Search for rare processes with a Z+bb signature at the LHC, using the matrix element method

This talk presents a detailed study of the final state with the Z boson decaying into two leptons, produced in the CMS detector at the LHC. In order to tag this topology, sophisticated b jet tagging algorithms have been used, and the calibration of one of them, the Jet Probability (JP) tagger, is exposed. A study of the tagger degradation at high energy has been done and led to a small gain of performance. This investigation is followed by the search for the associated production of the standard model (SM) Higgs boson with a Z boson and decaying into two b quarks (ZH), using the Matrix Element Method (MEM) and two b-taggers: JP and Combined Secondary Vertex (CSV). The MEM is an advanced tool that produces an event-by-event discriminating variable, called weight. To apply it, several sets of transfer function have been produced. The final results give an observed limit on the ZH production cross section times the H → bb branching ratio of 5.46 when using the CSV tagger and 4.89 when using the JP algorithm: a small excess of data is observed only for CSV. Finally, based on the previous analysis, a model-independent search of new physics has been performed. The goal was to discriminate all the SM processes to categorize the Zbb phase space, with a recursive approach using the MEM weights. Free parameters had to be tuned in order to find the best exclusion limit on the ZH signal strength. For the best configuration, the computed limit was 3.58, close to the one obtained with the dedicated search. To confirm the power of the method, a new physics signal motivated by 2HDM models with a Zbb final state was used leading to a similar conclusion than for the ZH case.(SC - Sciences) -- UCL, 2015Accès libre14/10/2015(SC - Sciences) -- UCL, 2015Anglai