In August I moved away from CERN, and I’ve been back and forth between CERN and Brussels quite a lot since then. In fact right now I’m sitting in the building 40 where people go to drink coffee and have meetings, and I can see the ATLAS Higgs Convener sitting on the next table. All this leaves me feeling a little detached from what is really happening at CERN, as if it’s not “my” lab anymore, and that actually sums up how many people think about particle physics at the moment. With LHC Run I we found the Higgs boson. It was what most people expected to see, and by a large margin it was the most probable thing we would have discovered. Things will be different for Run II. Nobody has a good idea about what to expect in terms of new particles (and if they say they do have a good idea, they’re lying.) In that sense it’s not “our” dataset, it’s whatever nature decides it should be. All we can do is say what is possible, not what is probable. (Although we can probably say one scenario is more probable than another.)
The problem we now face is that there is no longer an obvious piece that’s missing, but there are still many unanswered questions, which means we have to move from an era of a well constrained search to an era of phenomenology, or looking for new effects in the data. That’s not a transition I’m entirely comfortable with for several reasons. It’s often said that nature is not spiteful, but it is subtle and indifferent to our expectations. There’s no reason to think that there “should” be new physics for us to discover as we increase the energy of the LHC, and we could be unlucky enough to not find anything new in the Run II dataset. A phenomenological search also means that we’d be overly sensitive to statistical bumps and dips in the data. Every time there’s a new peak that we don’t expect we have to exercise caution and skepticism, almost to the point where it stops being fun. Suppose we find an excess in a dijet spectrum. We may conclude that this is due a new particle, but if we’re going to be phenomenologists about it we must remain open minded, so we can’t necessarily expect to see the same particle in a dimuon final state. It would then be prudent to ask if such a peak comes from a poorly understood effect, such as jet energy scales, and those kinds of effects can be hard to untangle if we don’t have a good control sample in data. At least with the discovery of the Higgs boson, the top quark, and the W and Z bosons we knew what final states to expect and what ratios they should exhibit. There’s also something a little unsettling about not having a roadmap of what to expect. When asked to pick between several alternative scenarios that are neither favoured by evidence nor disfavoured by lack of evidence it’s hard to decide what to prioritise.
On the other hand there is reason to be excited. Since we don’t know what to expect in LHC Run II, anything we do discover will change our views considerably, and will lead to a paradigm shift. If we do discover a new particle, or even better, a new sector of particles, it could help frame the Standard Model as a subset of something more elegant and unified. If that’s the case then we can look forward to decades of intense and exciting research, that would make the Higgs discovery look like small potatoes. So the next few years at the LHC could be either the most boring or the most exciting time in the history of particle physics, and we won’t know until we look at the data. Will nature tantalise us with hints of something novel, will it give us irrefutable evidence of a new resonance, or will it leave us with nothing new at all? For my part I’m taking on the dilepton final states. These are quick, clean, simple, and versatile signatures of something new that are not tied down to a specific model. That’s the best search I can perform in an environment of such uncertainty and with a lack of coherent direction. Let’s hope it pays off, and paves the way for even more discoveries.