José Manuel NievesSEGUIR Updated: Save Send news by mail electrónicoTu name *

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A team of researchers from the international collaboration XENON , one of the largest experiments underway for the detection of dark matter , has just announced that their latest data shows a surprising excess of events, which is interpreted as the first direct detection of an axion, a particle up to now hypothetical, and is not contemplated in the current models. The experiment consists of a tank of 3,500 litres of xenon ultra-pure liquid , surrounded by a multitude of sensors, specially calibrated to detect possible collisions of particles exotic with its atoms in the interior of the tank.

The researchers, who have shared their finding in the server prepublicaciones arXiv, they do not assert categorically the finding, but believe that the unexpected rate of collision events, whose source is still unknown, it could be the signal that reveals the existence of a new particle, the axion solar , although there are also other possibilities.

The detector XENON1T , installed nearly two miles of depth beneath the Apennines of italy, in the laboratory of Gran Sasso, was primarily designed to detect dark matter , the “other” type of matter in the Universe, five times more abundant than the ordinary matter, (of that are made all the stars and galaxies that we can see), but is undetectable because it does not emit any type of radiation. The depth at which the experiment prevents most of the particles of “normal”, from reaching the detector.

Until now, however, scientists have only been able to observe indirect evidence of dark matter. But the detector is also sensitive to other types of new particles and interactions theoretical, able to solve another series of important open questions in Physics. Last year, for example, the same researchers reported in Nature the observation of the nuclear decay more strange observed to date .

When a particle reaches its goal (the xenon liquid in the tank) and interacts with it, you can generate small flashes of light and release electrons in an atom of xenon. Most of these interactions happen with particles already known, but by comparing the data of XENON1T with the known history, the researchers found that, this time, with a surprising excess of 53 events over the 232 that were expected. Whence comes, then, that excess collisions?

Three possible explanations

A possible explanation would be that there is a new source not considered previously, caused by the presence of small amounts of tritium in the own detector. Tritium, a radioactive isotope of hydrogen, decays spontaneously by emitting an electron whose energy is very similar to that observed. And you only need about a few atoms of tritium for every 10 to 25 atoms of xenon (that is to say, for every atoms of xenon ), to explain the excess of events. Currently, however, there are no independent measurements that can confirm or refute the presence of the necessary amount of tritium in the detector as to cause those results.

In the second place, the excess of events could also be due to neutrinos, particles with almost no mass that pass through matter as if it did not exist. Trillions of them, in fact, go through every second every square inch of the Earth without that we don’t even realize it. It could be, therefore, that the magnetic moment (a property of all particles) of the neutrino to be greater than the value that was assigned in the Standard Model of physics, the grand theory that unites all the particles and forces that govern them. If so, this would be a clue of the existence of a “ new Physics ” beyond the established model.

But the third explanation, favored by the researchers, it is much more exciting, and imply the existence of a particle that is totally new. In fact, the excess observed has a spectrum of energy that is very similar to that expected of the axions produced by the Sun. The axions are hypothetical particles whose existence was proposed in order to preserve a symmetry of time reversal of the nuclear force, and the Sun can be a major source of them. While these axions solar are not candidates for the dark matter, its discovery would mark the first observation of a class of new particles, predicted but never observed, with a great impact to our understanding of the fundamental physics, and also to the astrophysical phenomena. In addition, some theories argue that the axions produced in the early universe could be, after all, the much sought-after source of dark matter.

A new particle, the best explanation

Among the three possible options, the researchers of the collaboration XENON believe that the most consistent is that the excess of events observed due to the action of axions solar. In statistical terms, this hypothesis has a significance of the 3.5 sigma, which means that there is a probability of 2 in 10,000 that the excess observed is due to a fluctuation random instead of a true signal. While this is a meaning quite high, it is not large enough to conclude that, indeed, we have detected axions. To officially announce his discovery, in effect, it would require a significance of 5 sigma. For its part, the hypotheses of the tritium and of the magnetic moment of the neutrinos have a statistical significance of 3.2 sigma. That is to say, they are also consistent with the data.

How to solve the issue? Currently, the experiment XENON1T, is being updated to its next phase, XENONnT , which means that it will triple the amount of xenon liquid in the interior of the tank, and hence the sensitivity of the detector. With some better data, then, researchers may determine by order whether the excess of events detected is a simple coincidence, statistics, or if, on the contrary, it really is the discovery of a new particle, beyond the known physical.