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Determination of top couplings in associated top pair events using ATLAS data

Final Report Summary - TOPCOUP (Determination of top couplings in associated top pair events using ATLAS data)

The top quark is the fundamental particle with the largest mass, about 35 times larger than the bottom quark and nearly as massive as a gold atom. As such it may play a special role in the Standard Model (SM) of particle physics and may open a window to new physics, commonly invoked to explain observed inconsistencies, but not yet observed in experiments. Top quarks are produced abundantly, predominantly in pairs, in proton-proton collisions at the highest centre-of-mass energies available (currently 13 TeV). So far the characterisation in terms of production, properties and decays in experiments at the Tevatron (Chicago) and CERN (Geneva) did not unveil any unexpected behaviour.

Top-quark couplings can be modified significantly in some extensions of the Standard Model and therefore studying the interactions of top quarks with bosons is expected to be sensitive to new effects. Interactions with gluons have been studied in some detail through the strong top quark pair-production and the tt+jets process, while interactions with the W boson have been studied in single-top production and weak top-quark decay. Interactions with photons (γ), Z and Higgs bosons, however, are essentially unexplored.

Within an international collaboration that operates the ATLAS detector at the Large Hadron Collider at CERN, the TopCoup project made important contributions to the development of data reconstruction algorithms (b-tagging), modelling of simulation (top-quark pair events), and to measurements of the top-quark pair charge asymmetry, the production cross-sections of the processes pp → ttγ, ttW, ttZ, and ttH using datasets at 7, 8 and 13 TeV.

The measured yield of top-quark pair production in association with a photon can constrain models of new physics, for example those with composite top quarks, or with excited top-quark production. The ttγ production cross section times the branching ratio of the single-lepton or dilepton decay channel has been measured via a template fit in a fiducial kinematic region. The result is in good agreement with expectation.

Measurements of the ttW and ttZ cross sections using the full 8 TeV and partial 13 TeV datasets have been performed. Depending on the decays of the top quarks, W and Z bosons, between zero and four prompt, isolated leptons may be produced. Channels with three and four leptons are particularly sensitive to the ttZ process. Each channel was divided into multiple analysis regions in order to enhance the sensitivity to the signal. A simultaneous fit was performed to all signal regions and selected control regions to extract cross sections in agreement with calculations, with an uncertainty of ~13%.

Because of its large mass, the top quark is the fermion with the largest Yukawa coupling to the Higgs field in the SM, with a value close to unity. Τhe only process where this coupling can be accessed directly is the production of a top-quark pair in association with a Higgs boson (ttH). A search with 8 TeV data selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and SM background. Presence of signal could not yet be confirmed, obtaining a cross-section of 1.6 ± 2.6 times the SM expectation. Another search for the tt¯H process, in this case using final states with multiple leptons, has been performed using data recorded in 2015 and 2016 at 13 TeV. The search included channels with tau-leptons, as well as two same-charge, three or four leptons. Together with other channels the first evidence, and later the observation of ttH production at more than 5σ could be claimed by ATLAS and CMS.