Standard Model Higgs pair production at e^+e^- colliders has the capability to determine the Higgs boson self-coupling lambda. I present a detailed analysis of the e^+e^- -> ZHH and e^+e^- -> ubar u HH signal channels, and the relevant background pr
ocesses, for future e^+e^- linear colliders with center of mass energies of sqrt{s}=0.5 TeV, 1 TeV, and 3 TeV. Special attention is given to the role non-resonant Feynman diagrams play, and the theoretical uncertainties of signal and background cross sections. I also derive quantitative sensitivity limits for lambda. I find that an e^+e^- collider with sqrt{s}=0.5 TeV can place meaningful bounds on lambda only if the Higgs boson mass is relatively close to its current lower limit. At an e^+e^- collider with sqrt{s}=1 TeV (3 TeV), lambda can be determined with a precision of 20-80% (10-20%) for integrated luminosities in the few ab^{-1} range and Higgs boson masses in the range m_H=120-180 GeV.
Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, t-bar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such part
icles. We present a detailed study of the discovery potential of the CERN Large Hadron Collider for Kaluza-Klein (KK) excitations of the gluon in bulk Randall-Sundrum (RS) models in the t-bar t -> ell^+/- nu b-bar bq-bar q (ell=e, mu) final state. We utilize final states with one or two tagged b-quarks, and two, three or four jets (including b-jets). Our calculations take into account the finite resolution of detectors, the energy loss due to b-quark decays, the expected reduced b-tagging efficiency at large t-bar t invariant masses, and include the background originating from Wb-bar b+jets, (Wb+W-bar b)+jets, W+jets, and single top + jets production. We derive semi-realistic 5 sigma discovery limits for nine different KK gluon scenarios, and compare them with those for KK gravitons, and a Z_H boson in the Littlest Higgs model. We also analyze the capabilities of the LHC experiments to differentiate between individual KK gluon models and measure the couplings of KK gluons to quarks. We find that, for the parameters and models chosen, KK gluons with masses up to about 4 TeV can be discovered at the LHC. The ability of the LHC to discriminate between different bulk RS models, and to measure the couplings of the KK gluons is found to be highly model dependent.
Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, tbar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such parti
cles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the tbar ttoell^pm u bbar bqbar q (ell=e, mu) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As a result, a large fraction of tbar ttoell^pm u bbar bqbar q events with an invariant mass in the TeV region contains less than four observable jets. Requiring one or two tagged b-quarks, we calculate the W+jets, Wb+jets, Wbbar b+jets, Wbt, and single top plus jets backgrounds for these final states, and identify cuts which help to suppress them. In particular, we discuss whether a cut on the jet invariant mass may be useful in reducing the background in the ell u+2 jets channel. We also investigate how next-to-leading order QCD corrections affect high p_T top quark production at the LHC. We find that the ell u+2 jets and ell u+3 jets final states with one or two $b$-tags will significantly improve the chances for discovering new heavy particles in the tbar t channel at the LHC.
