About four years ago, I wrote a short post about Columbia University professor Elena Aprile and her project to search for XENON dark matter particles. She then believed that “over the next couple of years, maybe five or six in total, we will either say for sure that WIMPs do not exist, or we will discover something.” And so, perhaps, they really discovered something!
XENON is the most sensitive detector at the moment, aimed at finding dark matter particles. Its current modification XENON1T is 3.2 tons of liquid ultrapure xenon (just to understand the technical complexity of the experiment: xenon can only be in liquid form in a very narrow temperature range from -108 to -111 ° C). The xenon tank is located deep underground in the mountains of Italy and surrounded by sensitive sensors.
Initially, the project was actually looking for WIMPs – these are such popular hypothetical candidates for dark matter particles with a large mass, which, it is assumed, can weakly interact with xenon nuclei. But for more than 10 years (before loading several tons of xenon, they started with small detectors) they have not found anything like that.
And then they realized that using the same setup, one could look for a slightly different type of hypothetical particles called axions. These particles are much lighter, and they are looked for by their effect on electrons. The problem here is that a lot of ordinary things interact with electrons: for example, electrons from the beta decay of radioactive atoms in a rock. But these ordinary events can be more or less accurately assessed and see if there is anything else in the signal.
It turned out that it seems as it is: instead of the expected 232 ± 15 events, we saw 285 – this is an excess of 3.5σ (this roughly means that the chance that the excess is a coincidence of circumstances is something about 0.02%). In the physics of elementary particles, this is not yet a discovery, but it is very close to it (for a discovery, 5σ is required, which corresponds to an error probability of ~ 10⁻⁵%).
Other explanations, of course, are also possible. For example, ordinary neutrinos can give the same signal if they suddenly have a large magnetic moment – no one knows its value, and if it really turns out to be so large, then this in itself will be extremely interesting and unexpected.
There is, however, a trivial explanation – the signal is caused by tritium contamination. This is the only banal reason that scientists could not completely exclude due to the difficulty of detecting tritium. In fact, this isotope of hydrogen, due to its high radioactivity, is extremely rare: in xenon, according to estimates, it should not have more than 1 atom per 10²⁵ of xenon atoms, but this may be enough to explain a good half of the observed signal. Apparently, the only way to exclude this explanation is to find a similar signal by other methods.
Preprint of the article: E. Aprile et al. (XENON Collaboration) Observation of Excess Electronic Recoil Events in XENON1T