Search for Higgs-Boson Production in Association with Top-Quark Pairs in $H\to b \bar{b}$ Decays at $\sqrt{s}$ = 8 and 13 TeV with the CMS Experiment

A search of the Higgs boson in association with a top-quark pair is presented, using pp collision data collected by the CMS detector at centre-of-mass energies of 8 and 13 TeV corresponding to integrated luminosities of 19 . 7 fb − 1 and 2 . 6 fb − 1 , respectively. Candidate t ̄ tH events are selected to enhance the dileptonic decay channel of the t ̄ t system and the decay of the Higgs boson into two b -quark jets ( H → b ̄ b ). The final state of the selected t ̄ tH ( b ̄ b ) processes is characterised by the presence of four b -jets, two originating from the t ̄ t system and two belonging to the Higgs-boson decay. The b -quark jet pair originating from the t ̄ t system is identified using multi-variate analysis techniques. In order to reduce the combinatorial background provided by the four b -jets in the final state, a generic method is developed to get an optimal separation between b and ̄ b jets. The remaining b -jets in the event are used to reconstruct the ( m b ̄ b ) invariant-mass distribution used to estimate the resonant signal above the non-resonant background and to determine the upper-limit on the t ̄ tH ( b ̄ b ) production. A combined fit to the data in two different categories results in an observed (expected) upper limit of μ < 10 . 9 (12 . 0) (for 8 TeV), and μ < 13 . 4 (14 . 4) (for 13 TeV) at the 95 % confidence level, where μ is the ratio of the observed t ̄ tH production cross section relative to the value expected for a Standard Model Higgs boson with m H = 125 GeV.