Top quark production cross sections

This analysis selects double top quark events that decay into four quark jets, an electron or muon, and a neutrino. After collecting a pure signal sample (the red part of the plot), the production properties of these events are extracted, including the cross section versus the top-antitop mass.

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It is now almost 20 years since the top quark was discovered in 1995 through the joint efforts of the CDF and DZero collaborations. As more data have been collected and new techniques developed, the properties of this most massive quark have been measured with increasing precision. Now, about two years since the last proton-antiproton collisions at the Tevatron, the DZero collaboration is releasing its final state-of-the-art results on a range of top quark measurements. This week we take a look at a new publication covering differential cross section measurements of top-antitop (double top) production.

At the Tevatron, top quarks are mainly produced in quark-antiquark pairs through the annihilation of two lighter quarks from the original proton and antiproton. This process is well understood in the Standard Model, which predicts both how often such events occur and their properties. However, there are many other models in which these characteristics change, and so analyzing the production of double top events is an important and sensitive way of searching for evidence of physics beyond the Standard Model.

The cross section of a process is a measure of its probability of occurring for particular collision conditions. If this is determined as a function of some parameter, such as the momentum of the top quark, then it is called a differential cross section. Such measurements are challenging, requiring precise knowledge of the number of initial proton-antiproton collisions. Furthermore, the measured distributions need to be corrected to account for the detector efficiency for recording and reconstructing these events, as well as for the experimental resolution, in a process called unfolding.

For this analysis, one of the top quarks must produce a final state of three quark jets, at least one of which must correspond to a bottom quark. The second top quark must produce another bottom quark jet, an electron or muon, and an undetected neutrino, which carries away some energy from the system. This complicated final state, with six distinct objects to be identified and reconstructed, requires information from all parts of the DZero detector and takes advantage of the mature and well-tested particle identification algorithms.

The final results include the double top cross section as a function of the top quark transverse momentum and rapidity (effectively the direction of the quark with respect to the initial proton beam). The results agree with the predictions of the Standard Model and are the most precise to date. Since cross sections depend on both the type and energy of the colliding particles, these results will remain uniquely relevant for many years to come.

Mark Williams

Andreas Jung of Fermilab was the primary analyzer for this measurement.
The year 2013 saw 12 DZero graduate students complete their Ph.D.s, representing all six physics sub-groups within the collaboration. Our congratulations and best wishes go to all these newly minted doctors as they start the next stage of their careers, whether staying at DZero or moving elsewhere.