Difference Between Neutrinos and Antineutrinos

Two researchers from the Institute of Corpuscular Physics in Valencia have presented a theorem to untangle the deceptive effect that the Earth produces in the oscillations of neutrinos and antineutrinos and makes it difficult to distinguish them. The theorem can be applied in future experiments such as DUNE in the US and T2HK in Japan, in addition to providing a new clue to explain the asymmetry between matter and antimatter in the universe.

Scientists José Bernabéu and Alejandro Segarra from the Institute of Corpuscular Physics (IFIC, a joint center of the University of Valencia and the CSIC) have just published in the journal Physical Review Letters the solution to a problem that had been discussed in neutrino physics for some time. decades.

By observing a phenomenon known as 'neutrino oscillations', science seeks an answer to why we live in a universe of matter and not antimatter, its identical replica. However, this process is affected by the Earth itself, made of matter, creating a tricky effect that was considered inseparable from genuine observation of the differences between matter and antimatter.

Now the two researchers propose a way to 'untangle' or separate both effects, with application in future experiments such as DUNE (Deep Underground Neutrino Experiment) in the United States and T2HK (Tokai to Hyper-Kamiokande) in Japan.

Neutrinos are special elementary particles: they have hardly any mass and rarely interact with other known matter. They abound in as yet undetected radiation produced in the early days of the universe, and are believed to hold the key to the matter-antimatter asymmetry, the explanation why matter prevailed over antimatter to form everything we see in the cosmos.

To study this question, one of the most important in Physics, the behavior of neutrinos and their antimatter replica, antineutrinos, produced in particle accelerators and detected hundreds of kilometers from their origin, are compared.
 
During this trip, the neutrinos 'oscillate', they transform between the three types that are known (electronic, muon and tau). This phenomenon, known as 'neutrino oscillations' and whose discovery led to the Nobel Prize in Physics in 2015, occurs inside the Earth, since neutrinos can pass through it by interacting very little with the matter that forms it.

The theorem allows us to differentiate the genuine effect of the differences between neutrinos and antineutrinos from the deceptive effect produced by the Earth.

"This creates a deceptive 'entangled' effect with the search for the genuine effect of the difference between neutrinos and antineutrinos as if they were propagated in a vacuum," explains José Bernabéu, emeritus professor at the University of Valencia and co-author of the work together with Alejandro Segarra, PhD student in the Department of Theoretical Physics and the IFIC.
 
“The two effects, the genuine and the deceptive, manifest themselves in the same way between neutrinos and antineutrinos, so it seems impossible to disentangle them. But they can be separated if they behave differently under other properties”, argues Bernabéu.

The two authors present a theorem for disentangling the two effects, which have different properties under other fundamental symmetries of Physics such as the so-called time inversion (T) and the combined charge, parity and time inversion (CPT), previously studied by Bernabéu in other physical systems.

This makes it possible to differentiate the genuine effect of the differences between neutrinos and antineutrinos from the deceptive effect, since the latter presents a CPT symmetry breaking that does not appear in the genuine one.

Magic energy to distinguish the deceptive effect

The first consequence of the Bernabéu and Segarra theorem is that the components that identify the two effects depend in a different way on the distance traveled by the neutrinos. However, the experiments that measure their oscillations cannot place different detectors throughout their journey on Earth, but instead build a single detector at a fixed distance ranging from the 300 kilometers of the T2HK experiment to the more than 1,000 of DUNE. .

What these detectors can measure is the energy of the oscillation, that is, the energy with which the neutrinos arrive. Thus, in this article the IFIC researchers explore the expected energy for each of the components, the genuine and the misleading, finding that, in fact, it is very different, which would provide an experimental signal to separate them.

Graph showing the different behavior of the two components of the asymmetry between neutrinos and antineutrinos, the deceptive one (in green) and the genuine one (in blue), around the so-called 'magical energy'. / Bernabéu, J. & Segarra, A./PRL
This last result has motivated a detailed study that the same authors publish in the Journal of High Energy Physics, where they analyze this energy dependence and discover the origin of its different behavior for the genuine component and the deceptive one.

The Valencian physicists find a 'magical energy' in which three properties coincide: the second maximum where neutrino oscillations occur, which offers an appreciable number of events to study; nullifies the deceptive component and provides a maximum of genuine effect for direct evidence of symmetry breaking between matter and antimatter.

In the 1,300 kilometers between the Fermi laboratory near Chicago and DUNE's detector under construction in South Dakota, that magical energy is 0.91 GeV.

"This 'magical energy' is accessible and reconstructable in the experiment even with a modest precision in determining its value with an uncertainty of 0.15 GeV," the researchers state.

On the other hand, at energies higher than this magical value, the deceptive component dominates, and the sign of the difference observed between neutrinos and antineutrinos offers the solution to another still open problem: ordering the levels from least to greatest mass of the three types. of neutrinos.

With these results it is now possible to carry out a rigorous simulation of the experiment and adapt its design to observe if there is a fundamental difference between the behavior of neutrinos and antineutrinos, something that the researchers are working on together with the IFIC group that participates in DUNE.