Abstract
The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy sqrt{s} = 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb−1 collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → ℓ+ℓ−γ + X (ℓ = e, μ) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the ℓ+ℓ−γ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered.
Highlights
Background estimationThe dominant source of background to the Z(→ + −)γ signal originates from Z + jets production in which a jet is misidentified as a photon
The inner tracking detector (ID) is composed of a silicon pixel detector and a silicon microstrip tracker (SCT), both of which cover the pseudorapidity range |η| < 2.5, together with a transition radiation tracker (TRT) with an acceptance of |η| < 2.0
The fraction of Z + jets background events relative to the number of Zγ signal events in the signal region can be derived from the number of observed events in the signal and control regions according to the methodology described in ref
Summary
The ATLAS experiment [23] at the LHC is a multipurpose particle detector with a forwardbackward symmetric cylindrical geometry and nearly 4π coverage in solid angle. Its major components are an inner tracking detector (ID) surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic (ECAL) and hadron (HCAL) calorimeters, and a muon spectrometer (MS). The ID is composed of a silicon pixel detector (including the insertable B-layer [24, 25] installed before the start of Run 2) and a silicon microstrip tracker (SCT), both of which cover the pseudorapidity range |η| < 2.5, together with a transition radiation tracker (TRT) with an acceptance of |η| < 2.0. The ECAL is composed of alternating layers of passive lead absorber interspersed with active liquid-argon (LAr) gaps and covers the pseudorapidity range |η| < 3.2. The second-level trigger is a software-based system which runs algorithms similar to those implemented in the offline reconstruction software, yielding a recorded event rate of about 1 kHz
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