The Gran Sasso, the hunter of the dark matter, detects unexpected events

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There is no traffic and under two kilometres of rock into the bowels of the Gran Sasso. More than expected. A number of events that are abnormal within the tool which searches for dark matter. Much to trigger a red light in the head of the scientists of the national laboratories of Gran Sasso of the national Institute of nuclear physics. Needless to say too much yet, you might just be a coincidence, a statistical fluctuation, or any unwanted residue. But among the hypotheses there is also a new particle ‘relative’ of the same dark matter, or, equally “exciting”, more of a crack in the building the theoretical the most solid ever built so far for sub-atomic physics: the the standard model.

The experiment Xenon1T

Let’s start from the beginning. The experiment Xenon1T is a big barrel filled with xenon liquid, inside there is a tool that tries to observe directly the interaction of particles of dark matter with ordinary matter: “it Is the largest detector of the xenon liquid that exists in the world, a noble gas that does not bind and remains pure – explain Marco Selvi responsible for national Infn of the experiment – was to search for dark matter. The xenon liquid is more dense, more mass gives greater chance that the particles interact”.


When a particle (like a neutrino that rains from the space or not) interacts with atoms of Xenon, produces two signals: one light (photons) and frees some of the electrons, which are ‘accompanied’ to the top from the magnetic field of the instrument and transformed to another signal light, more strong: “we Recognize as well the events we are interested in signals (spurious – adds Selvi – if it were to be dark matter, would have a score that is accurate, that we know, and we could distinguish it from interactions with particles that are already known”. So far, no dark matter particle has ‘shone’. However, is success of another.

The signal and the noise

XenoniT is also the most precise ever made, it means that the “background noise” from which you extract the signals, is very low, and any interesting phenomena stand out: “it Is a bit like looking at the stars during the day, when there is a lot of light, or from the center of a city. And then look at them from the summit of the Gran Sasso, from where you can see even the weakest ones, because the fund of brightness residual is lower,” continues the researcher. On the basis of its sensitivity, the scientists waited for 232 events, i.e. interactions, of varied type. They observed 52 in more. “A number that does not yet mean a new discovery, but that makes us raise an eyebrow”.

The hypothesis

Among the explanations more mundane there is a statistical fluctuation, that is, the case, which has produced an anomaly that is unlikely to be observed again. Or the presence of residual tritium in the instrument, which are not considered in the estimation of the initial fund. The tritium (radioactive isotope of hydrogen) decaying produces an electron with energy similar to that observed. One could, with some atoms about a million-billion-billion atoms of Xenon, to produce these effects.

But the hypothesis that best explains the data, presented today by researchers from the national Laboratories of Gran Sasso is a particle theorized, and never observed: the Axion solar: “Are the first candidates that best meet the analysis – stresses Selvi – predicted from the ’70s to explain some open problems in nuclear physics, not ever been observed, but there are models for their production and interaction with the other particles. They have their own energy in the region that we observe”.

Elusive as neutrinos, we would need instruments that are very precise, such as Xenon1T, precisely, to make them ‘shine’. Like neutrinos, they can come from space, through undisturbed kilometers of rock without interacting with anything: “The Xenon is so pure, that we may be able to see these events. It would be a very important discovery, to explain the various phenomena. In addition, another family of axions could be the one candidate to be the dark matter. Then if they were axions solar, perhaps, then, there are also their cousins, which could explain the material that is missing in the universe”.

A possible crack in the standard model

But there is another hypothesis, perhaps equally striking, the screening, and relates to a property of the neutrino: “The neutrino has an intrinsic property, the magnetic moment, explains Selvi – if it was related to what we observe, it would mean that this property, linked to the ‘spin’, is much larger than that predicted by the standard Model. It might be the signal of a new physics”. The standard Model is the theory that best explains three of the four fundamental interactions of matter (strong interaction, weak and electromagnetic, are left outside only the gravitational one) and the behavior of elementary particles. So far it has proven rock-solid. A ‘crack’ in this building that is almost perfect could open grasslands unexplored in the field of particle physics.

“In statistical terms, the hypothesis of axions solar has a significance of 3.5 sigma, equal to about a probability of 2 in 10,000 that the excess is due to a random fluctuation of the fund, instead of a new signal – explains the Infn in the press release that presents the data – although this significance is rather high, however, it is not yet sufficient to conclude the definitive observation of axions solar. The significance of the hypothesis of tritium and magnetic moment of the neutrino corresponds to 3.2 sigma, and then they, too, are well compatible with the experimental data”.

The new tool even more precise

Xenon1T has already finished his work. His successor, XenoNnT, will soon be talked about. The are installing in these months, and will take up service at the end of 2020. It will be more accurate and, while the researchers hope to see him turn it on for the dark matter, will also be used to find out whether these data are truly something new: “With XenoNnT the drug standard will be still less, if this excess return will be investigated far better, and understand if the responsible are the tritium, the axions or neutrinos. Or if you do not recur and we see only the interactions standard – ends Selvi – we will put an end to the excitement of this moment”, hoping to see him turn it on for the interaction with the dark matter.


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Mario Calabresi
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