Some Comments on Theory and Experiment at the LHC

In the next couple of days I hope to update a post I put up a short time ago, one that, as of today, 8/29/11, still holds true.  The issue that I addressed in that post was : What does the Large Hadron Collider (LHC) currently have to say about Supersymmetry?  I took a slightly polemical point of view, but you can look at the links in the post to longer, more pedagogical articles to see where the point of view comes from.

The problem with trying to answer a question like this one — What do LHC results imply for Theory X — is that it is ill-posed. An experiment searches for a phenomenon — not a theory. If a theory always predicts a certain phenomenon, then an experiment can search for that phenomenon, and, in finding it or not, give a definitive thumbs-up or -down to the theory. But often a theory has many versions, and although it will have a general set of predictions — supersymmetry, for instance, predicts superpartner particles — its details can look quite different to an experiment, depending on the version. [For instance, in supersymmetry, whether or not one sees the classic supersymmetry signature depends on the masses of the superpartner particles, on whether there are extra types of particles that are not required by the theory but might just be there anyway, etc.] So any given experimental result is just one important but incomplete piece of information, one that constrains some versions of the theory but not others. Typically, to entirely rule out a general theory like supersymmetry requires a large number of experimental searches for many different phenomena.

Conversely, though, many different theories may predict the same phenomenon. So an experiment that looks for but fails to find a certain phenomenon doesn’t just rule out just one version of one theory. It typically rules out several versions of many different types of theories.

And if it does find that phenomenon, it does not tell you which of those different types of theories it comes from. Remember that, when the LHC makes its first discovery! You’ll likely see claims from physicists in the press about what the discovery means that aren’t really merited by the data.

Crudely, this problem is much like the genotype-phenotype problem in biology. [More technically, the problem is that the mapping from theories to phenomena is highly non-linear and neither one-to-one nor onto, for those who know those terms.] Suppose you want to look for signs of bird genes on a deserted island. You might design an experiment to look for things that fly. But that’s problematic, of course.   Many different genetic codes can produce the phenomenon of flight — bat genes, hawk genes, dragonfly genes. So if you see something unfamiliar flying, you don’t know, without more information, whether its genetic code has anything to do with that of birds. Conversely, if you find nothing airborne, you’ve constrained not just bird genes but also those of many insects and various mammals. And yet you haven’t ruled out birds, or mammals, or insects, because some of them don’t fly. You’d be quite embarrassed to tell your scientific colleagues you’d proved there were no bird genes on the island, only to have a penguin or kiwi wander out from behind a bush.

This is the situation at the LHC.  This summer’s results were recently billed as “dashing hopes” of supersymmetry, and other such.  [I am glad that the BBC has adjusted its news webpage mini-headline for this article to read “LHC puts supersymmetry in doubt”, though I should point out it was in some doubt before the LHC too.]  But in fact (a) many versions of supersymmetry are not excluded by current LHC results, and, equally important, (b) many versions of other theories are excluded by these results.

Unfortunately this complexity is tough on reporters, and frankly on me too, because it is hard to summarize this kind of information in a short article. The relation between the empirical and the theoretical is very complicated, and hard to explain well.  I’ve given you a little taste of this in my articles on supersymmetry and what it predicts.

In the better press articles, such as this one, the LHC results have been reported as a failure to find the simplest version of supersymmetry, implying that more complicated versions must be necessary if supersymmetry is a part of nature.  Such a statement might be a bit premature, but is roughly true. However, I’m not sure the reporters (or all the physicists involved) understand what “more complicated” means. All that is needed for supersymmetry to evade the strongest current results from the LHC is one additional particle (and its superpartner). [In fact in some cases the number of additional particles required is zero, in the sense that the gravitino, the superpartner of the graviton, can be enough to muck things up.] Yes, that’s a bit more complicated than the simplest model. But not much. The theory already has (depending on how you count) several dozen particles and their superpartners; does one more make the theory so much more inelegant as to deserve disdain?

I will try, over the coming couple of weeks, to take stock of what the LHC has and hasn’t said about supersymmetry and some other theories. But before I do that in any detail, I can already tell you the basic answer. Recent LHC results put powerful and interesting constraints on the possibility of super-birds, extra-bats and little-insects. But conversely they do not exclude super-birds, extra-bats, or little-insects.

Thanks to the fantastic performance of the LHC and the great work done by the physicists on the ATLAS, CMS and LHCb experiments, we know a great deal more about what nature might and might not be like than we did last year. Nevertheless, because many types of experimental analyses of the LHC data have not yet been done, and because, for some questions, a lot more data is needed, we do not yet have nearly enough information to make any new, sweeping, existential claims about the nature of nature.  Patience; a thorough search that leads to definitive knowledge takes time.

17 responses to “Some Comments on Theory and Experiment at the LHC

  1. Amen 🙂

    I look forward to furter posts on this topic.

    In the light of what Lumo discusses here,

    I like to what the still allowed higgs mass range seems to converge at the moment, as explained by Phil Gibbs in his post “Higgs exluded from 130 to 480 GeV” here

    Such a light higgs would be not bad for SUSY I`ve learned 🙂


    • I have known Gordy Kane for many, many years, and I like him very much. He’s a very good scientist, and a good friend. I have also seen his predictions for supersymmetry switch with the winds… “just this moment, as data rules out the last thing we said, we’ve finally figured out what’s *really* going on, and it’s just out of reach…”. So. He could get lucky this time.

      As for what Phil Gibbs has to say — there is such a thing as digging for gold in a sandbox. Personally I think there’s not enough data yet for the conclusions he draws. For a person not directly involved with the data to informally combine such complex results as the Higgs search is fine, as long as he doesn’t truly believe what he is doing. For him to say that a 140 GeV Higgs is ruled out at this point is … well … look, of course he might be right, but if he turns out to be right it will be for reasons that he can’t possibly know right now.

      More importantly, as I have described in my Higgs-hints article,

    • , there are simple two-lepton/two-antilepton and two-photon analyses that don’t yet have enough data, and there are subtle lepton/anti-lepton/invisible-particle searches, both playing an important role in the current limits. It is very possible that the latter search is not being done properly, either because of theory issues or because of experimental issues. Should this turn out to be true, or should arguments ensue about those searches that cannot be resolved, you might have to reduce their weight. If you did that, the limits around 140 GeV would weaken considerably.

      You can play whatever statistical games you want, but nothing you do will turn insufficient data into knowledge.