Search for Higgs bosons with b-jets in the final state in proton-proton collisions with the ATLAS experiment

This thesis discusses searches for Higgs bosons in decays with b-quark jets in the final state in data from proton-proton collisions taken with the ATLAS detector at the Large Hadron Collider (LHC). First, a search for the Standard Model (SM) Higgs boson in the associated production with a vector boson and the decay to b-quarks, VH(bb), is presented. It is based on the data recorded during Run 1 of the LHC at a center-of-mass energy of sqrt(s) = 8 TeV, corresponding to an integrated luminosity of 20.3/fb. Other SM processes yield backgrounds with cross sections much larger than for the expected Higgs boson signal. Their contribution is efficiently separated from the signal using Boosted Decision Trees (BDT). The signal contribution is estimated using a profile likelihood fit of the BDT output distributions. The signal strength with respect to the SM expectation is measured to be 0.65+0.43-0.40, corresponding to an observed (expected) significance of 1.7 (2.5) standard deviations. In combination with the analysis performed using the data taken at sqrt(s) = 7 TeV the signal strength is measured to be 0.51+0.40-0.37, corresponding to an observed (expected) significance of 1.4 (2.6) standard deviations. Additionally, a sensitivity estimate of the SM Higgs boson produced in gluon-fusion and decaying to two b quarks, gg->H->bb, is presented. This feasibility study is based on the data taken at sqrt(s) = 8 TeV. The expected statistical significance has been estimated to be 0.7 standard deviations from a likelihood ratio based on the dijet mass. The second analysis presented in this thesis is a search for decays of the Higgs boson to Beyond Standard Model (BSM) particles that are invisible to the ATLAS detector. The search has been performed in the associated production with a hadronically decaying vector boson, V(qq)H(inv). Also this analysis is based on the data taken at sqrt(s) = 8 TeV. The analysis shares many aspects with the SM VH(bb) analysis, such that many techniques could be adapted. No excess of events over the expected SM background processes is observed. An observed (expected) 95% CL upper limit on the branching ratio for invisible decays of the Higgs Boson has been set as BR(H->inv) < 78% (86%), assuming SM production. The third analysis is a search for new and heavy vector bosons decaying via V'->VH(bb). The analysis is carried out with the first data taken during Run 2 of the LHC with sqrt(s) = 13 TeV, corresponding to an integrated luminosity of 3.2/fb. Boosted jet reconstruction techniques are employed for an optimal signal acceptance. No significant excess of events over the expected SM background processes is observed. Limits on the production cross section times branching ratio have been set independently for the W' and Z' hypotheses. They range from about 100 fb for resonance masses around 1 TeV to about 10 to 30 fb for the most sensitive region for masses between 2 to 5 TeV for both hypotheses.