A look into the heart of the standard model Searching for heavy Higgs bosons and constraining the Higgs boson width via its off-shell production

This thesis describes two analyses performed in the ATLAS collaboration. Both look at Higgs boson production at the Large Hadron Collider in the two-lepton-two-neutrino final state. One is a search for hypothetical heavy Higgs bosons, as predicted in various Beyond the Standard Model theories. No evidence is found for such bosons, and a limit is set on their production cross section. The other analysis uses the off-shell production of the Standard Model Higgs boson to measure the Higgs boson off-shell signal strength. This measurement is then interpreted as the first estimate of the Higgs boson lifetime in the ATLAS collaboration.