A Search for Dark Matter Through Invisible Decays of the Higgs Boson With the Atlas Detector at the LHC

Diboson physics provides insight into a wide variety of processes\nthat are produced copiously at the LHC. Of particular interest\nare events where leptons and neutrinos are produced, strong signatures\nof electroweak physics. The study of electroweak physics is\nessential to understanding the Standard Model and in searches\nfor phenomena outside the Standard Model.\nThe most sensitive channels that contributed to the discovery\nof the Higgs boson searched for decays to dibosons.\nThis thesis will present four analyses that utilize\ncharged leptons and neutrinos whose presence are\ninferred from the missing transverse momentum.\nTwo measurements of Standard\nModel cross sections will be presented. The WW cross section is measured\nusing 35 pb-1 of data collected in 2010 and is\none of the earliest measurements of diboson processes\nat the LHC. The ZZ cross section is measured using\n4.6 fb-1 of data collected in 2011. A search\nfor the Higgs in the WW decay channel is presented\nalso using 4.6 fb-1 of data collected in 2011.\nThis analysis did not have the sensitivity to discover the\nHiggs boson, but when data were added in 2012 this channel\ncontributed significantly to its discovery.\nFinally a search for anomalous invisible decays of the Higgs boson\nusing 4.6 fb-1 of\ndata taken in 2011 and 13.1 fb-1 of data taken in 2012\nwill be presented.\nThis search is sensitive to new physics that couples to the Higgs\nboson that would result in an increased rate of decays to invisible particles.\nOne possible scenario is that dark matter couples to the Higgs boson.\nIf the mass of the dark matter particle is less than half of the\nmass of the Higgs boson decays to dark matter increase\nits invisible branching fraction.\nDark matter is a significant contribution\nto the makeup of the universe, but very little is known about it.\nThis search provides additional limits on how dark matter can couple\nto the Standard Model.\nNo excess of events is observed in this search and\nlimits are placed on the allowed invisible branching fraction.\nThese limits are interpreted in the context of simple Higgs portal\nmodels to place constraints on the allowed dark matter mass and\ninteraction cross section. The results are compared with\ncurrent limits on dark matter and place significant restrictions\non the allowed dark matter mass and cross section within these models.