The top quark is the heaviest point-like particle known. It weighs about as much as an atom of tungsten yet is an elementary building block of the Standard Model of particle physics. Its mass is one of the model’s important parameters and is directly related via radiative corrections to the masses of the W and Higgs bosons. Precise knowledge of the top quark’s mass is therefore extremely valuable to constrain theoretical models.

The CMS collaboration has measured the top-quark mass by exploiting all possible final states originating from different decays of W bosons produced in the decays of top quarks. Final states where the W boson decays into leptons are particularly "clean" (see figure). Such events are selected by requiring energetic jets in the central region of the CMS detector, of which at least one must be compatible with originating from a bottom quark ("b-tagged jet"), together with one or two isolated and high-energy leptons. The selected samples are extremely pure in top-quark-pair events, with estimated purities greater than 95% for events containing at least one electron or a muon.

For hadronically decaying W bosons, the reconstruction techniques make use of kinematic fits to improve the energy resolution and the likelihood methods that can handle the combinatorial ambiguities in finding the triplet of jets corresponding to the top-quark decay. The use of b-tagging helps considerably in constraining these ambiguities further. For dilepton events, the presence of two neutrinos accompanying the charged leptons from the W-boson decays requires alternative techniques.

All of the methods and channels used give consistent measurements of the top-quark mass. The results are now fully dominated by uncertainties other than statistical, with major contributions coming from the uncertainty associated with the jet-energy scale and how well the Monte Carlo simulations model the details of the top decay. The best single measurement of the mass of the top quark, from the e/μ+jets channel, results in a statistical uncertainty of 0.4 GeV and a systematic uncertainty of around 1 GeV.

The combined CMS measurement, accounting for correlations between uncertainties obtained in the individual channels, yields a total uncertainty of about 1 GeV. This result is already competitive (and in agreement) with the combined measurement from the CDF and DØ experiments at Fermilab’s Tevatron, as the figure shows. For a further reduction of the uncertainty, it will become important to employ novel measurement techniques.

The CMS collaboration has also measured the difference in mass between the top quark and its antiquark – an important test of the symmetry between matter and antimatter. This is done by splitting the sample of events with e/μ+jets into two subsamples with opposite lepton charges. The difference in quark–antiquark masses is compatible with zero with an uncertainty of about 0.5 GeV. This is the best precision on this mass difference to date.

After more than 15 years of precision top physics at the Tevatron, the baton in the race to understand nature’s heaviest quark has now passed to the LHC. With an uncertainty on the top-quark mass of 1 GeV, CMS is now at the forefront of precision physics in the top sector.