[UPDATE 10/23/11: Blog traffic indicates that far more of you are reading this post than the one that follows. That's too bad, because in my opinion the more interesting information was in the second talk from CMS on the subject, not the first. But also it's too bad because you're not reading my very clear statement of what this small excess does and doesn't mean. So I've decided to copy that statement into this post. Here is my view, as stated 10/20/11.
But before we begin, maybe I should make my own opinion perfectly clear to the reader. How high does this story rate on the scale?
- Particle physicists perhaps should be interested, maybe even intrigued, but definitely not excited. As do most small excesses, this one will probably disappear as more data becomes available.
- Other scientists should basically ignore this. The excess will probably disappear soon enough.
- I can't see why the general public should pay any heed to this, as the excess will probably disappear --- with the exception of those who are specifically curious as to why particle physicists are paying close attention. Those of you who are in this category have a nice opportunity to learn why multi-lepton searches are a powerful, though tricky, way to look for new physics.
I go on in that post to talk about why I think this; you can read about it there. So -- buyer beware... The original post now follows (and please note words like "minor" and "somewhat". They are not there by accident. Bloggers: .)]
Finally, something at the Large Hadron Collider (LHC) that does not seem to agree that well with the predictions of the equations of the Standard Model of particle physics. Of course, we should not be surprised that it has taken a while for even a minor discrepancy of this type to see the light of day; as I emphasized in this article, there is always a tendency, during the early and middle years of an experiment, for the results that agree with expectations to appear first, while the results that don’t agree get extra scrutiny and take longer. For this very reason, many outside the Large Hadron Collider experiments have been waiting with great curiosity for the results of the search for “multileptons”: very rare proton-proton collisions which produce directly three or more of electrons, positrons (i.e. anti-electrons), muons, anti-muons, taus and/or anti-taus. These searches have been noticeably late.
Today, at the supersymmetry workshop at Berkeley, the CMS experiment has finally released some results on this search. And indeed, there is somewhat of an excess of events with three leptons, especially in events with no taus or anti-taus but also perhaps in events with one tau or anti-tau. Not enough for CMS to say anything other than “Observed data are essentially consistent with background expectations; no smoking gun for new physics yet.” But I think this is somewhat more interesting than their conservative statement implies. Here’s the table from their presentation, which I’ve marked with red dots where the number of events somewhat exceeds expectations. Importantly, also, there are very few entries below expectations to balance out these upward excesses; this really seems to be a true surplus. Of course, we absolutely cannot conclude this has anything to do with new physics; first, this still has rather low statistical significance, even combining some entries in the most optimistic way, and second, if CMS had a subtle problem understanding its detector backgrounds, or theorists had somehow missed or miscalculated a background, that would also potentially lead to an excess. But this is clearly something to watch closely over the coming months.
UPDATE: It has been pointed out to me that I might have mentioned that there is one very unusual four-lepton event, with MET>50, HT<200 and no Z. The background to such events is thought to be 0.014 +- 0.005. So this event should not be there. But there’s not much more to say; with this many collisions, occasional weird things do happen. Until we see three or four events like this, we have to assume this one might be a fluke. Remember the magnetic monopole of 1982.
I know something about how this search is done because the two experimental groups that are most involved are from Karlsruhe (whose member Fedor Ratnikov gave today’s talk) and my institution: Rutgers. [Colleagues at UC Davis inform me that one of their faculty and a student were also involved in part of the analysis.] The Rutgers group is headed by professor Sunil Somalwar, who, back in 2008 with his student Sourabh Dube, did a highly regarded comprehensive three-lepton search at the CDF experiment at the Tevatron (the now-closed U.S. predecessor to the LHC.) His extensive experience is being put to use again here at the LHC. Somalwar is assisted on the experimental side by postdoc Richard Gray (who will also be giving a talk about a related search tomorrow, where we may hope for more information) and faculty member Amit Lath and on the theory side by professor Scott Thomas, along with a number of students. One thing that’s important to understand is that this is an extremely complex search. You can get a sense for this simply from the number of entries in the table; obtaining the background expectation for each of the entries is a major chore, and there are dozens of them. And there are many subtleties in understanding backgrounds when one pushes the envelope to study leptons with rather low energies, which is part of the Rutgers’ group’s special expertise.
I’ll try to produce an explanation of why multi-lepton events are interesting places to look for new physics, and why understanding backgrounds in a search like this is tricky, over the next day or so. Stay tuned. UPDATE — a page on multi-lepton events, with support from preliminary pages (some lacking figures at the moment) on jets, taus, and supersymmetry multi-leptons, are now available if you are curious.