The Yukawa couplings of the Higgs boson to the quarks are key parameter of the Standard Model. After the Higgs boson discovery, the researcher interest has been focused on the study of its properties as its couplings to other particles, completely determined by the SM and consequently a very good handle for potential new physics discovery. With the new data that we are collecting at LHC a higher sensitivity to to Higgs boson coupling to top and bottom quarks will be reached and this will allow to identify new physics processes.
These results based on Run1 statistics[1] and the first attempt to use high center of mass ¿s= 13 TeV(Run 2) data[2] are incouraging to present this project.
The multivariate technique analysis will be implemented using more complex discriminating variables, like the Higgs boson visible mass with one lepton and one tau, or geometric variables like the distance between the light leptons and the hadronic tau. The performed an optimisation will allow to identify the most discriminant variables for each one of the main backgrounds separately. The preliminary results based on an integrated luminosity of 36.1 fb-1 may be presented at European Physical Society HEP2017(Venice, Italy). Further study a will be necssary to identitfy the tau source.
Moreover, our project needs to be extended in the next few years exploring other analysis method either pushing multivariate analysis further, either with more ambitious tools. We have considered the possibility to develop a discriminant based on the matrix-element approach for 2lss + 1¿ category, in order to improve the separation of the ttH signal with respect to the main backgrounds, namely ttZ and tt backgrounds. The computation of the discriminant is based on combining the knowledge of differential theoretical cross sections for the ttH signal and for background processes with the knowledge of the experimental resolution of the detector. This approach, could be extremelly powerful and help to the reach of a better significance.
[1]ATLAS Coll."Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS"
Physics Letters B 749 (2015) 519-541
[2]ATLAS Coll. "The ATLAS Collaboration Search for the Associated Production of a Higgs Boson and a Top Quark Pair in Multilepton Final States with the ATLAS Detector," ATLAS-CONF-2016-058