If a person takes a pen and draws on the palm of their hand a square measuring one centimeter on each side, this tiny surface area would immediately be traversed by 65 billion neutrinos, originating from the Sun’s nuclear reactions. And another 65 billion would cross the tiny square every second. Neutrinos are, along with light photons, the most abundant elementary particles in the universe. And yet, they are elusive and extremely difficult to detect because they possess no electrical charge and have a mass of practically zero, millions of times inferior to that of an electron.
The scientific community is spending hundreds of millions of euros on machines – like the Hyper-Kamiokande neutrino observatory in Kamioka, Japan – to try and capture neutrinos to measure their properties with precision. Researchers believe some of the greatest secrets in the universe are hiding in these ghostly particles. But an international team of scientists revealed an unpleasant surprise on November 24: the simulations being used up until now are riddled with errors. They need to be fine-tuned for us to find out why we exist.
The universe began with all its matter and energy concentrated in a point smaller than the full stop at the end of this sentence. Expansion started with the Big Bang, around 13.7 billion years ago. The problem with the theory is that at the origin of the universe, the same amount of matter and antimatter – particles with the same mass, but with opposite values of electric charge – would have had to be formed. And, if that was the case, the matter and antimatter would have annihilated one another upon coming into contact, and the universe as we know it would not exist at all. However, the reality is that antimatter represents less than 0.0000001% of the total matter in the universe. What happened after the Big Bang to allow matter to emerge victorious from its battle with antimatter?
Many physicists, including 34-year-old Spaniard Guillermo Megías, believe the neutrino holds the answer. “Something had to break this cycle. We have evolved to a universe in which we are surrounded by matter. There is no antimatter in a pen or a table,” says Megías, who recently joined the University of Seville after spending two years at the University of Tokyo. He adds that the key may lie in so-called neutrino oscillation: these particles change their identity as they pass through space and can adopt three different types, or “lepton flavors” (electron, muon or tau). They are chameleonic, which implies that they have mass, contrary to what was previously believed. The discovery of this phenomenon earned Takaaki Kajita and Arthur McDonald the 2015 Nobel Prize for Physics.
Matter’s victory over antimatter
Megías is taking part in the T2K experiment, an audacious bid designed to investigate this metamorphosis. Scientists working on the project generated a beam of neutrinos in Tokai on Japan’s eastern coast and sent them to Kamioka, 295 kilometers away on the western side of the country to try and capture them at the Super-Kamiokande, a subterranean detector built in 1996 inside an old zinc mine. Trillions of neutrinos pass through it without leaving a trace, but occasionally some collide with the material of a gigantic tank standing 41 meters high and filled with 50,000 tons of water. The changes observed in the composition and intensity of the neutrinos as they make this journey allow scientists to deduce their mysterious properties.
These measurements, however, rely on theoretical models that predict the way neutrinos will interact with the nuclei of atoms. A new study, published in the science journal Nature on November 24, reveals that the simulations that use these models are plagued with imprecisions. These need to be refined, especially now that huge new detectors are being built, like the Hyper-Kamiokande, which is eight times bigger than the Super-Kamiokande and will cost over €500 million, and the US-based DUNE, a similar project based in a former gold mine in South Dakota, which is valued at more than €900 million.
Neutrinos barely interact with matter. They can even pass through a lead barrier nine billion kilometers thick. In the course of current experiments, like the Japanese T2K or the US NOvA, scientists are only able to detect a single neutrino among the thousands of billions produced in particle accelerators. On those exceptional occasions when neutrinos interact with matter, for example when they collide with atomic nuclei in the water at the Super-Kamiokande facility, they generate three types of particles, depending on the flavor of the neutrino: the electron, muon (which are similar to electrons but 200 times heavier) and tau, which are 4,000 times heavier.
Ongoing experiments measure these resultant particles, which are easy to detect, to calculate the properties of the oscillations of the neutrinos, in an attempt to reconstruct the energy present in the processes of the theoretical models. The authors of the new study – led by Israeli MIT physicist Or Hen, have imitated these experiments but swapping out neutrinos for electrons, a particle that scientists have perfectly under their control. The results are both surprising and concerning. The data suggests that 70% of the interactions are badly reconstructed by the simulations currently in use, as stated by Megías, co-author of the investigation. Correcting the models will help to determine whether neutrino oscillation caused matter to get the better of antimatter after the Big Bang.
Physicist Pilar Coloma stresses the need to refine the models, above all in future experiments conducted by DUNE and at the Hyper-Kamiokande facility, which aim to measure the properties of neutrinos with previously unimaginable rigor. “To reach this level of precision you need to have systematic errors completely under control,” says Coloma, of the Institute of Theoretical Physics in Madrid.
Giant facilities like the Hyper-Kamiokande could also open the door toward a new kind of particle physics, one that goes beyond the Standard Model, the theory that has been in development since the 1970s and that describes the universe using 17 fundamental particles – the building blocks of nature – and the interactions between them. “An additional property could be discovered, or even a neutrino that we don’t know about,” says Coloma, who is also a co-author of the new study.
Over the last few years, several laboratories have tried unsuccessfully to find evidence of a hypothetical fourth neutrino – dubbed “sterile” because of its inability to interact with the rest of the known particles. Sterile neutrinos are one of the possible ingredients of dark matter – the enigmatic particles that are thought to comprise 85% of all matter in the universe, five times more than classical matter, which is what gives form to everything from the stars to human beings. “Physics beyond the Standard Model has to be there,” says Coloma. “The million-dollar question is whether we will discover it a few years from now.”