Supersymmetry holds that all the subatomic particles we know have counterparts that are almost exactly the same, only much, much heavier. But the Large Hadron Collider hasn't found any supersymmetric particles yet, and they're running out of places to hide. Will we have to come up with a new model for subatomics?
The theory of supersymmetry is a favorite of many physicists because it elegantly explains a lot of basic mysteries about the subatomic world. The larger, unstable particles are responsible for the quantum fluctuations that would otherwise force the particles we're familiar with to be much, much more massive than they otherwise are. (This is a short version of the problem - for a more detailed explanation, check out this Nature article, or check out our own primer on undiscovered particles for more on the so-called sparticles.) Some of the supersymmetric particles could also be responsible for dark matter, and supersymmetry could be key to uniting all forces into one all-encompassing theory.
But the Large Hadron Collider has now effectively ruled out masses for the sparticles below 700 gigaelectronvolts, and more and more mass ranges are getting eliminated all the time. This isn't just a practical problem. The tighter the mass range for the sparticles gets, the more physicists have to fine-tune their values to properly account for the observed quantum fluctuations. Scientists are averse to this sort of fine-tuning - indeed, the whole reason SUSY became popular in the first place was because it eliminated the need to fine-tune the mass value for the Higgs boson to resolve the fluctuation problem.
There's a lot of personal investment in the failure or success of supersymmetry, as physicist Alessandro Strumia explains:
"Privately, a lot of people think that the situation is not good for SUSY. This is a big political issue in our field. For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track."
Still, if supersymmetry is ruled out - and there won't be consensus on that until the Large Hadron Collider finishes up its run in 2012 - that's hardly the end of the world for physicists. All it means is that another theory will be needed to resolve the problems with the standard models. Science isn't about being absolutely right - it's about constructing the best theories from the available evidence, and being willing to revise them as new evidence comes to light. Even if that means throwing out one of the most elegant, even beautiful theories physics has ever seen.