Abstract
Measurements of normalized differential cross sections as functions of the multiplicity and kinematic variables of charged-particle tracks from the underlying event in top quark and antiquark pair production are presented. The measurements are performed in proton-proton collisions at a center-of-mass energy of 13~text {Te}text {V}, and are based on data collected by the CMS experiment at the LHC in 2016 corresponding to an integrated luminosity of 35.9~text {fb}^{-1}. Events containing one electron, one muon, and two jets from the hadronization and fragmentation of mathrm {b} quarks are used. These measurements characterize, for the first time, properties of the underlying event in top quark pair production and show no deviation from the universality hypothesis at energy scales typically above twice the top quark mass.
Highlights
The underlying event (UE) is defined as any hadronic activity that cannot be attributed to the particles stemming from the hard scatter, and in this case from tt decays
The main contribution to the UE comes from the color exchanges between the beam particles and is modeled in terms of multiparton interactions (MPI), color reconnection (CR), and beam-beam remnants (BBR), whose model parameters can be tuned to minimum bias and Drell–Yan (DY) data
The normalized differential cross sections measured as functions of Nch, pT, pT, | pT|, pz, pz, sphericity, aplanarity, C, and D are shown in Figs. 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13, respectively
Summary
The UE is defined as any hadronic activity that cannot be attributed to the particles stemming from the hard scatter, and in this case from tt decays. The study of the UE in tt events provides a direct test of its universality at higher energy scales than those probed in minimum bias or DY events This is relevant as a direct probe of CR, which is needed to confine the initial QCD color charge of the t quark into color-neutral states. The CR mainly occurs between one of the products of the fragmentation of the b quark from the t quark decay and the proton remnants This is expected to induce an ambiguity in the origin of some of the final states present in a bottom quark jet [4,5,6]. 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. [11]
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