Three decades of theorising and calculating. Entire careers spent constructing ideas. Nine billion dollars invested in an underground ring that spans two nations. Ten thousand dedicated scientists and engineers looking for the particle physics equivalent of a needle in a haystack. It’s all been leading to this moment. Small wonder that amid bated breath, you can hear a lot of nervous laughter.
“It’s got to be there, damn it!” Nobel prizewinning physicist Frank Wilczek chuckles in his office at the Massachusetts Institute of Technology in Cambridge. He’s talking about supersymmetry, endearingly known as SUSY, a theory that most physicists believe will lead them beyond the standard model of particle physics, the tried-and-true model of how particles and forces interact, and one big step closer to understanding how reality works.
Physicists are doggedly searching for it in the debris of particle collisions from ATLAS and CMS, two experiments at
CERN’s Large Hadron Collider near Geneva, Switzerland. A year into their runs, neither have glimpsed so much as a hint of SUSY particles at masses up to 700 gigaelectronvolts – well within the range theorists expect it to lurk (arxiv. org/abs/1103.1984, arxiv. org/abs/1102.2357, arxiv. org/abs/1102.5290, arxiv. org/abs/1101.1628).
Rumours are spreading of SUSY’s demise, and alternative theories are already waiting in the wings (see box below). But for many physicists like Wilczek, SUSY is just too beautiful to be wrong. “It would be really cruel of nature to get us this far, and have the next step in sight, and then it’s all just a joke on us.”
Supersymmetry suggests that the two basic types of particles that make up our world – fermions, the matter particles such as electrons and quarks, and bosons, the force-carrying particles such as photons and gluons – are merely two aspects of a single particle.
It’s an elegant idea and if correct, could solve some of the most perplexing problems in physics. It endows the elusive Higgs particle, which is believed to be responsible for giving every other known particle its mass, with just the right mass of its own to keep the whole edifice of particle physics from crumbling around us. Without SUSY, the Higgs mass is heavily influenced by the quantum behaviour of the vacuum.
As it interacts with the vacuum’s virtual particles, its mass skyrockets, growing so large that the standard model breaks down. SUSY saves the day – for every virtual interaction that drives up the Higgs mass, there is a svirtual interaction that drives it back down.
Just as importantly, SUSY unifies the three fundamental forces of the standard model, suggesting that electromagnetism and the strong and weak nuclear forces merge into a single superforce at high energies (see diagram).
What’s more, it provides an ideal candidate for the mysterious dark matter that seems to be holding galaxies together, accounting for approximately 80 per cent of all the matter in the universe. It even appears to be an essential ingredient in string theory, physicists’ leading contender for a theory of everything that will finally unite gravity with the other three forces.
No competing theory is able to solve all four problems in one fell swoop. That’s what makes SUSY so compelling and explains why many physicists are on tenterhooks.
Not everyone, though. “I never really believed in SUSY anyway,” says physicist Jonathan Butterworth of University College London, who works on the LHC’s ATLAS experiment.