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Ops! December 8, 2006

Posted by gordonwatts in physics.

Look what arrived just last week — a paper on heavy flavor production in W + Jets events. Dang. A W boson along with jets is the largest background for almost all of the high profile top quark analyses coming out of the Tevatron (yes, there is a value judgement in that statement). Top Mass measurement, top quark pair production cross section measurement, and single top.

In order to separate top quark events from other events we employ a so-called b-tagging algorithm. Top quark events all have a b-quark in them: the top quark decays to a W boson and a b quark nearly 100% of the time. W boson plus jet production, however, almost never has a b-quark. So this algorithm to look for b-quark jets is a fantastic separator — tagging top events and rejecting W boson plus jets events.

The fly in the ointment is that some W boson plus jets events do contain a b-quark. The question is: how many and what kind!? This is an amazingly important question because when our b-tagging algorithm accepts events with b-quarks in them it will amplify the # of W boson plus jets with a b-quark relative to the rest of the W boson plus jet events.

This paper describes a new calculation. The calculation predicts a x2 more b-quarks compared to the older calculation. This means our W boson plus jets background could be quite a bit larger than we expected. That could affect the results we are about to release!

Actually, it turns out not. We (and CDF) have known for a long time that the b-quark description of data isn’t right. Indeed, in this analysis we apply a scale factor to fix that problem which is close to this predicted x2.

When we get near releasing one of these results everyone gets nervous. Once the analysis enters the review process (which happened several months ago) it becomes a freight train. It is possible to derail it, but as long as that doesn’t happen it just picks up more and more speed. As you get closer and closer to the end more and more people in the collaboration are looking hard at the results. And anyone can ask a question in the collaboration, and anyone can put the brakes on. So we end up spending a lot of our time trying to anticipate questions. Holding our breath. And hoping that a paper likes this appears — or a paper that points out something that we’ve not taken into account!


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