What is the Higgs?
The Higgs gives all elementary particles their mass. Physics needs the Higgs. Without it the electron, for instance, would be massless. Just as a photon is the smallest amount (or quantum) of light, the Higgs boson is aquantum of the Higgs field, an all-pervading field thought to give elementary particles mass, via the Higgs mechanism.
Theory says that particles gain mass by interacting with the Higgs field: massless particles like photons slip through the field as if it wasn't there, whereas heavier particles such as quarks "feel" the Higgs field as if it were treacle. The Higgs boson is a key component of the standard model of particle physics, which explains all the known particles and forces except gravity.
Is a Higgs discovery technically possible?
Yes. The LHC is searching for the Higgs in the debris of proton-proton collisions. But the Higgs is too short-lived to be seen directly. Instead, its presence is inferred from the particles it decays into, but these particles are also produced by other so-called background processes that occur when protons collide. So, gathering data is the name of the game.
Only enough data can allow the separation of a Higgs signal from background noise. After the December announcement, the LHC shut down for its normal winter break. It then switched on again in April, smashing protons together at a higher energy than before. This high-energy data, combined with improved data analysis techniques, means researchers now have about twice as much information at their disposal than they did in December, says CERN physicist Albert De Roeck.
That is enough to either double the strength of the statistics obtained in December – or negate the December signal as a fluke. The latter would almost certainly be the death knell for the standard model Higgs, as physicists would have exhausted almost all their options. "It does swing the balance one way or another, so you can count on an interesting announcement," says De Roeck.
Wait a minute, so all this fuss could mean the Higgs isn't there at all?
Yes, it could. Perversely, a negative result is as exciting as finding the Higgs. That's because ruling out the Higgs would be a big blow for the standard model of particle physics. There's plenty of evidence from other experiments that the standard model is not a complete theory. Rethinking the Higgs mechanism in light of a negative result could provide strong clues to physics that lies beyond the standard model.
If the Higgs has been found, can everyone at the LHC leave and go home now?
No. The Higgs is the last remaining piece of the standard model – but we already know that there are particles and forces that lie beyond that model. Take gravity, dark matter – thought to make up about 85 per cent of the matter in our universe – and dark energy, which is credited with the universe's mysterious accelerating expansion. None of these can be explained by the standard model.
Dark matter particles, however, might show up at the LHC. These might come in the form of particles predicted by supersymmetry (SUSY), an extension to the standard model that posits a massive super-partner for every known particle. Some SUSY particles have the right properties to be dark matter, among other things.
What's the atmosphere like at CERN?
"I am very tired after two months of little sleep," says De Roeck, one of many who have been feverishly crunching the data in time for the ICHEP conference. "We feel tired but also excited." One thing that make things a little awkward is that the two rival Higgs-hunting experiments – ATLAS and CMS, both housed at CERN – are bound not to share their results with each other, as well as the general public, until Wednesday.
That's very important for the science: if they knew what the other had seen it might bias the results. But it also makes for uncomfortable encounters when ATLAS researchers pass CMS team members in the corridors of CERN. "We look suspiciously at each other, and we smile – and they smile back," says De Roeck.