A major step towards the explanation of the matter-antimatter asymmetry

The observed dominance of matter over antimatter in the Universe is among the most relevant open questions in science. According to our current understanding of the early stages of the Universe, at its origin an equal amount of matter and antimatter was generated. Its evolution toward a state where matter dominates over antimatter requires as necessary condition the violation of the so-called Charge-Parity (CP) symmetry.

Transforming elementary particles

In past experiments a violation of CP symmetry has been observed in the physics of quarks. However, its magnitude is not large enough to explain the observed matter prevalence over antimatter. For this reason other sources of CP symmetry violation are being searched for. The T2K experiment in Japan has been searching for CP symmetry violation in so-called neutrino oscillations. Neutrinos are elementary particles which travel through matter almost without interaction. They appear in three different types: electron- muon- and tau-neutrinos and their respective antiparticle (antineutrinos). In 2013 T2K discovered a new type of transformation among neutrinos, showing that muon-neutrinos transform (oscillate) into electron-neutrinos while travelling in space and time. The CP symmetry violation in neutrino oscillations manifests itself as a difference in the measured oscillation probability for muon neutrinos and muon antineutrinos into electron neutrinos or electron antineutrinos respectively.

Strongest evidence to date implying the breaking of the matter-antimatter symmetry

A new study by the T2K Collaboration published in the journal Nature shows the strongest constraint to date on the so-called δ_cp phase, a parameter implying the breaking of the symmetry between matter and antimatter in neutrino oscillations. The observed most probable value for the δ_cp phase is close to the value of maximal asymmetry. “This is a strong indication that the symmetry between matter and antimatter is violated in neutrino oscillations and therefore neutrinos also play a role in the creation of the matter-antimatter asymmetry in the Universe“, said Prof. Antonio Ereditato, director of the Laboratory of High Energy Physics of the University of Bern and leader of the Bern T2K group.

For its research program, T2K used beams of neutrinos or antineutrinos generated at the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai village on the east coast and detected 295 km away, at the Super-Kamiokande detector, located under a mountain in Kamioka, near the west coast (“T2K” stands for “Tokai to Kamiokande”).

The T2K experiment was constructed and is operated by an international collaboration which currently consists of nearly 500 scientists from 68 institutions in 12 countries.

Researchers of the University of Bern led by Prof. Antonio Ereditato are members of the T2K collaboration since 2006. They took important roles in the construction and operation of the “near detector” at J-PARC, on the data analysis and on coordination roles.