So, the 2011 run of the LHC is coming to a close, I mean the interesting part ;-). A 5 inverse femtobarn stash of data has been collected by each ATLAS and CMS. These data will by fully analyzed and scrutinized by the late winter 2012, while rumors should start popping up on blogs before the end of this year. One thing that is already clear is that new physics did not jump in our faces, which is hardly a surprise. And neither did the Higgs boson, which is more intriguing. Contrary to what I expected, the 2011 data may not yield a conclusive statement about the Higgs: neither a clear cut discovery nor excluding the entire low mass range appears likely at this point. We can now at least entertain the option, which as recently as last summer was unthinkable, that the LHC will not find the Higgs particle with the properties predicted by the Standard Model. What then?
First of all, it will be fun to watch the CERN management explaining the public that *not* discovering the Higgs is a success. For theorists, on the other hand, the best of all worlds will have been granted. In fact, we already have a deck of cards to play for that occasion, each very interesting as each pointing to exciting new physics within our reach. Here are the 3 main broad scenarios (not mutually exclusive):
- Higgs exists but has a smaller production cross section.
In the Standard Model the Higgs is produced mostly in gluon fusion, via a loop diagram with top quarks. One can easily imagine new particles meddling into Higgs production via a similar loop process; all they need is a color charge and a significant coupling to the Higgs. Thus, in every major new physics scenario modifying the Higgs production rate is possible without stretching the parameters too much. One interesting case is the composite Higgs, where the Higgs cross section is almost always suppressed, typically down to 70-90% of the Standard Model value. For experimentalists this is the simplest scenario, all they need to do is sit and wait a bit longer, and the Higgs will eventually show up. The matter should be sorted out after the 2012 data are analyze.
- Higgs exists but has non-standard decays.
For a low mass Higgs, around 120 GeV, the main discovery channel is the decay into 2 photons. Again, this is a loop process in the Standard Model so it's very easy for new physics to modify the branching fraction for that decay. As in the previous case, one may just sit and wait for the Higgs to eventually show up. However Higgs decays can be easily modified in a far more dramatic fashion than the production rate. For example, Higgs may be invisible, that is decaying into very weakly interacting particles whose only signature is the unbalanced momentum in the event. Or Higgs may dominantly decay into multiparticle final states (some popular model predict decays to 4 tau leptons or 4 b-quarks) and we'll never see the bastard in the diphoton channel. That would be a very interesting scenario not only for theorists but also experimentalist, as it would require clever new methods to spot the Higgs on top of the QCD background.
- There is no Higgs.
That would mean that the mechanism of electroweak symmetry breaking is inherently strongly coupled, somewhat resembling breaking of the chiral symmetry in QCD. This is the most challenging scenario for theorists and experimentalists, and the one that may require a lot of patience. In the optimistic case, the 14 TeV LHC run we will spot a number of resonances analogous to QCD mesons and little by little we'll understand the structure of the underlying gauge theory. But these resonances may well be too heavy or too wide to be efficiently studied at the LHC. Ultimately, we may need to probe the properties of the scattering amplitudes of W and Z bosons where, according to theory, these strong interactions must leave an imprint. The problem is that such a measurement is very non-trivial in the dirty LHC environment (the SSC or a linear collider would be a different story), so we may need some new theoretical or experimental ideas to make the progress. It's probably too early to bet large amounts on this scenario (which is currently disfavored by electroweak precision data) but if no hint of the Higgs is seen by the end of 2012 that will become the most promising direction.