When is a b-jet not a b-jet?

May 26, 2006 at 3:02 pm 1 comment

Tommaso recently wrote an excellent account of how to tag jets which contain a b-quark. As he explains, one uses the precise tracking made possible by silicon detector technology to look for sets of tracks which do not point back to the point where the proton and anti-proton collided. This technique is very well established with already two or three variations, and everyone in collider experiments knows something about it.

Another interesting and older technique to pick out jets which may contain a b-quark is to exploit the tendency for b-hadrons to decay semi-leptonically. A jet with a relatively energetic electron or muon inside is a good candidate for a b-quark jet.

Suppose you are making a measurement, of, say, a top-quark cross-section, and you need to understand quantitatively how well your silicon-tracker is helping you to tag b-quark jets. There will be two questions: (1) how often do I get a b-quark jet that is there? and, (2) how often do I get a jet which is not a b-jet (and may very well be a c-jet)?

One approach will be to find jets which are tagged by the silicon method, and then see what fraction of those are also tagged by the lepton method (and vice-versa of course). If you know what’s going on, then you can predict these fractions. Observing the values that you predict confirms your understanding, and one could then assume that your measurement is valid.

But suppose observation does not bear out your expectations! Usually that means you don’t understand something at a technical level, and you can feel frustrated. But there is also the possibility that some of your jets contain something new – perhaps even evidence for new physics! That would be truly wonderful!!

This is not just a series of what-if’s. This happened with CDF data taken in Run I and the team who came across this discrepancy tried hard to investigate the peculiar jets – which they called superjets – to decide whether new physics was present or not. Over the objections of many of their colleagues, they published their results which you can read at hep-ex/0109012.

I find this result remarkable not for the saga, on which I will not comment, but rather because it raises the very real possibility that new physics could arise almost anywhere, if one is alert and willing to keep an open mind. People sometimes pay too much attention to theoretical models and then overlook or neglect what might be there, right in front of them. What they regard as a “control region” or “cross-check” might in fact be the first glimpse of something exciting.

There is a corollary, too. If an anomaly like this is observed in one data set, then it is extremely important to check in another data set for confirmation. In fact, the CDF Collaboration did check their new data and compared the fraction of silicon-tagged jets with a muon to their expectation, and found good agreement. You can read that paper in hep-ex/0512065. Their conclusion is that there is no confirmation of a signal for new physics, in the new data.

It is sad that new physics is not there. But it is good that people did the first analysis, and that they did the second one, too. Now let’s see what the next alert person finds, and hope that it is genuine.


Entry filed under: Particle Physics.

Hello, World! mono-jets!

1 Comment Add your own

  • 1. dorigo  |  May 26, 2006 at 5:39 pm

    It was indeed a stimulating moment when CDF appeared to find a very strange rate of semileptonic b decays.
    For other readers of this blog: Michael and I were both internal referees (godparents) of that controversial analysis. I learned a lot by trying to figure out whether the analysis was wrong or not, and I am happy that the matter has been finally set by the new, higher statistics analysis.



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