Close but not equal

This plot shows the number of events observed in the data and expected various values of R as a function of identified b jets.

One of the most intriguing elementary particles physicists study is the top quark. Discovered at Fermilab in 1995, it is as heavy as a tungsten atom, and its large mass implies that it decays before joining with lighter quarks to form hadrons. This gives scientists the opportunity to study its properties in detail. Our understanding of the Standard Model predicts that most of the time — 99.83 percent — the top quark decays into a W boson and a b quark.

That’s the prediction. Researchers need to make a direct measurement to check it, so scientists at Fermilab and the LHC exploit their data to challenge theory and look for the unexpected.

To answer the question of how often top quarks decay into a W boson and b quark, CDF physicists looked at events in which Tevatron collisions produced a top-antitop quark pair and in which both resulting W bosons decay into an electron (or muon) and a neutrino. The electron and neutrino are accompanied by two additional jets in the final state, producing a peculiar topology.

The total sample consists of 286 events with an expected background of 55 events. (From this sample we measure the top-antitop cross section to be 7.64 ± 0.55 picobarns, which is in excellent agreement with previous measurements.)

As shown in the above figure, scientists at CDF counted how many times one, both or none of the two jets contain an identified b quark. Scientists compare this number with the total number of top decays. The comparison gives them the ratio, R, and if the Standard Model of elementary particles is correct, R should be almost 1. The result is that R is equal to 0.87 ± 0.07. It is tantalizingly close but not equal to the Standard Model expectation. The two largest contributions to the final uncertainty come from the limits in our understanding of the b-tagging efficiency and the statistical size of our sample.

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edited by Andy Beretvas

These physicists were responsible for this analysis. From left: Georgio Chiarelli, Camilla Galloni and Sandra Leone, all from INFN, Pisa.