| The Standard Model (SM) of Particle Physics predicts the existence of a fundamental scalar particle, the Higgs boson. The corresponding Higgs field is responsible for the breaking of the electroweak symmetry, and so far it remains undiscovered. It is one of the top priorities of current and future accelerator experiments to detect the Higgs, or rule out its existence. Identifying the Higgs sector requires precise theoretical predictions for many Higgs boson production and decay channels. In this thesis, we focus on next-to-leading-order (NLO) calculations for two important processes: the first is pp → H + 2 jets at the CERN Large Hadron Collider (LHC). The second is the gammagamma → H → bb process at the photon collider mode of operation of the International Linear Collider (MC).;For the pp → H + 2 jets process, we compute certain virtual NLO Quantum Chromodynamics (QCD) corrections to the Higgs production through gluon fusion. In particular, we use a bootstrap approach to calculate the one-loop Higgs + 4 partons amplitudes H qqQQ and Hqqgg analytically, for configurations with equal number of positive and negative helicities ("MHV"). The coupling of the Higgs boson to gluons is treated by an effective interaction in the limit of large top quark mass. The Higgs field is split into a complex field and its complex conjugate, simplifying calculations significantly. The cut-containing parts of the amplitudes are obtained using (generalized) unitarity, while the rational pieces are obtained by recursion relations. Throughout this process, only on-shell amplitudes are used. As a consequence, the method is very efficient and gives compact final answers. Using our results, we can compute the color-singlet piece of the gluon fusion-vector boson fusion interference at one-loop, as well as one loop amplitudes with a pseudoscalar instead of a scalar Higgs. We have verified the correctness of our expressions using a number of nontrivial checks and comparisons with previous calculations in the literature.;For the gammagamma → H → bb process at a photon collider, we study the interference between the resonant and continuum (gammagamma → bb) amplitudes. We take into account the virtual gluon exchange between the final state quarks and calculate the one loop corrections to the effective peak height for the case of a light (mH < 160 GeV) Higgs boson. We find that the interference is destructive, depends on the mass of the Higgs and the scattering angle but is only 0.1--0.2%, smaller by an order of magnitude to the experimental accuracy at a photon collider. |
