Abstract

Inclusive and differential cross-sections for the production of top quarks in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 139 fb−1. The data were collected by the ATLAS detector at the LHC during Run 2 between 2015 and 2018 at a centre-of-mass energy of 13 TeV. The measurements are performed in a fiducial volume defined at parton level. Events with exactly one photon, one electron and one muon of opposite sign, and at least two jets, of which at least one is b-tagged, are selected. The fiducial cross-section is measured to be {39.6}_{-2.3}^{+2.7} fb. Differential cross-sections as functions of several observables are compared with state-of-the-art Monte Carlo simulations and next-to-leading-order theoretical calculations. These include cross-sections as functions of photon kinematic variables, angular variables related to the photon and the leptons, and angular separations between the two leptons in the event. All measurements are in agreement with the predictions from the Standard Model.

Highlights

  • LHC during Run 2 between 2015 and 2018 at a centre-of-mass energy of 13 TeV

  • This paper presents a measurement of the fiducial inclusive and differential combined ttγ + tW γ production cross-sections in the final state with one electron and one muon, referred to as the eμ channel

  • Since showering and hadronisation effects are not considered in this calculation, the jets correspond to the b-quarks from the top-quark decay

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Summary

ATLAS detector

ATLAS [12,13,14] is a multipurpose detector with a forward-backward symmetric cylindrical geometry with respect to the LHC beam axis. The innermost layers consist of tracking detectors in the pseudorapidity range |η| < 2.5. The innermost layers consist of tracking detectors in the pseudorapidity range |η| < 2.5. By a thin superconducting solenoid that provides a 2 T axial magnetic field. It is enclosed by the electromagnetic and hadronic calorimeters, which cover |η| < 4.9. The outermost layers of ATLAS consist of an external muon spectrometer within |η| < 2.7, incorporating three large toroidal magnetic assemblies with eight coils each. The field integral of the toroids ranges between 2.0 and 6.0 Tm for most of the acceptance. The muon spectrometer includes precision tracking chambers and fast detectors for triggering. A two-level trigger system [15] reduces the recorded event rate to an average of 1 kHz

Signal and background modelling
Event selection
Analysis strategy
Fiducial region definition
Fiducial inclusive cross-section
Absolute and normalised differential cross-sections
Experimental uncertainties
Signal and background modelling uncertainties
Treatment of the systematic uncertainties in the measurements
Fiducial inclusive cross-section measurement
12 Post-Fit
Differential cross-section measurements
Conclusions
Findings
Normalised cross-section eμ
Full Text
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