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Can Neutrinos Solve the Mystery of the Matter-Antimatter Unbalance?

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Neutrinos are tiny subatomic particles that have sparked off a number of hugely consequential discussions in the past few months - but despite their huge scientific importance, some of the most basic particle properties remain unknown due to the fact that they do not interact with matter.

All evidence suggests that neutrinos have mass, but of such a small amount that it is assumed to be negligible. As they travel so fast, their mass must necessarily be tiny - this leads on to the physics controversy of 2011: the saga of the faster-than-light neutrinos. The team that initially recorded the superluminal speeds recently summarised two possible errors in their experiment that they believe could have caused these results: one raising the apparent neutrino speed (dodgy GPS wiring), and one lowering it (clock synchronisation). CERN released a statement earlier this month labelling the superluminal speeds recorded as "an artefact of the measurement" - all-in-all, the conclusion appears to be that no staggering physics-altering revolution will be occurring anytime soon. For now, Einstein still appears to be correct.


Particle tracks in a bubble chamber. Image courtesy of CERN/Science Photo Library

However, it is not just erroneous results that are neutrinos' claim to fame. The tiny particles are key players in another highly important unconfirmed concept in theoretical and particle physics - antimatter. Antimatter is an important part of the standard model of particle physics - something you might have heard about when reading article after article on CERN's hunt for the Higgs Boson. When matter meets antimatter - for example an electron and its positively-charged counterpart the positron - both particles are destroyed (or, to use the technical term, annihilated). Thus, we can learn a lot about particle behaviour and matter composition from observing neutrinos.
All the galaxies, stars, planets and other space objects we see are composed of matter. Image from a simulation: Virgo Consortium.


However, one of the most pertinent areas of neutrino research involves the Big Bang and neutrino astronomy. During the Big Bang - from which all matter, antimatter, energy, and everything else was created - one would expect an equal amount of matter and antimatter to have been created. And, bearing in mind what I just said about annihilation, this would lead to complete destruction of all kinds of matter, leaving the universe inhabited by only light and other forms of radiation. But this is not what is observed - it seems that there was a preference for creating matter over antimatter during the Big Bang, making it possible for us to exist at all. As far as we can see, we and everything else around us are made up of matter.

There are two possible explanations for this matter bias (very well explained in this New Scientist piece on the problem). One is that there actually was more matter formed than antimatter - leading to a surplus of matter after all the annihilation. Another explanation takes a different route, suggesting that there were actually equal amounts formed and that the antimatter somehow managed to escape. New Scientist words this well, proposing that "somewhere out there, in some mirror region of the cosmos, antimatter is lurking and has coalesced into anti-stars, anti-galaxies and maybe even anti-life". For a physicist, this is a very exciting prospect - but is it right?

Experiments have shown that, rather than being two completely removed worlds, antimatter and matter sometimes come together. Particles have the ability to change their flavour - a term used to describe certain different properties that particles hold - and can flick between matter and antimatter states. Back in 1998, CERN observed a particular particle, the Kaon, changing into its antiparticle more often than they expected.

Neutrinos have been observed doing the same thing, flipping between their three flavours: electron, muon, and tau - but only in certain combinations. In June last year, neutrinos were first observed flipping between muon and electron states, opening the doors for more in-depth neutrino physics. New research earlier this month observed that these changes were happening much more rapidly than expected, leading the researchers to question whether or not neutrinos and antineutrinos act differently. The detectors used by the researchers at the Daya Bay Reactor Neutrino Experiment in southern China were primed to detect electron antineutrinos, meaning that as they flipped between states they would be unobservable to the detectors. This led to detectors based in different positions detecting different numbers of electron antineutrinos. If these particles were observed to change state only rarely, this would have been quite an obstacle in terms of scaling this experiment to compare such antineutrinos to their matter counterparts. As it is, the next viable step for the research is to move into observing neutrinos, and comparing their behaviour to this previously-observed anti-neutrino activity.

This research really does lend potential to the field of neutrino physics. Observing the elusive neutrino flavour changes and interactions may be able to provide an explanation for this mysterious matter-antimatter imbalance - as well as helping to solve a spate of other particle physics mysteries.

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