CDF multi-muons, hmm

November 4, 2008

So I’m way behind the bloggy curve on this one (e.g. here and here), but I thought I would at least read the paper before talking about it.  The CDF collaboration (or two-thirds of it anyway) has submitted a paper to Physical Review D which claims that there are anomalous events with many muons, produced at a distance from the beam, and that these events cannot explained in terms of CDF’s understanding of physics and their detector.

Tommaso Dorigo promises that the paper “is guaranteed to have you fastened to the chair until you are done with its 70 pages”, but really, not so much – it’s actually terribly written, and you will most likely find yourself lost by page 28 or so.  Some of this is presumably because the paper wasn’t allowed to make a positive statement of a discovery, and so could only discuss things that look strange without developing much of an argument.  The paper presents a number of categories of odd events without too much of an obvious connection between them (except for having muon candidates at large impact parameter), and there’s an obvious selection bias in that you don’t hear about distributions that look normal.  It’s chock-full of detector jargon, and as a friend said to me, “it reads like someone dumped an internal CDF note on the arxiv.”

One serious objection I have with the writing style is that it plays somewhat subtle linguistic tricks on the reader.  For example, the events that they do model well are termed “QCD”, while those they don’t are “ghosts.”  Now there is every possibility that the “ghost” events are, in fact, due to particles produced via QCD, and they do discuss this, but after they’ve repeated “we subtract the QCD background” enough times you begin to believe that they know how to subtract all their Standard Model backgrounds perfectly based on data, which is not correct.

There are also amazingly few estimates of systematic uncertainty in the paper, which makes the quality of the numbers somewhat hard to judge.  Almost all the uncertainties you see are statistical.  Well, I assume they are – the paper doesn’t really say.

Do I have any physics comments?  Papers like these are exactly why we HEP experimentalists say that we should have exclusive control of the data – only the collaboration has the full knowledge of the detector, and their reputation is on the line.  In general this means that if a collaboration claims that something is inconsistent with what they know about their detector modeling, you take that extremely seriously.  On the other hand, if a paper explicitly says that there’s the possibility that the effect they’re seeing is not real, you also take that very seriously, and a large fraction of the collaboration seems uncomfortable with even that level of hedging.  Though it’s extremely hard for an outsider to judge (especially given the style of the paper, as I said before), my guesses would be

  • this isn’t a non-Standard Model effect;
  • their modeling of light hadron production is wrong;
  • their understanding of the secondary interactions of particles in the detector material is incomplete.

But, again, I have no special knowledge of any kind, so this is merely a layman’s opinion (albeit one who wasted a bunch of time looking at tracks with large impact parameters in a different detector…)

What would convince me that the effect is real?  First, observation of some electrons in the multi-muon events (it’s certainly possible for new physics to prefer muons to electrons, but not generic); secondly, confirmation by D0.

I should comment (though don’t take this as gospel, I’m just an experimentalist) that there are very strong constraints on whatever this could be already, should it be a new particle.  The proposed production rate is huge – comparable to that of b quarks – and to see those kinds of cross sections at a hadron collider suggests production in a QCD process, and that the new particle is fairly light (much lighter than the top, for example).  If it carries electromagnetic or weak charges, and is lighter than half the Z mass, it would have been seen already at electron-positron colliders; but it needs to couple to muons somehow…

Just for laughs, you can read arxiv:0810.5730, which manages to cite the CDF paper 39 times in six pages while attempting to give a phenomenological explanation for the anomalous events.  In short: there are particles in a mass ratio 4:2:1, which apparently have no SM couplings other than to the tau, and by the way are produced at a 100 nb rate with a huge boost.  Funny!