If yesterday’s newly discovered particle turns out to be the Higgs boson, as many think it might, it will be a landmark accomplishment for physics. Every other fundamental particle discovered to date – the quarks, leptons and gauge bosons of the standard model – has spin, an intrinsically quantum mechanical property that determines its fate. The Higgs, however, does not. It is an entirely new form of matter.
The spin of the quarks and leptons is ultimately responsible for the structure of matter, including the properties of nuclei and the electronic structures that govern all of chemistry. The spin of the gauge bosons gives rise to the forces of nature, ranging from electricity and magnetism to nuclear reactions and gravity.
The Higgs, though, is different; it has no spin. Its spinless state allows it to condense and fill the vacuum, much like steam condenses to form the sea. It is this Higgs condensate that is responsible for mass: particles travelling through the condensate experience a drag that slows their motion and gives them mass. The more the drag, the greater the mass.
But the Higgs does much more. Its discovery will mark a triumph of physics, and as with any discovery, it will open the door to a whole new range of questions.
For example, the Higgs condensate fills the vacuum, so empty space is not empty. Condensed matter physicists tell us that an analogous condensate forms inside a superconductor. Does this mean that the universe itself is a new type of superconductor? And if so, what new physics controls its properties?
Higgs-like particles are ubiquitous in theories of physics that extend beyond the standard model. They are predicted by supersymmetry and by theories of grand unification. Their condensates contribute to the dark energy that is accelerating the expansion of the universe, and they determine the geometry of the extra dimensions in string theory. Higgs-like particles might even be responsible for cosmological inflation, the change in time of dark energy, the missing dark matter, or even the puzzling properties of neutrinos.
Is yesterday’s discovery that of the Higgs, or is it something else? Time will tell, but signs so far are positive. Already the LHC experiments have shown that the new particle is different – it is not a quark, a lepton, or a gauge boson. But is it the Higgs? For a particle to be the Higgs, its properties must be exquisitely balanced – and that is difficult to check.
A linear collider, with its clean and controlled electron-positron collisions, offers the perfect environment to study Higgs bosons. Experiments at a linear collider can measure Higgs properties without assumptions and with unprecedented precision. A linear collider can serve as a Higgs factory, producing Higgs particles around the clock. A linear collider can tell whether a Higgs is really the Higgs – or whether it is an impostor, sharing some of its properties and not others. Is it the first of a family? Does it provide a portal to a world beyond? We need more experiments to know for sure.
Every physicist knows Faraday’s famous answer to Gladstone, when asked about the utility of electromagnetism: “Sir, I do not know what it is good for. But of one thing I am quite certain – some day you will tax it.” So why study the Higgs? To a physicist, the answer is clear: because it represents a new form of matter. It is something entirely different, fundamental to the world in which we live. But will it be useful? Again invoking Faraday: “It may be a weed instead of a fish that, after all my labour, I may at last pull up.” Time will tell, but I’d place my bet on the fish.