Dilepton azimuthal correlations in tt production

Abstract

The dilepton azimuthal correlation, namely the difference phi between the azimuthal angles of the positive and negative charged lepton in the laboratory frame, provides a stringent test of the spin correlation in tt production at the Large Hadron Collider. We introduce a parameterisation of the differential cross section dalpha=dphi in terms of a Fourier series and show that the third-order expansion provides a su ciently accurate approximation. This expansion can be considered as a `bridge' between theory and data, making it very simple to cast predictions in the Standard Model (SM) and beyond, and to report measurements, without the need to provide the numbers for the whole binned distribution. We show its application by giving predictions for the coeffcients in the presence of (i) an anomalous top chromomagnetic dipole moment; (ii) an anomalous tbW interaction. The methods presented greatly facilitate the study of this angular distribution, which is of special interest given the 3:2(3:7) deviation from the SM next-to-leading order prediction found by the ATLAS collaboration in Run 2 data.