Abstract
The top quark is unique among the known quarks in that it decays before it has an opportunity to form hadronic bound states. This makes measurements of its properties particularly interesting as one can access directly the properties of a bare quark. The latest measurements of these properties with the ATLAS detector at the LHC [1] are presented. Measurements of top-quark spin observables in top-antitop events, each sensitive to a different coefficient of the spin density matrix, are presented and compared to the Standard Model predictions. The helicity of the W boson from the top decays and the production angles of the top quark are further discussed. Limits on the rate of flavour changing neutral currents in the production or decay of the top quark are reported. The production of top-quark pairs in association with W and Z bosons is also presented. The measurement probes the coupling between the top quark and the Z boson. The cross-section measurement of photons produced in association with top-quark pairs is also discussed. These process are all compared to the best available theoretical calculations. The latest ATLAS measurements of the top-quark mass in lepton+jets, dilepton, and all-hadronic final states are also reported. In addition, measurements aiming to measure the mass in a well-defined scheme are presented.
Highlights
With the mass around 173 GeV, the top quark is the heaviest known elementary particle
Angles θa and θb are defined as angles between the momentum direction of a top-quark decay particle in its parent top-quark’s rest frame and the axis a or b respectively
Three different quantization axes are compared: helicity axis (k) - defined as the top-quark direction in the ttrest frame, transverse axis (n) - defined to be transverse to the production plane created by top-quark direction and the beam axis and r-axis (r) - axis orthogonal to the other two axes
Summary
With the mass around 173 GeV , the top quark is the heaviest known elementary particle. Due to its large mass, the top quark in the Standard Model (SM) has large decay width corresponding to very short mean life-time of around 0.5 × 10−24 s. The top quark is produced at very short distances - a characteristic strong coupling constant αs is of the order of 0.1 This makes top quark a perfect object to study perturbative QCD. The Large Hadron Collider (LHC) is referred as "a top factory", since the top quarks are produced abundantly during collisions. This makes the LHC an ideal place to measure the topquark properties. Depending on the subsequent decay of the W boson there are three decay channels of top-quark pair: all-hadronic - both W bosons decay into pair of quarks, semi-leptonic - one W boson decays into pair of quarks and the other W boson decays into charged lepton and its corresponding neutrino and dileptonic - both W bosons decay into charged leptons and their corresponding neutrino
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