About a year ago, the CMS collaboration released its first publication on studies of the top quark – the measurement of the tt production cross-section at 7 TeV. The measurement was based on a data set of only 3 pb–1 of integrated luminosity and the top quarks were identified through the leptonic decay channels of the W boson. Now, a plethora of results on the top quark based on luminosities of 1–2 fb–1 have been released for the summer conferences, in particular for the TOP2011 workshop, held at the end of September at Sant Feliu de Guixol, Spain.
Top quarks decay almost exclusively into a W boson and a b-flavoured quark jet, leading to different event final states that can be used for selecting tops. Figure 1 gives an overview of the CMS results, which use more or less all of the decay modes. The most precise single measurement is the analysis where one W boson decays into leptons while the second W decays into hadrons and b-quark identification is used, giving a cross-section of 164.4 ± 14.3 pb, i.e. a precision of 8.5%. Precise measurements of the cross-section can also be converted into measurements of the top quark’s mass, within a given theoretical scheme. Currently, the CMS cross-section measurements allow for a precision on the top mass of about 7–8 GeV in such data extractions.
Further new analyses include a measurement of the difference in mass of the t and t, which is an interesting test of CPT invariance. For this study, data are used where one of the W bosons decays into a muon, allowing the event to be classified as t or t decay, depending on the charge of the muon. The difference in mass between the t and t is found to be 1.2±1.3 GeV, i.e. the result is compatible with equal mass within the uncertainty. This is the most precise result on this quantity to date.
Another interesting measurement concerns the charge asymmetry in top production. The experiments at Fermilab’s Tevatron reported asymmetries that are larger than expected. At the LHC, tt production is also slightly asymmetric in rapidity as a result of the different roles that the valence and sea quarks play in the production. CMS has studied this asymmetry by measuring the different widths of the rapidity distribution for t and t. The result gives an asymmetry of 1.6% with an uncertainty of about 3.5%; an asymmetry of about 1.3% is expected from theory. The agreement with the Standard Model is good within the measured uncertainties.
Finally, a challenging new measurement on the electroweak production of single top has been undertaken, namely tW associated production. While single top production in the top-quark channel was reported by the LHC experiments earlier this year, this measurement analyses a different final state; also, this channel is not accessible at the Tevatron. CMS finds an excess over expected background events with a significance of 2.7 σ, and is compatible with the expectation for tW production.
With several tens of thousands of top-quark pairs recorded so far, the detailed study of the properties of the heaviest quark is merely starting. Results based on the full 2011 data sample should be ready in time for the 2012 winter conferences.
Further reading
For more information, see the CMS papers:
CMS-TOP-11-003.
CMS-TOP-11-008.
CMS-TOP-11-014.
CMS-TOP-11-019.
CMS-TOP-11-022.
