Major international experiments generate key insights into neutrino behaviour

Two experiments involving Imperial have created the world's most precise measurement of the difference between the mass of two types of neutrino.

The current laws of physics do not explain why matter persists over antimatter – why the universe is made of ‘stuff’. Scientists believe equal amounts of matter and antimatter were created at the beginning of the universe, but this would mean they should have wiped each other out, annihilating the universe as it began.

Instead, physicists suggest there must be differences in the way matter and antimatter behave that explain why matter persisted and now dominates the universe. Each particle of matter has an antimatter equivalent, and neutrinos are no different, with an antimatter equivalent called antineutrinos.

Now, researchers have worked together to combine data from two different experiments, T2K (Tokai to Kamioka) and NOvA (NuMI Off-Axis νe Appearance).  They found evidence for the first time that in one of the two ways neutrino masses can be ordered, matter and antimatter neutrinos behave differently.

The study is published today in Nature.

Imperial College London researchers have been involved in this seven-year work since day one and the university is unique in having people working on this result from both T2K and NOvA (see box, right).

Detecting neutrinos in Japan

T2K is a neutrino experiment designed to investigate how neutrinos change from one flavour to another as they travel – known as neutrino oscillation. It is powered by the J-PARC nuclear physics facility located on the east coast of Japan. An intense beam of muon neutrinos is generated at the J-PARC site and is directed across the country to the Super-Kamiokande neutrino detector in the mountains of western Japan.

The beam is measured once before it leaves the J-PARC site, using the near detector ND280, and again at Super-K. The change in the measured intensity and composition of the beam is used to provide information on the properties of neutrinos.

Patrick Dunne, co-lead of the working group for T2K, said: "Working together has allowed us to make the world's most precise measurement of the mass difference between the neutrino flavours one of a handful of parameters that governs our current theory of how fundamental particles behave. Not only have we done this but we’ve also shed light on how this phenomenon of neutrino oscillations could be linked to why the universe has matter in it not an equal mix of matter and antimatter, which is one of the biggest open questions in physics today."

Firing neutrinos 500 miles across the United States

NOvA is managed by Fermi National Accelerator Laboratory in Illinois, in the United States. It generates a beam of neutrinos to send 500 miles to a 14,000-ton detector in Ash River, Minnesota. Scientists created the NuMI (Neutrinos at the Main Injector) neutrino beam by firing protons from Fermilab’s Main Injector into a graphite target. The neutrino beam is aimed downward at a 3.3° angle. Although the beam starts 150 feet/45.72 meters below ground at Fermilab, it passes as much as six miles below the surface as it travels toward Ash River. The particles complete the trip in less than three milliseconds.

Because neutrinos rarely interact with other matter, they travel straight through the Earth without a tunnel. Scientists detect a small fraction of the neutrinos in a 300 metric-ton near-detector at Fermilab and in the much larger far-detector in Minnesota, looking for signals that the neutrinos are changing from one type to another on their trip.

Both experiments can run this setup using either neutrinos or their antimatter counterparts, antineutrinos. Because they operate at different distances and energies, their sensitivities to the properties of neutrinos differ and complement each other.

Linda Cremonesi, Associate Professor in Particle Physics, said of the results: "We first discussed this joint analysis over eight years ago. Covid caused some delays, but during these years, the two collaborations have developed a deeper understanding of each other’s research and established a strong partnership. We are very pleased with this first joint result, and I personally look forward to our future joint analyses."

'Evidence that matter and antimatter neutrinos behave differently'

Before this work, it was thought that the results of the two most precise long-baseline neutrino oscillation experiments, T2K and NOvA, might be in tension. Working together and using both experiments' full capabilities, combined in a consistent analysis, has confirmed that the data are not in tension, and researchers can therefore use both together to increase our knowledge of neutrino oscillations.

The two experiments have agreed to carry on this collaboration and are working towards an updated analysis with additional data and more advanced analyses.

Further into the future, Imperial is working on the successor experiments to T2K and NOvA, Hyper-Kamiokande and DUNE, that will definitively probe the evidence opened up by this work.

This work represents a large multi-national effort, with Imperial’s contributions being supported by STFC and UKRI, GridPP, the Royal Society and the Royal Commission for the Exhibition of 1851.

'Joint neutrino oscillation analysis from the T2K and NOvA experiment' is published in Nature.