Top of the world

This week saw a historic collaboration between scientists from CERN and Fermilab, as the world combination of the top quark mass was reported, using inputs from the ATLAS, CDF, CMS and DZero collaborations.

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The top quark has only ever been produced and detected in two places on Earth: at Fermilab, where the DZero and CDF collaborations first discovered this most massive particle in 1995, and at CERN, where the Large Hadron Collider generates so many top quarks that the ATLAS and CMS experiments are often described as top factories. Now, in an historic act of trans-Atlantic collaboration, analyzers from all four of these experiments have worked together to combine their measurements and produce the most precise determination of the top quark mass.

The act of collating and averaging measurements from different sources is very common in high-energy physics. In fact there is a dedicated organization charged with doing just this, the Particle Data Group. However, combining the various top quark mass measurements is nontrivial, because many of the procedures and inputs are shared between different experiments. This means that a naive average of the different results is no longer optimal. To illustrate this, consider a group of measurements of some object’s weight, using a set of scales. If the scales under- or overestimate the weight every time, this will affect all the measurements in the same way. The uncertainty on the result caused by the calibration of the scales will remain unchanged however many measurements are performed.

This world combination, a year in the making, takes a total of 11 individual measurements from all four experiments (six from the Tevatron and five from the LHC), and performs a weighted average that properly accounts for all correlations between different parts of the analyses. All measurements use top-antitop events in one of four possible decay patterns: lepton + jets, dilepton, all jets, or missing transverse energy + jets. A significant challenge is separating the uncertainty on every measurement into standardized categories, as well as assessing their correlations both within each experiment and between the different collaborations and accelerators. Scientists test different assumptions of the correlations to ensure that the results are stable with respect to these tricky decisions.

The combination is designed to minimize the total uncertainty on the top quark mass, and it gives a final result 173.34 GeV, with a total uncertainty of just 0.76 GeV, or around 0.4 percent. This is the most precise evaluation of the top quark mass ever made and the first time that the full global combination has included a complete assessment of the correlations. Reaching such excellent precision is important, as the top quark mass plays a vital role in the Standard Model of particle physics and also has cosmological implications through the “stability” of the universe. Given that collaborations from both colliders continue to work on improved individual measurements, it looks like there is plenty more trans-Atlantic collaboration to come, which bodes well for the future precision of this and other fundamental Standard Model parameters.

Mark Williams

Measurements from the CDF and DZero experiments represent the Tevatron in the top mass combination.
The LHC top mass measurements came from both ATLAS and CMS collaborations.