Search for new physics using top quark pairs produced in association with a boosted Z or Higgs boson in effective field theory.

A data sample containing top quark pairs (t ̄t) produced in association with a boosted Z or Higgs boson is used to search for signs of new physics within the framework of effective field theory. The data correspond to an integrated luminosity of 138 fb−1 of proton-proton collisions produced at a center-of-mass energy of 13 TeV at the LHC and collected by the CMS experiment. Selected collision events contain a single lepton and hadronic jets, including two identified with the decay of bottom quarks, plus an additional large-radius jet with high transverse momentum identified as a Z or Higgs boson decaying to a bottom quark pair. Machine learning techniques are employed to discriminate t ̄tZ and t ̄tH events from background processes, which are dominated by t ̄t + jets production. The signal strengths of boosted t ̄tZ and t ̄tH processes are measured, and upper limits are placed on the t ̄tZ and t ̄tH differential cross sections as a function of the Z or Higgs boson transverse momentum. In addition, effects of physics beyond the standard model are probed using a framework in which the standard model is considered to be the low-energy effective field theory of a higher- scale theory. Eight possible dimension-six operators are added to the standard model Lagrangian and their corresponding coefficients are constrained via a fit to the data.