Perhaps you’ve heard of the Standard Model of Particle Physics. (If you’ve been following Flip’s ongoing series, you’ve certainly heard a lot.) In a nutshell, the Standard Model is a theory describing the fundamental particles of matter and the forces that mediate their interactions, and it’s the theoretical framework in which we at CERN generally understand our experimental data. In the three decades since its formulation, the Standard Model has been extraordinarily successful and accurate in its predictions, and with the notable exception of the SM Higgs boson, all of the pieces to this particle puzzle are already in place.
The Standard Model is so boring.
However, for a variety of reasons that I’ll keep outside the purview of this post, we believe that there’s physics “beyond” the Standard Model. As with most things in high-energy physics (HEP), this has been given a practical acronym: BSM (Beyond the Standard Model) physics. Many BSM theories predict the existence of one or more new (non-SM) particles. Here’s a scandalously abridged list:
- Supersymmetry (SUSY) predicts “superpartners” or “sparticles” corresponding to every current SM particle
- in addition to the three generations of matter particles already observed, there could be a fourth, more massive generation consisting of two quarks and two leptons
- because the SM Higgs just isn’t enough! Little Higgs, Fat Higgs, Composite Higgs, Hidden Higgs …
- W’ and Z’
Theorists are very creative! This list could go on for quite a while…
Given that I’m an experimentalist, however, I’m more directly interested in how one goes about detecting these hypothetical particles. And as an entirely practical matter, the first thing you have to do is actually choose which of these particles you’ll be searching for. This isn’t as simple as it sounds! Here are some general issues to take into consideration when making your decision:
- How much data is needed for a discovery? Or, alternatively, How large is the production cross-section? If your particle has a large production cross-section, it will be produced in copious amounts by the LHC, and therefore less data is required for you to find it. Since the LHC is still relatively young, and the number of collisions recorded by the experiments is only a tiny fraction of the expected total, many analyses — BSM searches especially! — are limited by statistics. We hope to have about 1 fb-1 of data by the end of this year. If you expect to be sensitive to your particle only with 100 fb-1, then you will be waiting an awfully long time…
- Is the particle model-independent? It pays to be general. If your particle is entirely dependent on the viability of a particular BSM model, and that model is subsequently ruled out by experimental data, then where does that leave you? Back at the beginning, looking for a new particle to discover. Furthermore, the more general the particle, the less fine-tuned your analysis will be. Many BSM particles resemble each other, at least in a general sense, so you can imagine a situation where you’re so focused on finding one specific particle that you don’t notice another one lurking in the data.
- Are other people already looking for your particle? Collaboration is essential in a field like ours, but do make sure that there’s room for you on this search before you barge in. Perhaps the other analysts are very territorial. Maybe there are simply too many of them already. Or could it be that you don’t want to share the Nobel Prize when you discover your new particle…? Ask around ahead of time, even email the folks in charge (“conveners”) to find out who is working on what. Given the aforementioned creativity of theorists, there’s sure to be particles still in need of analysts!
- Is it available at The Particle Zoo? Just sayin.
In truth, these are only a few of the many considerations that go into choosing a new particle to search for. From pure theory to pure politics, this decision is not one to be taken lightly! Think carefully before setting your heart on (and devoting many months of work to) your new particle — then go for it.