Study of ttH production at sqrt(s) = 13 TeV in the all-jets channel with CMS

Measuring the associated production of the Higgs boson and a top quark-antiquark pair is one of the major goals for the Run 2 of the LHC. The aim of this thesis is to report on a first search for t ̄tH, using pp collision recorded with the CMS detector in the whole 2016, at a centre-of-mass energy of 13 TeV, and corresponding to an integrated luminosity L = 36 fb^−1. In particular, the analysis is performed and optimised on 2015 data, corresponding to an integrated luminosity L = 2.63 fb−1 , and then a projection of the results on the full data set is given. Candidate t ̄tH events are selected with criteria which enhance the all-jets decay channels of the t ̄t system and the decay of the Higgs boson into a bottom quark-antiquark pair (H -> bb). Despite being the channel with the highest branching ratio and potentially fully reconstructable, such decay mode is particularly challenging, due to the completely dominating background contributions. In order to refine the selection, signal and background events are separated using a multivariate approach, using a Boosted Decision Tree. A first estimation of the BDT sensitivity is performed. Such estimate is presented as an upper limit at 95% confidence level on the t ̄tH production cross section, with respect to the SM expectations, through the signal strength (μ), and results in μ = 8.2^{+3.4}_{−2.4}. The projection of the result on the full data set is μ = 2.1^{+2.1}_{−1.6}.