The Science of Discworld IV

some particles are more fundamental than others. A proton, for example, is made from three smaller particles called quarks. The same goes for the neutron, but the combination is different. Electrons, neutrinos and photons, however, remain fundamental; as far as we know, they’re not made out of anything simpler. fn1
    One of the main reasons for constructing the LHC was to investigate the final missing ingredient of the standard model, which despite its modest name seems to explain almost everything in particle physics. This model maintains, with strong supporting evidence, that
all
particles are made from sixteen truly fundamental ones. Six are called quarks, and they come in pairs with quirky names: up/down, charmed/strange, and top/bottom. A neutron is one up quark plus two down quarks; a proton is one down quark plus two up quarks.
    Next come six so-called leptons, also in pairs: the electron, muon, and tauon (usually just called tau) and their associated neutrinos. The original neutrino is now called the electron neutrino, and it is paired with the electron. These twelve particles – quarks and leptons – are collectively called fermions, after the great Italian-born American physicist Enrico Fermi.
    The remaining four particles are associated with forces, so they hold everything else together. Physicists recognise four basic forces of nature: gravity, electromagnetism, the strong nuclear force and the weak nuclear force. Gravity plays no role in the standard model because it hasn’t yet been fitted into a quantum-mechanical picture. The other three forces are associated with specific particles known as bosons in honour of the Indian physicist Satyendra Nath Bose. The distinction between fermions and bosons is important: they have different statistical properties.
    The four bosons ‘mediate’ the forces, much as two tennis players are held together by their mutual attention to the ball. The electromagnetic force is mediated by the photon, the weak nuclear force is mediated by the Z-boson and the W-boson, and the strong nuclear force is mediated by the gluon. So that’s the standard model: twelve fermions (six quarks, six leptons) held together by four bosons.
    Sixteen fundamental particles.
    Oh, and the Higgs boson –
seventeen
fundamental particles.
    Assuming, of course, that the fabled Higgs (as it is colloquially called) actually existed. Which, until 2012, was moot.
    Despite its successes, the standard model fails to explain why most particles have masses (for one particular technical meaning of ‘mass’). The Higgs came to prominence in the 1960s, when several physicists realised that a boson with unusual features might solve one important aspect of this riddle. Among them was Peter Higgs, who worked out some of the hypothetical particle’s properties and predicted that it should exist. The Higgs boson creates a Higgs field: a sea of Higgs bosons. The main unusual feature is that the strength of the Higgs field is not zero, even in empty space. When a particle moves through this all-pervasive Higgs field it interacts with it, and the effect can be interpreted as mass. One analogy is moving a spoon through treacle, but that misrepresents mass as resistance, and Higgs is critical of that way of describing his theory. Another analogy views the Higgs as a celebrity at a party, who attracts a cluster of admirers.
    The existence (or not) of the Higgs boson was the main reason, though by no means the only one, for spending billions of euros on the LHC. And in July 2012 it duly delivered, with the announcement by two independent experimental teams of the discovery of a previously unknown particle. It was a boson with a mass of about 126 GeV (billion electronvolts, a standard unit used in particle physics), and the observations were consistent with the Higgs in the sense that those features that could be measured were what Higgs had predicted.
    This discovery of the long-sought Higgs, if it holds up, completes the standard model. It could not have been made without big science, and it represents a major triumph for the LHC. However, the main impact to date has been in theoretical physics. The existence of the Higgs does not greatly affect the rest of science, which already assumes that particles have mass. So it could be argued that the same amount of money, spent on less spectacular projects, would almost certainly have produced results with more practical utility. However, it is in the nature of Great