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Run top quark run

The CMS collaboration has measured for the first time the variation, or “running”, of the top-quark mass


A candidate event for a top quark–antiquark pair recorded by the CMS detector.
A candidate event for a top quark–antiquark pair recorded by the CMS detector. Such an event is expected to produce an electron (green), a muon (red) of opposite charge, two high-energy “jets” of particles (orange) and a large amount of missing energy (purple) (Image: CMS/CERN)

Dive into the subatomic world, into the heart of protons or neutrons, and you’ll find elementary particles known as quarks. Measuring the mass of these quarks can be challenging, but new results from the CMS collaboration reveal for the first time how the mass of the top quark – the heaviest of six types of quarks – varies depending on the energy scale used to measure the particle.

The theory of quantum chromodynamics, a component of the Standard Model, predicts this energy-scale variation, known as running, for the masses of all quarks and for the strong force acting between them. Observing the running masses of quarks can therefore provide a way of testing quantum chromodynamics and the Standard Model.

Experiments at CERN and other laboratories have already measured the running masses of the bottom and charm quarks, the second and third heaviest quarks, and the results were in agreement with quantum chromodynamics. Now, the CMS collaboration has used data from high-energy proton–proton collisions at the Large Hadron Collider to chase out the running mass of the top quark.

The CMS physicists looked for how often pairs of particles comprising a top quark and its antimatter counterpart were produced in the collisions. They did this measurement at three different energy scales, between about 400 GeV and 1 TeV, and then compared the results with theoretical predictions of the top quark–antiquark production rate. From this comparison, they obtained the top-quark mass at those three energy scales.

The result? The top-quark mass does seem to run as predicted by quantum chromodynamics – that is, it decreases with increasing energy scale. However, the result is based on only three experimental data points. More data points, as well as improved theoretical predictions, should be able to tell with more precision whether that’s indeed the case.

Find out more on the CMS website.