Z Boson Decay into Two Leptons and Photon, Charge Asymmetry and Running Weak Mixing Angle

Abstract

In this paper, we consider the lepton and photon invariant mass distribution in the Z boson decay into two leptons and photon. The measurements were performed with the ATLAS detector and have best accuracy today, the systematic uncertainty does not exceed 0.6%. Quantitative comparison of the measurement results with the predictions of modern event generators implementing the calculation of the Standard Model for this process was performed. The discovered deviations from the predictions of the generators are interpreted using the effective interaction of the Z boson with two leptons and photon. The addition of the effective interaction contribution turned out to be significant at the confidence level of \( > \,5\sigma \) for PowHeg + PHOTOS and \( > \,4.7\sigma \) for Sherpa 2.2. The charge asymmetry for this process was calculated at different values of the Weinberg angle sine. The asymmetry reaches a maximum and minimum at sine values of about 0.2 and 0.7, respectively. Numerically, the asymmetry is 0.03 at maximum and –0.03 at minimum, which does not allow one to explain the experimental value of the asymmetry of \(0.088 \pm 0.022\) at lepton and photon invariant mass equal to the W boson mass.