Pat Symonds's guide to neutrino physics

In January this year, the Tribunal de Grande Instance overturned the FIA's decision to impose a lifetime ban on Flavio Briatore for his part in the Crashgate saga. Also annulled was the five-year suspension meted out to Renault's erstwhile director of engineering Pat Symonds. Despite the FIA's protestations that they would appeal this decision, an out-of-court settlement between the parties was announced this week, and as part of this settlement, Symonds was permitted to begin work immediately for any Formula One team, via his consultancy company, Neutrino Dynamics. This is good news, for most reliable Formula One observers consider Symonds to be a good egg, who simply made an error of judgement on this occasion.

Moreover, now that Symonds has clearly diversified into elementary particle physics, McCabism would like to present Pat with a handy, question-and-answer crib-sheet, which can be used to field any smart-arse questions he might be asked about neutrinos in the coming months.

Q: Pat, what's a neutrino?
A: Well, it's an elementary particle with zero electric charge and spin 1/2. It was named by Italian physicist Enrico Fermi, and means 'little neutral one' in Italian. It was originally thought to have zero mass, but it is now known to have a small positive mass. Nevertheless, individual neutrinos interact very rarely with anything else. Portuguese physicist Joao Magueijo has recently compared them to scooters, zipping through the streets of Palermo. This rather underestimates things, however, for there are 10 trillion neutrinos passing through your body every second.

Q: What's the significance of the fact that neutrinos have mass?
A: Well, for a start, it means that they don't quite travel as fast as light in a vacuum. Secondly, like many elementary particles, neutrinos possess a property called 'handedness', or 'chirality'. This means that they can be left-handed or right-handed. If neutrinos were massless, the handedness of a neutrino would be an invariant property of a neutrino throughout its lifetime, part of the definition of the particle type. Originally, it was held that neutrinos were right-handed, and anti-neutrinos were left-handed. In contrast, if neutrinos have mass, then the handedness of neutrino is a variable property; an individual neutrino can be initially right-handed, but change to being left-handed at a later time.

Q:How did scientists discover that neutrinos have mass?
A: Well, there are three different so-called neutrino 'flavours': electron-neutrinos, muon-neutrinos, and tauon-neutrinos. Oscillations between these different flavours are only possible if neutrinos have mass. Scientists found that there was a deficit in the expected number of electron-neutrinos produced by the fusion reactions in the Sun, and detected on the Earth. This deficit was eventually explained by the fact that the electron-neutrinos emitted by the Sun are oscillating between the different flavours, so that only a third of them arrive at the Earth as electron-neutrinos. Neutrino flavour oscillations are only possible if neutrinos have mass, hence neutrinos have mass. I actually got the idea of using a mass damper in Formula One from the concept of neutrino oscillations.

Q: How has the Standard Model of particle physics accommodated this revised understanding of neutrinos?
A: Well, massless neutrinos were represented by things called Weyl spinor fields, which were required to satisfy an equation called the Weyl equation. Weyl spinor fields assign an element of C² to each point in space. In contrast, if neutrinos have mass, then they are either Dirac spinor fields or Majorana spinor fields. Dirac spinor fields assign an element of C⁴ to each point in space, whilst Majorana spinor fields assign an element of R⁴ to each such point. Anything represented by real number fields in particle physics equals its own anti-particle, hence if neutrinos are Majorana spinor fields, then there is no distinction between neutrinos and anti-neutrinos.

Q: But Pat, how can you determine if neutrinos are Majorana spinor fields?
A: Well, if they were, a process called neutrinoless double-beta decay would be possible. Double-beta decay results in the twin emission of two neutrinos; if a neutrino equals its own anti-particle, then it is possible for the two neutrinos in double-beta decay to annihilate each other. Hence if we detect neutrinoless double-beta decay, neutrinos must be Majorana spinor fields. In fact, we could also infer the electron-neutrino mass from neutrinoless double-beta decay.

Detecting neutrinos, however, is a difficult task at the best of times. At Neutrino Dynamics, we've installed a large tank of ultra-pure water in a mine deep beneath Enstone, where the detector is shielded from cosmic rays. Whenever a neutrino collides with an electron in the water, the electron initially recoils faster than the speed of light in water, creating a shock wave of visible light called Cerenkov radiation. We can detect these flashes of light with photomultipliers, and thence infer the neutrino reactions. In effect, we're trying to get neutrinos to deliberately crash into atomic electrons in our tank of water.

(Additional information from Dark Side of the Universe, Iain Nicolson, Canopus, 2007.)