Electroweak and QCD physics


I performed the world’s first measurement of the A2 and A3 angular coefficients of the W boson, produced in a hadron collider. These are 2 of the significant 4 leading angular coefficients. These angular coefficients are ratios of the helicity cross sections of the W and the total unpolarized cross section.

Here θ and φ are the angles of the charged lepton from the W decay in the Collins-Soper W rest frame. (the frame where the z-axis bisects the proton and minus the antiproton directions in the W rest frame). The quantities qT and η and are the transverse momentum and rapidity of the W in the lab frame. In this frame, the φ angle is always known, and the ambiguity in the value of the neutrino longitudinal momentum leads to only a sign ambiguity in the polar angle θ of the lepton. The following picture shows the transformation from the laboratory frame to the Collins-Soper frame: The angular coefficients with significan value are A0,A2,A3 and A4, as can be seen in my W Helicity page. Their value is sensitive to the production of the W boson in association with new exotic particles or the Higgs boson. The presence of jets balance the transverse momentum of the W , which is necessary for non-zero A2 and A3 values. At the same time, they introduce QCD effects that we would like to also test.

The measurement was performed using CDF W+jets data and by reconstructing the φ angle of the charged lepton as a function of the W boson transverse momentum (4 bins). The result agrees well at low momenta, with a slight high deviation at the higher momenta. The analysis was carried out separately for electrons and muons and the results were combined at the end. The final measurement is consistent with the Standard Model, with a ~1 σ deviation at the higher pT bin:

For more information, please visit the PRD publication (Phys. Rev. D 73, 052002 (2006)).