ATLAS sensitivity to top quark and W boson polarization in t(t)over-bar events

Stringent tests on top quark production and decay mechanisms are provided by the measurement of the top quark and W boson polarization. This paper presents a detailed study of these two measurements with the ATLAS detector, in the semileptonic (t (t) over bar -> WWb (b) over bar -> lvj(1)j(2)b (b) over bar) and dileptonic (t (t) over bar -> WWb (b) over bar -> lvlvb (b) over bar) t (t) over bar channels. It is based on leading- order Monte Carlo generators and on a fast simulation of the detector. A particular attention is paid to the systematic uncertainties, which dominate the statistical errors after one LHC year at low luminosity (10 fb(-1)), and to the background estimate. Combining results from both channel studies, the longitudinal component of theW polarization (F-0) can be measured with a 2% accuracy and the right- handed component (F-R, which is zero in the Standard Model) with a 1% precision with 10 fb(-1). Even though the top quarks in t (t) over bar pairs are not polarized, a large asymmetry is expected within the Standard Model in the like- spin versus unlike-spin pair production. A 4% precision on this asymmetry measurement is possible with 10 fb(-1), after combining results from both channel studies. These promising results are converted in a sensitivity to new physics, such as tWb anomalous couplings, top decay to charged Higgs boson, or new s-channels (heavy resonance, gravitons) in t (t) over bar production.