The searches for a Beyond Standard Model (BSM) heavy CP-odd Higgs boson A decaying to Zh in the context of two-Higgs-doublet models (2HDM), and Minimal Supersymmetric Standard Model (MSSM) neutral Higgs bosons H/A decaying to a tau pair are presented. The search for the heavy CP-odd Higgs boson, A, is conducted through the A -> Zh -> ll T[subscript]lep T[subscript]had decay channel using 20.3 fb−1 of proton-proton collision data collected by the ATLAS detector at a center-of-mass of 8 TeV. The search for the neutral MSSM H/A decaying to a tau pair in the T[subscript]lep T[subscript]had b-veto final state is done using 3.2 fb−1 of proton-proton collision data recorded by the ATLAS detector at a center-of-mass of 13 TeV. The observed data agrees with the Standard Model background prediction, and upper limits are set on cross-section times branching ratio of neutral BSM Higgs bosons.

Despite its tremendous success in describing the available high energy physics data to high precision, the standard model (SM) of particle physics is known to be incomplete. The most popular extension of the SM is supersymmetry. It provides elegant solutions to various problems of the SM, e.g. the fact that in the SM the mass of the Higgs boson is sensitive to large radiative corrections driving its mass to the Planck scale. The reported results are based on data samples of proton-antiproton collisions at a center-of-mass energy of (square root)s = 1.96 TeV provided by the Tevatron. The analyzed data is recorded by the D0 detector [1]. All reported limits are calculated at the 95% confidence level (CL) based on the modified frequentist approach [2]. Recent searches by the D0 collaboration for Higgs bosons in extensions of the Standard Model at the Tevatron are reported with emphasis on neutral Higgs bosons in supersymmetry.

A search for neutral MSSM Higgs bosons in pp collisions at the LHC at a center-of-mass energy of 7 TeV is presented. The results are based on a data sample corresponding to an integrated luminosity of 36 inverse picobarns recorded by the CMS experiment. The search uses decays of the Higgs bosons to tau pairs. No excess is observed in the tau-pair invariant-mass spectrum. The resulting upper limits on the Higgs boson production cross section times branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, yield stringent new bounds in the MSSM parameter space.

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1 sigma. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is estimated to be 1.8 sigma. More data are required to ascertain the origin of this excess.

This dissertation presents a search for physics beyond the Standard Model using pairs of muons with equal electric charge. The search is performed in a proton-proton collision data sample collected during 2011 at sqrt(s) = 7 TeV by the ATLAS experiment at the Large Hadron Collider. The total data sample corresponds to an integrated luminosity of 4.7fb-1. Events are selected by requiring pairs of prompt and isolated like-sign muons with transverse momentum greater than 20 GeV. The observed dimuon invariant mass distribution is compared to the Standard Model expectation, searching for any deviation between the observed and predicted yields. No evidence for an excess beyond the Standard Model expectation is observed. The results are interpreted as inclusive cross-section limits on the production of like-sign muon pairs from non-Standard Model sources as function of the dimuon invariant mass. The cross-section limits range between 30 fb and 1.2 fb. The data are also searched for a new narrow resonance which decays to like-sign muon pairs. No evidence for such resonant production is observed and the results are interpreted as upper limits on the mass and production cross section of doubly charged Higgs bosons. Assuming pair-production and a 100% branching ratio to muons, masses below 398 GeV (306 GeV) are excluded assuming coupling to left-handed (right-handed) fermions.

Searches for invisible decays of the Higgs boson are presented. The data collected with the CMS detector at the LHC correspond to integrated luminosities of 5.1, 19.7, and 2.3 inverse femtobarns at centre-of-mass energies of 7, 8, and 13 TeV, respectively. The search channels target Higgs boson production via gluon fusion, vector boson fusion, and in association with a vector boson. Upper limits are placed on the branching fraction of the Higgs boson decay to invisible particles, as a function of the assumed production cross sections. The combination of all channels, assuming standard model production, yields an observed (expected) upper limit on the invisible branching fraction of 0.24 (0.23) at the 95% confidence level. The results are also interpreted in the context of Higgs-portal dark matter models.