THE RIDGE OF mountains marking the Electroweak Symmetry Breaking Scale follows a rift between continental plates which runs north–south across all latitudes of our map, even into the southern hemisphere. As we start to descend from the pass we have taken, and into the forests below us, we expect surprises.
The significant changes in physics at this longitude affect many experiments and measurements. Perhaps the clearest is in the process that we last encountered on our train journeys around Hadron Island and the Isle of Quarks – the scattering of electrons off protons, which first revealed the presence of quarks inside hadrons.
In these experiments, a beam of leptons – usually electrons or positrons – scatters off a hadron – usually a proton – and transfers so much energy and momentum that the hadron shatters. Usually this is an electromagnetic interaction, meaning that a virtual photon is exchanged between the electron and a quark.
However, electron–proton collisions can also be mediated by the weak force, meaning the exchange of the W or the Z boson. Since the W carries away the electric charge of the electron, in those events the emerging scattered lepton is a neutrino.fn1 As might be expected, these ‘charged-current’ events are much rarer than the electromagnetic scatters, reflecting the fact that the weak force is much … well, weaker, than the electromagnetic force.
As the energy scale increases, and we move eastwards, the rates of these two different types of scattering converge, until, as we descend, they become roughly equal. As we descend the ridge of the Electroweak Symmetry Breaking Scale, they become equal. And, if we can pull together what we have learned on our journeys so far, we already know enough to understand why.
In the west, at lower energies, most of the difference in strength and range between electromagnetism and the weak force is due to the different masses of the exchanged bosons. The photon has zero mass, while the W and Z have masses of just below 1011 electrovolts – about a hundred times the mass of a hydrogen atom.
The range and strength of the force depend on the mass of the exchanged particle because the particles in these exchanges are virtual and do not have the correct mass. In fact they cannot have the correct mass – there is no way that an electron can suddenly emit a real photon, a W or a Z, and conserve energy. The only way that can work is if the emitted particle has the wrong mass. And we already know that the further the virtual particle is from the correct mass, the lower is the probability of it being emitted or exchanged.
In the low energy interactions of Atom Land, and indeed everyday life westward, the massive W and Z bosons are much further from their correct mass than is the photon, so are much less likely to feature. This is why the electromagnetic force is stronger, and more commonly observed.
Go further east though, where energies are higher, and the difference in mass between the W, Z and photon becomes less and less relevant, and the weak force and the electromagnetic force become very similar in strength.fn2 In terms of the transport network on our map, beyond this ridge of mountains flying has become just as common as travelling by road, mainly because the roads are becoming more difficult to negotiate.
This serves to make the mass problem even more important. The W and Z masses play an absolutely crucial role in determining the characteristics of the weak force, and we still have the problem that they ought to break renormalisation and let in the infinities. As indicated by clues on our frozen windscreen, and the name of the Electroweak Symmetry Breaking Scale mountains we have just passed through, a broken symmetry is behind this.
People suspected this might be the case decades ago, before the rest of the Standard Model was established and even before the Isle of Quarks was discovered. If fundamental particles – whatever they might turn out to be – were to have mass, something had to be done. Something strange and different has to be going on in the forests we are now entering.