The second of two articles, in which as his five-year mandate as Director-General of CERN nears its end, Chris Llewellyn Smith reflects on what particle physics has achieved, and where it may be going. The focus is on conceptual advances building on the Standard Model.
The very successful Standard Model summarizes our present understanding of the constituents of matter and the forces that control their behaviour. Descriptions of this model usually emphasize its content quarks, leptons, W and Z bosons, and gluons and do not stress that it embodies three very important conceptual advances.
First, according to the Standard Model all forces are generated by the exchange of particles. For example, when an electron scatters from a proton, energy and momentum are carried from one to the other by a particle of light, called a photon. Thus, force is not a separate concept given the existence of particles, and of interactions that allow particles to be emitted and absorbed, forces follow!
Second, Nature elegantly allows all conventions to be fixed locally, and this determines the form of all the known forces. For example, quarks (the constituents of nuclear particles such as the proton) are distinguished by different labels, but their properties are unchanged when the labels are switched in certain ways. The assignment of these labels is therefore a matter of convention (such independence of the choice of a convention is known generally as a symmetry). Remarkably, this convention does not have to be fixed once and for all, but can be chosen differently at different times and places. The possibility of choosing conventions locally requires the existence of the observed force carrying particles and fixes their interactions.
Third, the Standard Model contains “hidden symmetries”, symmetries in the underlying mathematical description that do not show up in Nature. In particular, there is excellent evidence for a symmetry that relates the electromagnetic and the weak forces. But it must be hidden otherwise the choice of the labels “W”, “Z” and “photon” would be a matter of convention, and the massive W and Z particles, that carry the weak force, would be massless like the photon, which carries the electromagnetic force. The exciting discovery that Nature hides symmetries opens the possibility that they are hidden links between other phenomena.