Differences between neutrino matter and antimatter revealed

Researchers from the Institute of Corpuscular Physics (IFIC), from the Physics Institute of Other Energies (IFAE-Barcelona) and from the Autonomous University of Madrid, have contributed to reveal a basic property of neutrinos that had not been measured until now. It represents an important step towards learning whether neutrinos behave differently in their matter and antimatter forms. The obtained results have been published as the front-page article of journal Nature.T2K

T2K (Tokai a Kamioka) is a particle physics experiment located in Japan, which is studying neutrino oscillations and which is being worked on by research teams from 12 countries. The T2K collaboration has published new results, the most accurate ones obtained to date: the parameter that governs the rupture of the matter-antimatter symmetry in the oscillations of these particles. For the first time, the experiment starts to reveal a basic property of neutrinos that had not yet been measured. This is an important step to learn whether neutrinos and antineutrinos behave differently and not in a symmetrical way, as described by the laws of most physical phenomena in this field. These results, that use data compiled up to 2018, are the ones published in scientific journal Nature.

A majority of physical phenomena are described as laws that predict a symmetric behaviour for matter and antimatter. In physics jargon it is known as charge-parity symmetry, or simply CP symmetry. However, this symmetry is not universal, as is clear from the current make-up of the universe, whose antimatter content is very small. The Big-Bang Theory assumes that the universe was created with identical amounts of matter and antimatter. To reach the current situation, there must be a violation of the CP symmetry.

Until now, violation of the CP symmetry had only been observed in the physics of subatomic particles known as quarks, but the magnitude of this violation is not large enough to explain the make-up of the current universe. T2K seeks a new source of violation of the CP symmetry in the oscillations of neutrinos, which would manifest itself as a difference in the probability of oscillation for neutrinos and antineutrinos.

The T2K experiment uses neutrino and antineutrino beams created by using the proton beam from the Japan Proton Accelerator Research Complex (J-PARC), located in Tokai, on the east coast of Japan.

Spanish contribution

The T2K experiment has been created and operated by an international collaboration which is currently composed of over 500 scientists from 68 institutions of 12 countries (Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, United Kingdom, United States and Vietnam). It is mainly funded by the Ministry of Culture, Sports, Science and Technology (MEXT) of Japan. Spain contributes with three research groups, two of which – IFAE and IFIC – have participated in the design, construction and operation of the experiment for over 15 years, conducting relevant contributions to the study of neutrino oscillation. The Autonomous University of Madrid was recently added to the team. Spain funded the research activity through the Ministry of Economy and Finance and the Government of Catalonia, with the support of the National Centre of Nuclear Particles and Astroparticles (CPAN).

Paper signees on behalf of the IFIC are M. Antonova, A. Cervera, P. Fernández, A. Izmaylov and P. Novella.

Reference

“Constraint on the Matter-Antimatter Symmetry-Violating Phase in Neutrino Oscillations”. DOI: 10.1038/s41586-020-2177-0. Nature Vol. 580, pp. 339-344