John Ellis, a particle theorist at Cern and King’s College London, has been working on supersymmetry for more than 30 years, and is optimistic that the collider will find the evidence he has been waiting for. But when would he give up? “After you’ve run the LHC for another 10 years or more and explored lots of parameter space and you still haven’t found supersymmetry at that stage, I’ll probably be retired. It’s often said that it’s not theories that die, it’s theorists that die.”
Ellis counsels his colleagues, some of whom he admits are getting spooked by the lack of evidence for supersymmetry, to have patience. “There can be a big gap between a proposal of a theoretical idea and the experimental confirmation. Let’s not forget that the Higgs boson was proposed in 1964 and it took 48 years for it to be discovered,” he says. “Realistic supersymmetric theories probably date back to 1973, so we’ve only been looking for supersymmetry for 40 years. So keep the faith.”
Theorists are well aware that anything they come up with on paper, scientifically speaking, is subject to confirmation by experiment. “Theorists never take anything for granted — your theory is just as good as anybody else’s, until it’s been proven wrong,” says Shears.
But if the LHC doesn’t find any compelling proof for supersymmetry in the next few years, physicists will be left with some uncomfortable possibilities. It could be that the accelerator is just not powerful enough to produce the new particles, and a future accelerator will discover them. Or it could be that we do not live in a supersymmetric universe.
“Then,” says Shears,” we have the third alternative, which is going to be the most frustrating of all, which is that we could live in a supersymmetric universe and just never know.”
In that case, what fills the theory gap? “If nothing else shows up — we’ve got a Higgs and nothing else — then it’s not at all obvious what the next experiment ought to be,” says Butterworth. In other words, if supersymmetry doesn’t work out, theorists do not have a ready alternative to take its place.
If the worst happens, and supersymmetry does not show itself at the LHC, Allanach says it will be a wrench to have to go and work on something else. “I’ll feel a sense of loss over the excitement of the discovery. I still feel that excitement and I can imagine it, six months into the running at 14TeV and then some bumps appearing in the data and getting very excited and getting stuck in. It’s the loss of that that would affect me, emotionally.”
Ellis, though confident that he will be vindicated, is philosophical about the potential failure of a theory that he, and thousands of other physicists, have worked on for their entire careers.
“It’s better to have loved and lost than not to have loved at all,” he says. “Obviously we theorists working on supersymmetry are playing for big stakes. We’re talking about dark matter, the origins of mass scales in physics, unifying the fundamental forces. You have to be realistic: if you are playing for big stakes, very possibly you’re not going to win.