Abstract
Measurements of differential top quark pair mathrm{t}overline{mathrm{t}} cross sections using events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV containing two oppositely charged leptons are presented. The data were recorded by the CMS experiment at the CERN LHC in 2016 and correspond to an integrated luminosity of 35.9 fb−1. The differential cross sections are presented as functions of kinematic observables of the top quarks and their decay products, the mathrm{t}overline{mathrm{t}} system, and the total number of jets in the event. The differential cross sections are defined both with particle-level objects in a fiducial phase space close to that of the detector acceptance and with parton-level top quarks in the full phase space. All results are compared with standard model predictions from Monte Carlo simulations with next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD) at matrix-element level interfaced to parton-shower simulations. Where possible, parton-level results are compared to calculations with beyond-NLO precision in QCD. Significant disagreement is observed between data and all predictions for several observables. The measurements are used to constrain the top quark chromomagnetic dipole moment in an effective field theory framework at NLO in QCD and to extract mathrm{t}overline{mathrm{t}} and leptonic charge asymmetries.
Highlights
Channel of the tt process is utilised
The finite resolution introduced by the detector response, parton shower, and hadronisation lead to migration of events across bins when correcting the data to both the fiducial phase space based on particle-level objects or the full phase space based on the parton-level top quarks
In order to probe the modelling of the top quark pT as thoroughly as possible, various differential cross sections related to the pT of top quarks are measured. These include: the separate pT of the top quarks and antiquarks in the laboratory frame and, in order to suppress the effects of initial- and final-state radiation (ISR and FSR), in the tt rest frame (RF), and the largest and second-largest pT top quark or antiquark in an event
Summary
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Forward calorimeters extend the pseudorapidity coverage provided by the barrel and endcap detectors. Events of interest are selected using a twotiered trigger system [30]. The first level, composed of custom hardware processors, uses information from the calorimeters and muon detectors to select events at a rate of around 100 kHz within a time interval of less than 4 μs. The second level, known as the highlevel trigger (HLT), consists of a farm of processors running a version of the full event reconstruction software optimised for fast processing, and reduces the event rate to around 1 kHz before data storage. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [31]
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