Why does the Universe exist instead of having been annihilated shortly after its birth? This fundamental question in physics finds new illumination thanks to an unprecedented collaboration between two major scientific experiments.
Neutrinos, these ghostly particles that pass through matter with almost no interaction, could hold the key to this mystery. Their precise study allows us to explore why matter survived antimatter during the first cosmic moments.
Interior of the Super-Kamiokande detector.
Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo
For the first time, the T2K experiment in Japan and the NOvA experiment in the United States have combined their data. They produce beams of neutrinos sent to near and far detectors, over hundreds of kilometers (hundreds of miles). This method allows the observation of how these particles change type while traveling, a phenomenon called oscillation (see below).
The combined results, published in
Nature, offer very precise measurements of this behavior. They help determine the order of neutrino masses, i.e., which types are the lightest. This information influences the possibility of a symmetry violation between matter and antimatter (explanation at the end of the article).
If neutrinos and antineutrinos behave differently, this could explain the imbalance observed in the Universe. Despite significant progress, current data does not yet allow for a statistically significant conclusion. This study represents a global effort involving hundreds of researchers from many countries.
The experiments continue to collect data to refine future analyses.
Neutrino oscillation
Neutrinos exist in three forms, or flavors: electron, muon, and tau. During their travel, they can spontaneously switch from one flavor to another. This phenomenon, called oscillation, depends on their masses and the distances traveled.
Oscillation occurs because each flavor is a mixture of three distinct mass states. These states evolve differently in space, leading to periodic changes in the detected flavor. It's a bit like a color shifting to another depending on the path taken.
Experiments like T2K and NOvA measure these oscillations by sending neutrino beams over long distances. By comparing the flavors at the source and at the detection point, physicists can deduce key parameters, such as the mass differences between the states.
This understanding is essential for exploring broader questions, such as the asymmetry between matter and antimatter. Neutrino oscillation thus represents a window into fundamental processes that shaped the Universe.
CP symmetry and its cosmic role
CP symmetry is a principle in particle physics that postulates that the laws should be identical for matter and antimatter after a charge and parity inversion. In other words, a process and its mirror image with antiparticles should occur with the same probability.
If this symmetry is violated, it means that matter and antimatter do not behave exactly the same way. Such violations have already been observed in other particles, but they are too weak to explain the predominance of matter in the observable Universe.
Neutrinos offer a promising ground for detecting a more significant CP symmetry violation. If the oscillations of neutrinos and antineutrinos differ, it would indicate an asymmetry that may have influenced early cosmic evolution.
Current research aims to precisely measure these behavioral differences. The results could help explain why the Universe today contains mostly matter, allowing for the existence of galaxies, stars, and life.