Measurements of the W boson helicity in t anti t decays

The subject of this thesis is the measurement of the fraction of W bosons with positive helicity in top-antitop events recorded by the D0 detector at the Fermilab Tevatron collider during the years 2002-2004. The data corresponds to an integrated luminosity of about 230 inverse pb. The measurement makes use of the decay angle theta between the charged lepton and the (negative) b quark direction in the rest frame of the W boson. To obtain the angle theta, the top-antitop system and its decay products are reconstructed using a kinematic fit with constraints on the mass of the W boson and of the top quark. By comparing the expectation for cos(theta) from Monte Carlo samples with different fractions of right-handed W bosons, f+, with the distribution obtained in data, the following result for f+ has been obtained: f+= -0.10±0.17(stat)±0.07(syst). This effectively rules out theoretical models which predict a pure or very high V+A contribution in the weak charged current interaction of the top quark. (Orig.)