If a person takes a pen and draws in the palm of his hand a square one centimeter on a side, that small surface will immediately be crossed by a few 65 billion neutrinos coming from the nuclear reactions of the Sun. And every second another 65,000 million will pass. Neutrinos are, along with photons in light, the most abundant elementary particles in the universe. And yet they are elusive, extremely difficult to detect, lacking electrical charge and having almost zero mass, million times less to that of the electron. The scientific community is building machines worth hundreds of millions of euros, such as the future Japanese detector Hyper-Kamiokande, to try to hunt down neutrinos and accurately measure their properties. Researchers believe that these ghostly particles hide some of the biggest secrets about the universe. An international team of researchers revealed an unpleasant surprise on Wednesday: the simulators used so far are riddled with errors. You have to fine-tune them to understand why we exist.
The universe began with all matter and energy concentrated at a point smaller than the one at the end of this sentence. The expansion began with the Big Bang, about 13.7 billion years ago. The problem with the theory is that at the origin of the universe the same amount of matter should have been formed as antimatter: particles with the same mass, but with opposite values of electric charge. And if this were so, matter and antimatter would have annihilated each other on contact, and the known universe would not exist. However, the reality is that antimatter represents less than 0.0000001% of the total matter of the universe. What happened after the Big Bang for matter to be victorious in its fight against antimatter?
Many physicists, like Spanish Guillermo Megías, they believe that the neutrino has the answer. “Something had to break that cycle. We have evolved into a universe in which we are surrounded by matter. There is no antimatter in a pen or on a table ”, explains Megías, recently rejoined the University of Seville after two years at the University of Tokyo. The physicist, a 34-year-old from Seville, details that the key may be in the call neutrino oscillation: these particles change their identity as they move through space, and can adopt three types or flavors. They are chameleonic, which implies that they have mass, contrary to what was thought. The discovery of this phenomenon won the Nobel Prize in Physics in 2015 for the Japanese Takaaki kajita and the canadian Arthur McDonald.
The victory of matter over antimatter
Guillermo Megías participates in the T2K, a daring experiment designed to investigate this metamorphosis. Scientists generate a jet of neutrinos on the east coast of Japan and try to hunt them on the other coast, about 300 kilometers, in the Super-Kamiokande: an underground detector opened in 1996 inside a former zinc mine. Trillions of neutrinos pass through it without a trace, but every now and then one collides with the matter of a gigantic 41-meter-high tank filled with 50,000 tons of water. The changes observed in the composition and intensity of the neutrinos in that kilometer trip allow their mysterious properties to be deduced.
The measurements, however, depend on theoretical models that predict how neutrinos interact with the nuclei of atoms. The new study, published this Wednesday in the magazine Nature, reveals that the simulators using these models are plagued with inaccuracies. They have to be fine-tuned, especially now that huge detectors are being built, such as the Japanese Hiper-Kamiokande, five times larger than the Super-Kamiokande and at a cost of more than 500 million euros, and the American DUNE, a similar project in a former South Dakota gold mine, with a committed investment of more than 80 million euros.
Neutrinos hardly interact with matter. They could even break through a lead barrier of nine trillion kilometers thick. In current experiments, such as the Japanese T2K or the American NOvAscientists are only able to detect one neutrino for every several billion trillion produced in particle accelerators. On those rare occasions in which neutrinos interact with matter, for example when colliding with the atomic nuclei of the water of the Super-Kamiokande, three types of particles are generated, depending on the taste of the neutrino: electrons, muons (similar to electrons, but 200 times heavier) or taus (4,000 times heavier).
Current experiments measure these resulting easily detectable particles to deduce the properties of neutrino oscillations, trying to reconstruct the energies of the process with theoretical models. The authors of the new work – led by Israeli physicist Or Hen, from the Massachusetts Institute of Technology (USA) – have imitated these experiments, but exchanging neutrinos for electrons, a particle perfectly controlled by scientists. The result is surprising and disturbing. The data suggests that up to 70% of interactions are poorly reconstructed with current simulators, according to Megías, a co-author of the research. Correcting the models will help to find out if the oscillation of the neutrinos caused matter to beat antimatter after the Big Bang.
The new physics has to be there. The million dollar question is whether we will discover it in a few years “
Pilar Coloma, physics
Physics Pilar Coloma, born in Santa Cruz de Tenerife 37 years ago, highlights the need to fine-tune the models, especially in future DUNE and Hiper-Kamiokande experiments, which aim to measure the properties of neutrinos with unprecedented rigor. “To get to that level of precision you need to have your systematic errors very controlled,” says Coloma, from the Institute of Theoretical Physics, in Madrid.
Giants like the Hyper-Kamiokande could also open a door to a new physics, beyond the Standard Model, the theory developed since the 1970s that describes the universe using 17 fundamental particles – the building blocks of nature – and the interactions between them. “It could discover some additional property or even some neutrino that we do not know,” explains Coloma.
In recent years, several laboratories have unsuccessfully searched for a hypothetical fourth neutrino, named sterile, due to its alleged inability to interact with other known particles. Sterile neutrinos are one of the possible ingredients of dark matter, enigmatic particles that apparently make up 85% of the matter in the universe, five times more than the classical matter, the one that forms from the stars to the human beings. “The new physics has to be there,” says Coloma. “The million dollar question is whether we will discover it in a few years.”
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