To live happily, let’s live hidden. This adage also applies to dark matter detection experiments. These detectors are often installed in underground laboratories several hundred meters or kilometers below the surface. The thickness of the rock blocks most of the cosmic radiation that would disrupt the measurements. But there is a flow of particles that even the best shielding cannot stop: neutrinos. For a long time, experiments did not have sufficient sensitivity to see this “neutrino fog”. But two of the most sensitive detectors to date, PandaX-4Tin China, and XENONnTin Italy, have just announced that they have, for the first time, penetrated this fog.
Dark matter remains one of the major enigmas of cosmology. Its existence has been postulated to explain several observations, such as the abnormally high rotation speed of spiral galaxies and the formation of large structures in the Universe (galaxies and galaxy clusters). Its nature remains obscure. Among dozens of candidates, the “wimps” hypothesis (weakly interacting massive particle) is one of the most studied. She predicts that, from an experimental point of view, these particles should have the advantage of interacting, weakly, with matter, which makes it possible to imagine experiments to test their existence. The possibility of detecting wimps depends on two unknown parameters, the mass of the wimp and the cross section, that is to say the probability of interaction with an atomic nucleus.
In twenty-five years, the sensitivity of experiments has been improved by several orders of magnitude. But, if no trace of dark matter has been revealed, the instruments are now capable of observing a rare phenomenon: coherent elastic diffusion between neutrinos and nuclei (CEvNS, for coherent elastic neutrino-nucleus scattering). The term “coherent” here means that the neutrino interacts with the nucleus as a whole and not with one of the protons or neutrons that compose it. This process was first detected in 2017 in an experiment at a particle accelerator at the American Oak Ridge laboratory.
Neutrinos interact very weakly with matter, but the Sun produces a colossal flow of them: on Earth, we are crossed by approximately 64 billion neutrinos per second and per square centimeter. To give an idea, among all the neutrinos passing through your body, only one per week interacts with one of your atoms. To measure such a tiny effect, experiments like PandaX-4T et XENONnT are equipped with tanks filled with several tons of liquid xenon and surrounded by a myriad of ultrasensitive detectors.
The team of XENONnT analyzed 300 days of data (collected between 2021 and 2023). “Thanks to an artificial intelligence algorithm, we identified 37 promising events,” explains Luca Scotto Lavina, from the LPNHE (Laboratory of Nuclear Physics and High Energy), in Paris. “Then we determined that, of these, 26 are from background noise, while the other 11 are likely CEvNS. » More precisely, based on the energy and number of these events, physicists suggest that they were caused by neutrinos produced during the beta decay of boron-8. The latter being formed during fusion reactions in the heart of the Sun. The results of PandaX-4T confirm those of XENONnT. The Chinese team gets more events, but at the cost of more background noise.
From a statistical point of view, these results do not yet reach (barely) the “three sigma” threshold (which corresponds to less than 0.3% risk that this result is due to a statistical fluctuation in the noise of bottom). However, because it is a phenomenon predicted by theory, the researchers are confident in the reality of their discovery.
This breakthrough raises a crucial question. When a neutrino interacts with an atomic nucleus, it causes the latter to recoil minutely. This movement is detected by the instruments of the experiment. However, this is the same type of recoil that we expect when a wimp hits a xenon core. Therefore, if dark matter is composed of wimps and their interactions with matter are rarer than those of neutrinos, do these events not risk being drowned in the fog of neutrinos? Specialists have long feared that this situation would make the future detection of dark matter impossible. But two factors may help researchers achieve this. First, the energy spectra of wimps and neutrinos could be different, but it is not certain that this difference is sufficient to be exploitable. The other solution is perhaps more interesting. “The wimps that we are tracking belong to a halo that encompasses the entire Milky Way,” explains Luca Scotto Lavina. As the Sun moves through the Milky Way, this movement induces a form of dark matter “wind” with a specific direction. This flow of wimps, in the detector, is then modulated by the movement of the Earth around the Sun. It is maximum in June, when the Earth is moving in the same direction as the Sun (and minimum in December when the Earth is moving in the opposite direction). For neutrinos, the flux is maximum when the Earth is closest to the Sun, i.e. in January. These two modulations are offset by six months, a difference which should be measurable. »
But if physicists want to continue to increase sensitivity and plunge into the neutrino fog, it will be essential to find another solution: to design detectors capable of determining the initial direction of the particles before the collision. The idea would therefore be to be able to say whether the direction of the particle corresponds to that of the wimps wind or to the solar flux of neutrinos. The experiences PandaX-4T et XENONnT cannot measure this information. “For the moment, different techniques giving the direction of the particle are being explored, but none can achieve the sensitivities required to detect neutrinos,” emphasizes Luca Scotto Lavina. Significant development work will be necessary for the next generation of detectors, if it does not want to get lost in the fog…
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