• b quark
• BDT
• CMS
• HH production
• Higgs
• multivariate analysis
• tau quark

This thesis reports the optimisation of a multivariate analysis technique for the search for Higgs boson pair (HH) production. HH production gives an access to the Higgs boson trilinear self-coupling and is sensitive to the presence of physics beyond the Standard Model. Both resonant and nonresonant production mechanisms are investigated exploring events with one Higgs boson decaying into two b quarks and the other decaying into two tau leptons (HH->b bbar tau+ tau-). This process is studied through the examination of the three decay modes of the tau+ tau- system, with one or two tau decaying into hadrons in the final state. The search uses proton-proton collision data collected at centre of mass of 13TeV with the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 35.9 fb-1.
The main effort has been devoted to design and develop a multivariate technique to separate the signal from the ttbar background. This technique has been applied for the first time to all the three final states and has proved to be an essential element to enhance the sensitivity. No evidence for the presence of a signal has been found and results are found to be consistent, within uncertainties, with the standard model background predictions. Upper limits are set at 95% confidence level on resonant and nonresonant HH production. The expected and observed upper limits are about 15 and 10 time the SM prediction, respectively, corresponding to the most stringent limits set so far at the LHC.