| We have applied a general approach for extracting information from data to a study of top quarks produced in proton-antiproton (pp¯) collisions in the process pp¯ → tt¯ . This reaction can be calculated in the Standard Model (SM), in which the top (or antitop) quarks decay into b quarks and W bosons: t →W+ b, t¯ → W− b¯. We examine the decays of the W boson in these events in order to establish how the spin of the W correlates with its momentum vector. This is defined by the helicity of the W boson (projection of its spin along its line of flight), which is also predicted by the SM. The analysis is based on a direct calculation of a probability for each event as a function of the helicity of the W bosons in top-antitop events in the lepton + jets final state. These events correspond to one W decaying into a lepton and its neutrino, and the other W into a quark-antiquark pair, with the quarks from the W and the two b quarks evolving into jets of particles. The probability is calculated by convoluting the differential cross section with the resolution and acceptance of the detector. This measurement uses top quarks collected by the DØ experiment in 125 events/pb of data in pp¯ collisions at = 1.8 TeV during Run I of the Fermilab TeVatron. Assuming the “V-A” coupling of the SM decay, we obtain a longitudinal helicity fraction of F0 = 0.56 ± 0.31(stat) ± 0.07(syst) for the W, which is consistent with the prediction of the Standard Model of F0 = 0.70 for a top-quark mass of 175 GeV/c2. The method employed in this analysis offers the possibility of increasing statistical precision by using both of the decays of W bosons in these events. Also Monte Carlo studies indicate that the approach provides an unbiased result in the limit of poor statistics. Although our measurement is severely limited by the small event sample of Run I, this powerful technique will provide far greater sensitivity to any departures from the SM in the data anticipated from Run II. |
