Closer to Understanding the Lack of Antimatter

Neutrinos are the most abundant particles in our Universe, carrying crucial information for particle physicists, astrophysicists, and astronomers alike. Three types are known, they have no electric charge, and although their exact masses remain unknown, strong evidence indicates that their masses are not zero. However, their masses — and the differences between the masses of the three types — are so small that measuring them poses a major challenge for science. Why is it so important to measure the masses of neutrinos and the differences between them? Because it may help us better understand how the Universe works and bring us closer to revealing why, after the Big Bang, more matter than antimatter was formed.

Neutrinos possess another remarkable property: they can change from one so-called flavor (“type”) to another. This phenomenon is called neutrino oscillation. Intense neutrino beams generated by particle accelerators allow more precise studies of neutrino oscillations, and these oscillations in turn can provide insight into neutrino masses. Both the T2K experiment in Japan and the NOvA experiment in the United States use accelerator-produced neutrinos for this purpose, although the two experiments operate with different baseline distances and energy ranges.

“Neutrinos are produced from decays of pions and other particles generated when proton beams interact with a graphite target. To understand their properties and to uncover why antimatter is missing from the Universe, a great number of neutrinos must be produced, and the interactions between neutrinos and atomic nuclei must be well understood,” — explained earlier Yoshikazu Nagai, Assistant Professor at the Department of Atomic Physics of ELTE, leader of the T2K beam group and head of the ELTE Neutrino Physics Research Group.

Although the two experiments have traditionally been considered competitors, their complementary features have now made it possible for the research teams to work together and carry out high-precision oscillation measurements. Their analysis has reduced the uncertainty on neutrino mass-splitting to below 2%, resulting in the most precise measurement yet of the difference between neutrino masses. This constitutes a major advance in particle physics, as the findings bring us closer to determining whether neutrino behavior is related to the emergence of matter–antimatter asymmetry in the Universe.

Although the ordering of the masses of the three types of neutrinos is still unknown — that is, we cannot yet determine which type is the lightest and which is the heaviest — the results show that the difference in behavior between particles and antiparticles (the so-called CP violation) can be strongly constrained depending on the mass ordering. This achievement represents an important step toward determining whether CP violation occurs in neutrinos and brings us closer to understanding the origin of the matter–antimatter asymmetry in the Universe. In a perfectly symmetric scenario, matter and its corresponding antiparticles would behave identically — yet, according to the measurements, this is not the case.

“ELTE plays a leading role in the T2K experiment,” - said Yoshikazu Nagai, referring to the contribution of the Hungarian physicists. — “We are fundamentally contributing to achieving record-breaking accelerator beam power and to measuring the properties of the neutrino beam with unprecedented precision — crucial factors for the neutrino oscillation analyses.”

Nagai has also played a key role in ensuring the reliability of the joint T2K–NOvA analysis by thoroughly studying potential correlations between input parameters originating from the two collaborations, thereby guaranteeing the validity of the conclusions.

The future goal of the ELTE Neutrino Physics Research Group is to take advantage of the increased beam power and newly improved detectors to accomplish the main physics objective of the experiment: finding evidence for CP violation in the lepton sector.

The published analyses used ten years of data collected by T2K since 2010 and six years of data collected by NOvA since 2014. Beyond representing a major scientific breakthrough, this result is a compelling example of how two experiments located on opposite sides of the Earth — historically rivals — can combine their expertise to achieve outstanding scientific success.

Source of the pictures: T2K and NOvA collaboration

The original press realese of the T2K and NOvA collaboration: available here