Neutrinos shed light on the standard model and beyond
Physicists have spent a lot of effort exploring the properties of these mysterious particles. By the end of the last century, they had discovered that neutrinos come in three types or 'flavours': electron, muon and tau. Moreover, neutrinos can switch flavour through a process called oscillation. This surprising fact indicated physics beyond the standard model. Within the EU-funded project NUTRAILS (On the trails of new neutrino properties), physicists geared up for neutrino studies that will open a window on new physics hidden in the subatomic world. They concentrated their research on standard and non-standard neutrino properties, such as the charge-parity (CP) symmetry violation and the impact of sterile neutrinos. According to current understanding of the Big Bang, matter and antimatter formed in equal amounts when our Universe was born. If that was the case, every smidgen of matter and antimatter should have annihilated each other as they were made. To explain the predominance of matter, physicists searched to find the right kind of CP symmetry violation. Previous studies found a difference in the behaviour of quarks and their antiparticles. However, this CP symmetry violation does not explain the overall matter-antimatter imbalance. Exploiting a wealth of neutrino data from experiments around the world, the NUTRAILS project provided the first hint in favour of the CP symmetry violation for neutrinos, particles of the family of leptons. Additionally, NUTRAILS scientists studied oscillations of neutrinos between the three flavours in search of sterile neutrinos. The existence of this type of neutrinos beyond the three known types, when proven, will have a profound impact on our understanding of the Universe. The new results set limits on the mixing of electrons with sterile neutrinos and on light sterile neutrinos with mass in the sub-eV scale. The NUTRAILS project has provided ongoing experiments with valuable boundaries for the searched mass range for the fourth possible neutrino state. In addition, the project has pointed out, for the first time, that present and future long-baseline experiments are sensitive to CP symmetry violation induced by the sterile neutrinos. Overall, the new findings on neutrinos make it clear that it is important to search for other neutral particles that contribute to the matter-antimatter balance of our Universe.