Measurement of the cross-section and charge asymmetry of W bosons produced in proton–proton collisions at $$\sqrt{s}=8~\text {TeV}$$ with the ATLAS detector

Abstract

Measurement of the cross-section and charge asymmetry of W bosons produced in proton-proton collisions at √ s = 8 TeV with the ATLAS detectorThe ATLAS Collaboration This paper presents measurements of the W + → µ + ν and W − → µ − ν cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2 fb −1 . The precision of the cross-section measurements varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis coinciding with the axis of the beam pipe. The x-axis points from the interaction point to the centre of the LHC ring, and the y-axis points upward. Polar coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The rapidity y of a system is defined in terms of its energy E and its longitudinal momentum p z as y = (1/2) ln[(E + p z )/(E − p z )]. Angular separations between particles or reconstructed objects are measured in η-φ space using ∆R = (∆η) 2 + (∆φ) 2 . The ATLAS detectorThe ATLAS detector [20] at the LHC covers nearly the entire solid angle around the collision point. It consists of an inner tracking detector (ID) surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS) incorporating three large superconducting toroid magnets. The ID is immersed in a 2 T axial magnetic field and provides charged-particle tracking in the range |η| < 2.5. A high-granularity silicon pixel detector typically provides three measurements per track and is followed by a silicon microstrip tracker, which usually provides four three-dimensional measurement points per track. These silicon detectors are complemented by a transition radiation tracker, which enables radially extended track reconstruction up to |η| = 2.0.The calorimeter system covers the pseudorapidity range |η| < 4.9. Electromagnetic calorimetry is provided by barrel and endcap high-granularity lead/liquid-argon (LAr) sampling calorimeters in the |η| < 3.2 region. A thin LAr presampler, covering |η| < 1.8, corrects for energy loss in the material upstream of the calorimeters. Hadronic calorimetry is provided in the |η| < 1.7 region by the steel/scintillator tile calorimeter, segmented into three barrel structures, and in the endcap region by two copper/LAr calorimeters. The forward ...

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