Abstract
The Higgs boson was discovered on the 4th of July 2012 with a mass around 125 GeV by ATLAS and CMS experiments at LHC. Determining the Higgs properties (production and decay modes, couplings,...) is an important part of the high-energy physics programme in this decade. A search for the Higgs boson production in association with a top quark pair (tt̄H) at ATLAS [1] is summarized in this paper at an unexplored center-of-mass energy of 13 TeV, which could allow a first direct measurement of the top quark Yukawa coupling and could reveal new physics. The tt̄H analysis in ATLAS is divided into 3 channels according to the Higgs decay modes: H → Hadrons, H → Leptons and H → Photons. The best-fit value of the ratio of observed and Standard Model cross sections of tt̄H production process, using 2015-2016 data and combining all tt̄H final states, is 1:8±0:7, corresponds to 2:8σ (1:8σ) observed (expected) significance.
Highlights
The Higgs boson production in association with a top quark pair ttH (Figure 1a) represents 1% of the total Higgs production making the observation of ttH very challenging with limited available statistics in Run 1
The post-fit yields of signal and total background are compared to data where the signal is normalised to the best fit value and to the excluded value
A good agreement is seen between data and expectation in all bins within the uncertainties
Summary
The Higgs boson production in association with a top quark pair ttH (Figure 1a) represents 1% of the total Higgs production making the observation of ttH very challenging with limited available statistics in Run 1. The observation of the ttH production mode allows a direct measurement of the top Yukawa coupling (referred as gttH): the strongest coupling to the Higgs boson, giving that the top quark is the heaviest fundamental particle in the SM, with a mass equal to 173.21 GeV [2]. Its measurement is estimated using the ggF production (Figure 1b) where the Higgs is indirectly coupled to the top quark via a loop but no new physics is assumed. New physics could be hidden in the loops mediating the Higgs production via ggF This issue could be solved by performing the measurements in ttH production mode involving a direct, tree-level, Higgs-top coupling. It is worth to mention that all results, discussed in this paper, are obtained from Refs [4,5,6,7,8]
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