After a long and occasionally difficult road to turning on, most people are just thrilled that the Large Hadron Collider is up and running smoothly. But already in its young life the LHC has collected enough data to yield impressive physics results. Unfortunately, those have mostly been of the form “we haven’t seen anything new yet.”
One new thing we would like to see is supersymmetry. The two big multi-purpose detectors at the LHC, named ATLAS and CMS, have both done searches for SUSY in the new data and come up empty-handed. That doesn’t mean it’s not there, actually; a “search for SUSY” is typically a search for a particular kind of signal, often in a particular kind of model. There is so much data already that it’s takes time and more than a bit of ingenuity to search through it effectively. But we could have seen evidence by now, and we haven’t.
Here is a paper from CMS, and a paper from ATLAS. There’s also a great blog post describing the results by Flip Tanedo at US/LHC Blogs. If you like your exclusion plots a bit sassier, check out Résonaances, where Jester reproduces a plot from Alessandro Strumia.
Here’s one way of thinking about the results, from the ATLAS paper (via Flip’s blog post). They look at collisions that produce a particular kind of signal — one lepton, jets (collimated collections of strongly-interacting particles produced by quark or gluon decay), and missing energy (indicating particles like neutrinos that aren’t measured by the experiment directly).
Supersymmetry predicts lots of particles and lots of parameters, so you can’t realistically explore the whole parameter space and put it on a single plot. Instead, you fix some parameters and set others to zero, leaving a two-dimensional space of possibilities. In this case, the horizontal axis is a scalar mass parameter, and the vertical axis is a fermion mass parameter, in terms of which everything else is determined (within this highly constrained and frankly unrealistic parameterization).
The solid-color regions are the places where previous experiments (the Tevatron and LEP) had already ruled them out. The dark black line is the new limit from CMS, and the dark red line is the new limit from ATLAS. Everything below those lines is ruled out.
So: the LHC has ruled out a lot of parameter space that was previously allowed for supersymmetry. Which is too bad. Except that it’s very difficult to quantify what one means by “a lot” in this case. There are many different ways to parameterize the theories, and many different searches one can do. Nature could be surprising us; it wouldn’t be the first time. Bottom line: it’s too bad we haven’t found SUSY yet, but there’s certainly no reason to declare it dead.
But supersymmetry might just be out of reach, or completely irrelevant. Theorists have to keep an open mind, and watch what happens as the experimenters push forward. My hope has always been that we’ll discover something that nobody thought of ahead of time — that possibility is very much open!