Can neutrinos shed some light on the imbalance of the universe?

One of the Universe’s greatest mysteries revolves around antimatter, or rather the lack thereof.  Physicists are convinced that when our Universe began, matter and antimatter should have been created and dispersed in equal amounts. However, it is evident that we live in a Universe that is almost entirely dominated by matter.

It’s just as well things worked out the way they did. If there had been equal amounts of matter and antimatter in the early Universe, particles and their antimatter counterparts would have annihilated each other and left the Universe containing nothing but leftover energy

Something must have tipped the balance for matter to overtake antimatter, and we could be getting closer to finding out what this might have been. Research published earlier this month suggests that differences between certain particles – neutrinos and antineutrinos – might help unravel this mystery.

Inside the Super-Kamiokande neutrino detector during work on the detectors. Image credit: Kamioka Observatory, ICRR, University of Tokyo

What’s the (anti)matter?

Each basic particle of matter has a corresponding antimatter particle with the same mass but with certain properties, like electric charge, being opposite. For example, a positively charged proton has a negatively charged antiproton counterpart. Likewise, a negatively charged electron corresponds to a positively charged antielectron – usually referred to as a positron.

Antiparticles do appear occasionally, and can be created naturally or artificially. Positrons, for example, are commonly produced as a byproduct of radiation. Interestingly enough, a banana emits a positron every 75 minutes or so. This is because bananas are high in potassium, a very small amount of which will be a naturally occurring radioactive isotope called potassium-40.

Antiparticles can be created from high-energy collisions, such as those in particle accelerators, where the excess energy generates pairs of particles and antiparticles in a process called pair production. In the opposite process, when a matter particle and its antimatter equivalent come together, the two particles annihilate each other resulting in a burst of energy. This would explain why, in such a matter-dominated Universe, we see so little antimatter.

But it hasn’t always been this way. 13.8 million years ago, at the time of the Big Bang, every particle and its antiparticle should have been created simultaneously in equal amounts, which would inevitably have led to all of these particles being annihilated. This has left physicists puzzled, to say the least.

New clues from T2K

The neutrino was first experimentally discovered in 1956 by physicists Clyde Cowan and Frederick Reines, who described it as “the smallest bit of material reality ever conceived of by man”. These tiny particles exist in three ‘flavours’, electron, muon and tau neutrinos, and are notoriously difficult to detect because they have no electrical charge and interact very little with any other forms of matter.

Evidence published earlier this month from a particle physics experiment called Tokai to Kamioka, or T2K, has suggested that differences between the behaviour of neutrinos and antineutrinos could help physicists in their bid to explain the matter-antimatter asymmetry problem.

Muon neutrinos and their equivalent antiparticles are generated at the Japan Proton Accelerator Research Complex (J-PARC) at Tokai, on Japan’s east coast. They are then fired 295km underground towards the west coast where they reach a neutrino observatory called Super-Kamiokande. A very small portion of these neutrino (or antineutrino) beams can then be detected from the pattern they leave in the detector’s 50,000 tons of water.

Neutrinos can change flavour, or oscillate, as they propagate. In this experiment, the probability that a neutrino would oscillate on its journey from J-PARC to Super-Kamiokande was measured and compared with results from the same experiment on antineutrinos. If matter and antimatter are perfectly symmetrical as originally thought, these probabilities should be identical.

The results from T2K indicated that neutrinos had a higher probability of changing flavour than their antimatter counterparts. This suggests that matter and antimatter might hold different properties.  

5σ isn’t everything

From the total of around 1020 (yes, that’s 100000000000000000000) neutrino-generating collisions at J-PARC, it took T2K ten years to detect just 90 neutrinos and 15 antineutrinos. The finding is not statistically large enough to satisfy the 5σ (5-sigma) level of confidence that would usually be required to qualify this as a discovery.

It is, however, an important step forward for physicists understanding of matter and antimatter. New generations of experiments such as T2Ks successor, Hyper-K, will provide more data in the years to come. If T2K’s results are confirmed, we could be closer than ever to solving the mystery of why our Universe exists the way it does.  

Written by Anna Purdue and edited by Ailie McWhinnie.

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