’t be a dark matter candidate, if confirmed this would still mark the very first detection of any kind of axion. That in itself would be a huge discovery, and could suggest that other types of axions are more likely to be dark matter than other hypothetical particles.
The third explanation is that these signals are from previously unknown properties of neutrinos. These ultra-light elementary particles are everywhere and rarely interact with other matter, but occasionally they do. If they are interacting with the xenon in this experiment, the signal suggests they have a larger magnetic moment than the Standard Model of particle physics describes. That in itself would require new physics to explain.
In terms of likelihood, the team says that the solar axion is the frontrunner. This hypothesis has a statistical significance of 3.5 sigma, meaning there’s a 2-in-10,000 chance that the observation is random. As high as that is, scientists usually require a 5-sigma significance to be “sure.”
The other two hypotheses were deemed slightly less likely, with both having significances of 3.2 sigma.
Exactly what was observed in that big underground tank remains a mystery for now, but an upgraded version of the experiment could hold the answers. The next phase, known as XENONnT, will have an active xenon mass that’s three times larger than the previous version, and should have less background noise.
The research is still pre-print, but is currently available on ArXiv (PDF).
Source: Kavli IPMU
