A crack in the mirror of nature

The symmetry of nature is broken: the Universe is made of matter, while antimatter, which was originally produced in equal parts, has almost completely disappeared. For decades, physicists have been trying to measure this crack in the perfect mirror of the laws of physics (or "CP symmetry" break). In 1964, they detected for the first time a difference in behaviour between particles, called quarks, and their antiparticles, the antiquarks. But it was too small to explain the observations and has not been followed by any other discovery of the same nature since. This could change thanks to the efforts of a broad scientific collaboration co-led by a physicist from the University of Geneva (UNIGE). Using the Super Kamiokande detector in Japan, scientists discovered that neutrino, one of the most difficult particles to detect, also behaves differently than antineutrino. However, more data - i.e. years of measurements - are still needed before it can be said that the CP symmetry breakage in neutrinos is greater than in quarks. This major breakthrough in particle physics can be read in the journal Nature.

Matter and antimatter are very similar. An antimatter particle is even identical to a matter particle in every way, except for its electric charge (and a few other differences in quantum numbers). An anti-electron, for example, is positively charged, an anti-proton negatively charged. The problem is that when a particle meets its antiparticle, the two immediately annihilate each other by emitting light. Matter and antimatter cannot coexist.

THE DISAPPEARANCE OF ANTIMATTER

"If we go back to the origins of the Universe, matter and antimatter were produced at the same time and in equal quantities," explains Federico Sánchez Nieto, professor in the Department of Nuclear and Corpuscular Physics at the UNIGE Faculty of Sciences and international co-spokesman of the T2K collaboration, which carried out the experiment and took over the work of UNIGE Honorary Professor Alain Blondel. Today, however, all we see around us is matter. Antimatter seems to have completely disappeared, except for a few products of nuclear disintegration of interaction between high-energy particles."

So the beautiful symmetry of nature is obviously broken. Physicists call it the "violation of charge-parity symmetry" (CP). It is a necessary condition to explain the current dominance of matter over antimatter, and implies that particles, despite appearances, must behave in a different way from their antiparticles, if only slightly.

In 1964, physicists discovered such a difference between quarks and antiquarks (the elementary particles that make up neutrons and protons). But it is very small, too small to explain the imbalance observed in the abundance of matter compared to antimatter. This is why some researchers have turned to neutrinos, which are the most ghostly particles known. Impressive quantities of neutrinos travel across the Earth every second without interacting with any of its atoms. Except a few times.

Detect neutrinos

The Super Kamiokande detector, built under a mountain in Japan, is designed to detect these few neutrinos at the rate of a handful of particles per year. It is a particle accelerator located in Tokai, nearly 300 km away, which provides it with a beam of neutrinos or antineutrinos at will.

There are three types, or flavours, of neutrinos: electron, muon and tau," says Sánchez Nieto. A surprising feature of this particle is that it sometimes changes flavour spontaneously, for example from electron neutrino to muon neutrino. This is called an "oscillation". What we have done is to see if neutrinos oscillate at the same rate as antineutrinos, in this case from muon to electron flavor. And it turns out they don't."

It took 10 years for the T2K (Toka to Kamioka) experiment to collect enough data and get a statistically significant result. It also had to correct for any biases related to the fact that the environment of the experiment is obviously matter and not antimatter. In concrete terms, the physicists observed a total of 90 electron neutrinos and 15 electron antineutrinos meeting their criteria. After careful analysis, this result is compatible with maximum CP symmetry breaking in favour of neutrinos and against antineutrinos.

"Our results show a strong preference for matter," notes Federico Sánchez Nieto. But they are not yet sufficient to formally assert that CP symmetry has been violated. However, we are on the right track. To reach our goal, we will improve the sensitivity of our experiment, increase the intensity of the neutrino beam and accumulate more data."

The T2K experiment was built and is being conducted by an international collaboration currently comprising nearly 500 researchers from 68 institutions in 12 different countries (Canada, France, Germany, Italy, Japan, Poland, Russia, Spain, Switzerland, UK, USA and Vietnam). In particular, the National Fund for Scientific Research has, notably through the Universities of Geneva and Bern as well as the Swiss Federal Institute of Technology in Zurich, financially and scientifically supported this project for many years.