Read more: 鈥�Instant Expert 35: The Higgs boson鈥�
So a particle has been spotted, but is it what we were looking for? Checking whether we really have found the Higgs boson predicted by the standard model will be high on the agenda over the coming years, as will looking for clues that it is a more exotic version. That would lift the lid on physics beyond the standard model
Is it a Higgs?
When the LHC powers up again, probably early in 2015, it will produce collisions at a higher rate, and at nearly twice the energy that was possible before the upgrade. This will allow it to probe several properties of the newly discovered particle to check that it really is the particle that gives all others mass.
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One such property is spin. The Higgs is classed as a boson because it is expected to carry an integer amount of spin, putting it in the same category as force-carrying particles such as photons. All bosons so far discovered have a spin of 1; matter-making particles such as quarks and electrons, meanwhile, carry half a unit of spin.
But the Higgs isn鈥檛 just another force carrier. As the particle produced by the background field that gives all other particles mass, it must be able to interact with all other particles regardless of their spin 鈥� something only possible if, uniquely for a boson, its own spin is zero. The evidence for this is now pretty overwhelming, but more precise measurements of the angular distributions of the particles produced in Higgs decays will tell us if anything untoward is going on.
Another crucial question is how the newly discovered particle interacts with W and Z bosons. It is through these interactions that the Higgs is thought to break apart the electroweak force (see 鈥�Higgs boson: Why do we need it?鈥�). Here we are already on more solid ground: the new particle decays to W and Z bosons at roughly the rate predicted for a Higgs by the standard model (see diagram). Further measurements may reveal subtle differences from the standard model, or reveal additional Higgs bosons expected in some extensions of it. But we already know enough to call this a Higgs boson of some sort.
Is it a standard-model Higgs?
If we accept that what we have snared is the Higgs boson, then there is no room for manoeuvre: the standard model predicts everything else about it.
While we are pretty sure the new particle decays into force-carrying bosons as a Higgs should, we are less sure about decays into matter-making fermions. The upgraded LHC should be able to precisely measure these rarer (or better hidden) Higgs decays into bottom quarks, tau leptons and even muons.
The standard model also makes concrete predictions for how the Higgs should interact with the top quark (it cannot decay into the top quark because the top quark is more massive). Any significant deviation would be a sign of new physics.
The most intriguing questions surround the particle鈥檚 mass. In the standard model, the interactions of the Higgs with itself and the particles around it seem to imply it should have a huge mass. But the particle discovered by the LHC is far smaller.
Order can be restored by 鈥渇ine-tuning鈥� the standard model, tweaking things so that two incredibly large numbers almost, but not quite, cancel each other out, leaving the Higgs with a small mass. Many are unhappy with this fix, believing it makes the theory a little unnatural.
A popular proposal to get around this problem is supersymmetry, which extends the standard model via a symmetry between fermions and bosons. This predicts a slew of new particles, a fermion for every boson and vice versa, whose interactions would naturally cancel out those that cause the Higgs mass to balloon (see diagram).
The trouble is that neither the LHC nor any other machine has yet seen any evidence for these particles, or indeed for anything predicted by any other extension to the standard model. If we have a Higgs and nothing else, maybe we have to live with a seemingly unnatural nature. Or maybe we are just missing some subtlety of the standard model itself. Most exciting of all is the prospect of a whole new layer of structure in the universe just waiting to be uncovered.
Unanswered questions
The standard model is a huge success. Yet even with the Higgs as its crowning glory it is incomplete. Gravity is conspicuously absent, and it cannot explain dark matter, discernible in astronomical observations only by its gravitational influence. Then there is the mystery of why there is so much more matter than antimatter, when the standard model predicts there should be roughly equal amounts.
The next steps in particle physics must be to explain these mysteries. We might make dark matter particles in the proton collisions at the LHC, for example, or observe them at one of several experiments hidden away in mines and tunnels far from the cosmic rays that could confuse their results. Alternatively we may observe dark matter indirectly in space by seeing the energetic particles thought to result when two dark matter particles annihilate, for instance at the AMS experiment on the International Space Station.
As for antimatter, experiments at CERN have made and stored it, and we even use it in PET scanners to help diagnose cancer. The LHCb experiment will look for clues to why it is so rare by examining the decays of ephemeral particles created during proton-proton collisions.
Neutrinos might offer some help, too. The three types of this ghostly particle morph from one form to another as they travel through space. Measurements of the degree of this mixing by experiments in China and South Korea suggest that matter-antimatter imbalances might occur among neutrinos, too. This might close the gap between the matter-antimatter difference observed in nature and that predicted by the standard model.
Even weirder, the neutrino may not even get its mass from the Higgs mechanism at all. Because it has no charge, it could be its own antiparticle. If so, its mass could come from interactions with itself rather than with the Higgs. Sensitive underground experiments are looking for very rare nuclear decays that might tell us the answer.
This article appeared in print under the headline 鈥淲hat鈥檚 next?鈥�
