Measurements of top-quark properties were among the many new results shown by the ATLAS collaboration at Lepton-Photon 2011. The very large mass of this quark relative to the others leads many physicists to believe that it plays a special role in physics beyond the Standard Model.
At the luminosity recently achieved at the LHC, a top quark is produced on average approximately every second. Because of the large number of top quarks produced and the excellent detector performance, the ATLAS experiment is able to measure precisely the quark’s properties, thereby providing stringent tests of the Standard Model as well as probing for the subtle effects of new physics. So far all measurements are consistent with the Standard Model, but further data will bring increased precision and with it greater sensitivity to new effects.
ATLAS has measured the production cross-section of top pairs in the single lepton decay channel to be 179±12 pb. This precision of 7% is better than the uncertainty on the theoretical prediction, providing an excellent testing ground for perturbative QCD. A combination of the measurements in the different channels will further increase the precision.
Electroweak production of single top quarks is sensitive to the element Vtb of the quark-mixing matrix and also to a potential flavour-changing-neutral current component in top-quark couplings. ATLAS has measured top production in the t-channel – first measured only a couple of years ago at the Tevatron – with a significance exceeding 7σ. ATLAS has also placed limits on the production of single top in the s-channel and the Wt final state, laying the groundwork for the eventual measurement of these processes.
As a result of the excellent calibration of the detector, ATLAS has also measured precisely the mass of the top quark at 175.9 GeV, with a total uncertainty of just 2.8 GeV. This precise measurement, together with the W mass and electroweak radiative corrections, implies that the Higgs boson is lurking at low mass – if it is indeed a Standard Model Higgs.
ATLAS has further probed for new physics with the most precise measurements to date of the fraction of longitudinally polarized W bosons in the decay of top quarks and of the degree of spin correlation. The results of these measurements are consistent with Standard Model expectations, as are those of production asymmetries similar to those recently reported to be anomalous at the Tevatron.
Further reading
For ATLAS results presented at Mumbai, see https://twiki.cern.ch/twiki/bin/view/AtlasPublic/AtlasResultsEPS2011.
