Study of the associated production of the Higgs boson with a top quark pair in a boosted regime in the ATLAS experiment

The measurements of the pp→ tt¯ H signal strength (μ=σobs/σtt¯HSM) and its upper limit at a center-of-mass energy of pp collision of 13TeV are presented in this paper. The pp data collected with the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.5 fb-1, have been subjected to a detailed analysis. The tt¯ H signal strength and its upper limit have been measured searching for the decays H→ bb¯ and tt¯ → lνbqq¯ b¯. Two different analysis approaches have been used in order to analyse the same data in two different ways. In the first (namely “resolved”), the procedure uses standard identification and reconstruction algorithms, meaning that the individual partons from the initial hard process are reconstructed as separate objects. In the second (namely “combined”), the events are separated in two exclusive regimes, including events containing the hadronically decaying top quark (t→ qqb) with a low transverse momentum (pT< 250 GeV) and the boosted ones with the opposite requirement, where the partially overlapped jets coming from high pT tops are suitably identified, reconstructed and tagged. The boosted sample is sensitive to a significantly different kinematic region and provides additional information. The measured signal strength is 1. 4 ± 0. 5 and 1. 2 ± 0. 5 , respectively, for the resolved and the combined analysis. A signal strength larger than 2.3 and 2.0 can be excluded at the 95% confidence level respectively for the resolved and the combined analysis. Both results are compatible with the prediction of the Standard Model (μ= 1). Previous results in the search for the tt¯ H process were reported by the ATLAS Collaboration and a boosted reconstruction in the channel was previously done by the CMS Collaboration.