Measurement of polarization angular coefficients in Z boson leptonic decays with ATLAS at the LHC
- PDF / 217,298 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 84 Downloads / 239 Views
asurement of Polarization Angular Coefficients in Z Boson Leptonic Decays with ATLAS at the LHC O. Fedin St. Petersburg Nuclear Physics Institute, NRC Kurchatov Institute, Gatchina, Leningrad oblast, 188300 Russia e-mail: [email protected] Abstract—This paper presents the complete set of polarization angular coefficients A0 −7 describing lepton angular distributions in Z boson decay, which were measured at the ATLAS experiment in proton–proton collisions with the energy s = 8 TeV. Theoretical values for the difference A0 − A2 calculated in the fixedorder QCD perturbation theory 2(α 2s ), demonstrate significant deviation from the measured data, which indicates the necessity of taking into account higher order corrections. The evidence of nonzero coefficients A5,6,7 was obtained for the first time, in accordance with theoretical calculations in 2(α 2s ) approximation. Measurement of the polarization angular coefficients Ai is important for subsequent precision measurement of parameters of the electroweak model at the LHC, such as the sine of Weinberg electroweak mixing angle sin 2 θW and the W boson mass. DOI: 10.1134/S1063779617050161
1. INTRODUCTION Measurement of angular distributions for leptons produced in gauge boson decays Z → , in proton– proton collisions via the Drell–Yan process, allow a detailed study of the production mechanism and lepton–hadron correlation effects. In practice, these correlations are described by the set of nine hadronic “structural functions”, or polarization angular coefficients which can be calculated in the framework of the parton model, using the perturbative QCD. Theoretical formalism of these calculations is described in detail in [1]. The expression for the differential cross section of this process can be written as
dσ cos θ d φ
2 Z d pTZ dy d
{
U +L = 3 d σ 2 Z (1 + cos 2 θ) + 1 A0(1 − 3 cos 2 θ) 16π d p Z dy 2 T
+ A1 sin 2θ cos φ + 1 2 A2 sin θ cos 2ϕ 2
(1)
+ A3 sin θ cos φ + A4 cos θ + A5 sin θ sin 2φ 2
⎫ A6 sin 2θ sin φ + A7 sin θ sin φ⎬ , ⎭
cross sections without polarization and are the functions of the Z boson kinematic variables, namely, the transverse momentum pTZ , the rapidity y Z , and the mass m Z . The dependence of these angular coefficients on pTZ and y Z is determined by the choice of the quantization axis, i.e., the z axis of the lepton pair rest frame. In this study, we used the Collins–Soper rest frame [2] most widely spread in measurements. 2. MEASUREMENT METHOD The polarization angular coefficients were extracted [3] from the data by fitting the measured two-dimensional angular distributions in the (cos θCS , φCS ) space with nine template distributions corresponding to polynomials in formula (1). Each template distribution had one free parameter determining the measured coefficient Ai . One additional fitting parameter σ, common for all template distributions, was introduced; it corresponds to the value of the nonpolarized differential cross section, integrated over the variables cos θCS and φCS . The fitting was performed by minimization
Data Loading...
