“It’s such an attractive idea,” says Shears. “But,” she warns, “if that idea is going to be reality, you need to have experimental proof.”
One way to prove that supersymmetry is a true picture of nature is to find some of its predicted superpartner particles at the LHC. But, as yet, there has been nothing. All the results from experiments match up neatly with the Standard Model.
To find new particles, scientists at the LHC accelerate protons to near the speed of light and smash them together. As the wreckage fires out in all directions, it produces an array of particles. The higher the energy of the collision, the more massive the particles that can be formed. So far, the collider has been smashing together particles at a peak energy of around half its maximum output, and has uncovered the Higgs boson, which has a mass of around 125 gigaelectronvolts (GeV).
The masses of the predicted supersymmetric particles are not fixed in the theoretical models. But scientists know that the superpartner of the top quark, for example, should not be much bigger than the Higgs boson.
“Even the best supersymmetry fans would say that if — when — we go to higher energies, we see no sign of anything, and we manage to exclude the superpartner of the top quark up to a few TeV [teraelectronvolts], it’s significantly less interesting,” says Professor Jon Butterworth, who works on the Atlas detector at the LHC and is head of the physics and astronomy department at University College London.
“In my view ... it’s already less credible than it was before the LHC turned on. If we don’t find the top superpartner in the next run, then I think it’s dead, to be honest.”
The LHC is currently switched off, part way through a technical upgrade that will allow proton collisions near the machine’s maximum-rated energy, of 14TeV, by the start of 2015. If supersymmetry is real, then scientists might expect the particles it predicts to show up in the detectors as jets of hadrons (composite particles made of quarks) coming out of the collision that are not balanced, in terms of momentum, by the jets going off in the opposite direction. The missing momentum could be a sign that a neutralino has been created but, because it interacts so weakly with normal matter, it can be detected only indirectly.
“Supersymmetry is a bit late to the party, but I don’t think it’s lost yet,” says Professor Ben Allanach, a particle theorist at the University of Cambridge who works on supersymmetry. “That’s not necessarily a view held by many of my colleagues. Many of the experimentalists — and some theorists — have got disheartened with it and are giving up already. It begs the question: when would I give up if there’s no new evidence?”
Keeping the faith
Allanach says he will wait until the LHC has spent a year or so collecting data from its high-energy runs from 2015. And if no particles turn up during that time? “Then what you can say is there’s unlikely to be a discovery of supersymmetry at Cern in the foreseeable future,” he says.