Six measurements of top quark pair production

The six input σ{tt } measurement from the CDF and DZero experiments, along with the CDF-only and DZero-only combination results and their combination for the Tevatron result.

Fermilab Today has recently reported on several combined measurements. Today we look closely at one of them: the production cross section of a top quark and antiquark by a collision of a proton beam with an antiproton beam, where the energy in the center-of-mass frame is 1.96 TeV.

The aim of combining six different measurements from CDF and DZero is to make the measurement as close as possible to the true value and to have the best possible precision. The results are summarized in the above table. We see that there is very good agreement between CDF (7.63 ± 0.50 picobarns) and DZero (7.56 ± 0.59 picobarns). The combined measurement of 7.60 ± 0.41 pb is also in good agreement with the Standard Model expectation of 7.35 +0.28/-0.33 pb. Here, the production cross section is quoted for top quark mass of 172.5 GeV/c2.

The measurement is difficult for several reasons, yet scientists have achieved a precision of about 5 percent. The main problem was that only one in about 10 billion collisions produced a top quark pair. The data for Tevatron Run II was taken over a long time period from 2002 to 2011, and thus the equipment had to perform essentially perfectly for this time period. Thanks to the Accelerator Division, the instantaneous luminosity of the beams increased over the years with a peak value of 4×1032 cm-2s-1 in April 2010. This is 400 times the original Tevatron design luminosity.

A top quark almost always decays into a W boson and a b jet. The W can decay into a charged lepton and a neutrino or into two jets. Thus for top quark pair production there are, in principle, three different decay modes: two b jets and either two charged leptons and two neutrinos (dilepton); one lepton, one neutrino and two jets (lepton+jets); or four jets (all jets).

CDF reports two measurements in the best mode for measuring top quark pair production, the lepton+jets mode. The first method (LJ-ANN) requires events with at least three jets and applies an artificial neural network to distinguish the signal from the background. The second method (LJ-SVX) uses a subsample of these events, ones that have at least one b jet, which reduces the dominant W+jet background substantially. The DZero measurement proceeds by selecting events with at least three jets and splitting them into many subsamples according to the total number of jets and the number of identified b jets. Scientists used a sophisticated statistical method to separate the signal from the background in the background-dominated subsamples.

It is important to note that the systematic uncertainty from the luminosity is reduced in the CDF lepton+jets measurements by simultaneously measuring the top quark pair and the cross section for the production of a different particle, the Z boson. This method takes advantage of the smaller theoretical uncertainty for Z boson production. Future measurements with the fully integrated luminosity can also use this technique to achieve even better precision and accuracy.

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edited by Andy Beretvas

This CDF analysis was done by Evelyn Thomson (University of Pennsylvania).
These DZero members contributed to this analysis. Top row, from left: Frederic Deliot (CEA, Saclay, France) and Yvonne Peters (The University of Manchester, UK, and DESY, Germany). Bottom row, from left: Christian Schwanenberger (The University of Manchester, UK) and Lisa Shabalina (Georg-August-Universtat Gottingen, Germany).