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Probing Fundamental Physics with Antineutrinos at the NOvA Experiment

Final Report Summary - ANTINEUTRINONOVA (Probing Fundamental Physics with Antineutrinos at the NOvA Experiment)

Neutrinos are ghostly fundamental particles that are all around us yet incredibly hard to detect. They are given off by the sun when it shines, in nuclear power stations and can be produced in a laboratory when an accelerated beam of protons is smashed into a target. Studying neutrinos is hard and there is much we don’t yet know. Our theories of how neutrinos behave suggest that they hold a key to understanding why the universe is made of matter and not antimatter. To determine if the theories are correct, we are making measurements of neutrinos and antineutrinos to look for differences.

In this ERC project, data from the NuMI neutrino beam at Fermi National Accelerator Laboratory (Fermilab) in the USA was collected using the two huge detectors of the NOvA experiment. The “Near” detector is located 1 km from the NuMI beam and the “Far” detector is 810 km away, allowing us to measure how the neutrinos change in flight. The NuMI beam produces neutrinos (or antineutrinos) by accelerating protons to close to the speed of light and then smashing them into a long, thin graphite target. Exotic, short-lived particles are produced that then decay to give neutrinos. A magnet system is used to focus either the positively or negatively charged exotic particles, producing either a neutrino or antineutrino beam.

One way that neutrinos and antineutrinos are produced is in association with a particle such as an electron or the antimatter equivalent called a positron. By definition, an electron-type-neutrino is produced alongside a positron and an electron-type-antineutrino is produced alongside an electron: there is always one matter particle and one antimatter particle produced in every case. While they don’t exist much in everyday life, there are two fundamental particles called a muon and a tau that are almost exactly like an electron except hundreds of times more massive. When these heavy particles or their antimatter equivalents are produced, they also have neutrinos or antineutrinos created alongside them. So, finally, by extension, we define muon and tau types of neutrinos and antineutrinos. The beam at Fermilab produces either muon neutrinos or muon antineutrinos depending on how we set the magnets.

Twenty years ago it was discovered that neutrinos produced as one type could oscillate into another type, back and forth. This ERC project was conceived to measure whether neutrinos and antineutrinos oscillate differently.

The probability of a muon neutrino oscillating into an electron neutrino was known to be less than about 10% at the start of the project. We have now measured this probability by detecting 58 candidate electron neutrinos with a background of 15 events when the expected range was 30-70 candidates. Furthermore, in the summer of 2018 we released the world’s first evidence of muon antineutrinos oscillating into electron antineutrinos. With the number of antineutrinos produced by the beam and predictions of the oscillation probabilities we expected to measure between 11-23 candidate electron antineutrinos on a background of 5.3 events. We detected 18 candidates, which is by far the world’s strongest ever signal of this crucial process.

Measurement of these oscillation probabilities has provided fundamental information about neutrinos. There are 3 aspects of the standard theory remaining to be measured and the results of this project impact all those areas. They are, the matter-antimatter asymmetry, the ordering of the masses of the neutrinos and whether there is a symmetry between muon and tau neutrinos. With the data collected so far, the results have a preference for the “normal” ordering of the neutrino masses. Over the next few years further data will be collected and analysed by the group, with the sensitivity to make a potential 4 sigma measurement of the neutrino mass ordering.