No unknown unknowns

Proton and antiproton collide to produce a top-antitop pair. R is the ratio of the branching fraction of top → W+b to top → W+q, where q is any quark (d, s, b). If there is no background, the number of events with one tag is proportional to 2R(1-R), and the number of events with two tags is proportional to R2.

By studying the properties of known particles and comparing their actual measurements to calculated predictions, scientists can investigate whether there are as yet unknown particle physics processes.

The top quark was discovered at Fermilab in 1995 and, as part of the third and final generation of quarks, completes the quark family. Because of its large mass, it decays before joining other quarks to form hadrons. This peculiarity makes the top quark a unique particle to study. When the top quark decays, the Standard Model predicts that most of the time (more than 99.83 percent) it decays into a W boson and a b quark.

What if the top decays into a W and a b quark less often than expected? To answer this question, CDF scientists looked at events where a top-antitop pair is produced. These events are identified by the presence of a W boson decaying into a charged lepton (electron or muon), a neutrino and several jets. The number of jets originating from a b quark depends upon the number of tops decaying into W+b with respect to the number of top quarks decaying into W+q where q is any kind of quark with charge -1/3 (d, s, b). This ratio is called R. If the Standard Model is correct, then this ratio should be almost one.

Scientists at CDF have looked into the full data set, collected during the 10 years of Tevatron Run II. They measured the number of times the top decays into a W boson and a b quark with respect to the total number of top decays and compared this number with expectations. R is measured as a function of lepton type, the number of jets in the event (three, four, five or more) and identified b-tags (one, two). This measurement was performed simultaneously with a measurement of the cross section for top-antitop pair production (the cross section is a measurement of the number of times an event occurs). The results are shown in the lower plot.

CDF measured that R is equal to 0.94 ± 0.09, which is consistent with Standard Model expectation. This shows there are no unknown processes at the level indicated by the accuracy of the measurement. An important number for physicists is Vtb, which gives the strength of the coupling between the t and the b quark. Making some standard assumptions, |Vtb|= 0.97 ± 0.05, or |Vtb| > 0.89 at the 95 percent credibility limit.

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edited by Andy Beretvas

The fit results for the simultaneous measurement of the top-antitop cross section and R. The X near the center of the graph corresponds to the data. The point with an error corresponds to a Standard Model calculation.
These physicists, all from INFN Pisa, were responsible for this analysis. Top row from left: Pierfrancesco Butti, Georgio Chiarelli. Bottom row from left: Sandra Leone and Federico Sforza.