“Within a year or so we will be able to discover the Higgs boson or to exclude it in its simplest form,” says Guido Tonelli, lead scientist of one of the two main Higgs-hunting projects, called the CMS. “Both results will change our current understanding of nature. A positive discovery will give us an elegant explanation of the origin of mass of elementary particles. But if we can instead exclude the Higgs in its simplest form, we'll need to rewrite our books.”
That is true. But it doesn’t dispel a certain nervousness. The £2.6bn LHC was planned and built in a climate of confidence about the existence of the Higgs boson – but at this point there’s no credible sign that it has been seen.
This particle provides the last remaining piece of the theory known as the standard model, which explains all of the particles currently known. This theory states that two of the four forces of nature were once the same force, but split shortly after the Big Bang to become the electromagnetic force – the familiar force associated with magnetism and electrostatic attraction – and the weak nuclear force, an interaction involved in radioactive decay. Most physicists suspect it should ultimately be possible to unify all four of the fundamental forces – to show that not only these two but also gravity and the strong nuclear force, which binds the nuclei of atoms together, were once all the same force very early in the Big Bang.
When two objects feel a mutual force, in effect a particle passes between the two. For the electromagnetic force, these are particles of light, little packets of energy called photons. For the unified electromagnetic-weak (electroweak) force, there are also associated particles called W and Z bosons, discovered in the 1970s. But W and Z bosons have mass, whereas photons don’t. The question is where, when the electroweak force split in two, this mass came from.
The answer, according to British physicist Peter Higgs, is that there is a force field, now known as the Higgs field, that gave them mass. (Higgs wasn’t the only one to have this idea, but it has become associated with his name.) W and Z bosons feel the Higgs field, and get dragged back by it – they acquire inertia, and thus mass – whereas photons don’t. It’s not just the weak-force bosons that get their mass this way; many other fundamental particles do too. And like other force fields, the Higgs has a particle associated with it: the Higgs boson.
But there is no way of predicting how massive a Higgs should be. At this scale, particle masses are typically measured in billions of electronvolts (gigaelectronvolts, GeV). Previous results have narrowed the possible mass to a window between about 114-150 GeV, and at the moment the money is on a value of around 129 GeV.
So the hunting of the Higgs is being squeezed into an ever smaller space, and still there’s no scent. That’s why it is now no heresy to be wondering if the thing just ain’t there. Two separate experiments at the LHC are searching for it: CMS and ATLAS, with detectors situated on opposite sides of the circular tunnel. They use slightly different methods to search for new particles produced in the collisions. Some say there’s a chance we’ll have the answer by Christmas. Others agree with Cern’s press officer James Gillies that “I think it’s safe to say that by this time next year we'll know.”
What if the Higgs doesn’t exist? There are alternative theories, but they are even harder to fathom than Higgs’s solution. Take the so-called technicolor theory. This offers a way for W and Z bosons to get mass without a Higgs, by invoking other types of particle. It doesn’t sound like much of a gain, but technicolor could also help explain the nature of the mysterious ingredient of the cosmos called dark matter.
Alternatively, you could try braid models. Here the allegedly basic particles quarks and leptons (such as electrons) are composed of smaller components called preons which are themselves braided strands of spacetime itself. The Australian particle physicist Sundance Bilson-Thompson has shown how braided preons could allow the standard model to emerge out of a theory that unifies gravity (in other words, Einstein’s general relativity) with quantum theory. This theory is called loop quantum gravity, and it represents spacetime as a chainmail network of loops made from the gravitational field itself: a kind of quantum macramé.
If we do end up needing an alternative to the Higgs, the problem is that there are in fact too many of these mind-boggling candidates, and no simple experiment to winnow them. That will be messy for physics. But the win-win defence of the LHC remains valid; indeed, a vanishing Higgs would be the most interesting result of all.
Philip Ball’s latest book is Unnatural: The Heretical Idea of Making People. www.philipball.co.uk