We present the latest developments of the MadGraph/MadEvent Monte Carlo event generator and several applications to hadron collider physics. In the current version events at the parton, hadron and detector level can be generated directly from a web interface, for arbitrary processes in the Standard Model and in several physics scenarios beyond it (HEFT, MSSM, 2HDM). The most important additions are: a new framework for implementing user-defined new physics models; a standalone running mode for creating and testing matrix elements; generation of events corresponding to different processes, such as signal(s) and backgrounds, in the same run; two platforms for data analysis, where events are accessible at the parton, hadron and detector level; and the generation of inclusive multi-jet samples by combining parton-level events with parton showers. To illustrate the new capabilities of the package some applications to hadron collider physics are presented:I. Higgs search in pp → H → W + W − : signal and backgrounds. II. Higgs CP properties: pp → Hjj in the HEFT. III. Spin of a new resonance from lepton angular distributions. IV. Single-top and Higgs associated production in a generic 2HDM. V. Comparison of strong SUSY pair production at the SPS points. VI. Inclusive W +jets matched samples: comparison with the Tevatron data.

Stop particles are expected to be the lightest squarks in supersymmetric theories and the search for these particles is an important experimental task. We therefore present the cross sections for the production processes pp/pp →t 1t1 andt 2t2 at Tevatron and LHC energies in next-to-leading order supersymmetric QCD. The corrections stabilize the theoretical predictions for the cross sections, and they are positive, thus raising the cross sections to values above the leading-order predictions. Mixed t 1t2 /t 1t2 pairs can only be generated in higher orders at strongly suppressed rates.

In the light of the LHC, we revisit the implications of a fourth generation of chiral matter. We identify a specific ensemble of particle masses and mixings that are in agreement with all current experimental bounds as well as minimize the contributions to electroweak precision observables. Higgs masses between 115-315 (115-750) GeV are allowed by electroweak precision data at the 68% and 95% CL. Within this parameter space, there are dramatic effects on Higgs phenomenology: production rates are enhanced, weak-boson-fusion channels are suppressed, angular distributions are modified, and Higgs pairs can we observed. We also identify exotic signals, such as Higgs decay to same-sign dileptons. Finally, we estimate the upper bound on the cutoff scale from vacuum stability and triviality.

Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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At the LHC associated top quark and Higgs boson production with a Higgs boson decay to bottom quarks has long been a heavily disputed search channel. Recently, it has been found not to be viable. We show how it can be observed by tagging massive Higgs bosons and top jets. For this purpose we construct boosted top and Higgs taggers for standard-model processes in a complex QCD environment.

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Higgs boson production via weak boson fusion at the CERN Large Hadron Collider has the capability to determine the dominant CP nature of a Higgs boson, via the tensor structure of its coupling to weak bosons. This information is contained in the azimuthal angle distribution of the two outgoing forward tagging jets. The technique is independent of both the Higgs boson mass and the observed decay channel.The CERN Large Hadron Collider (LHC) is generally regarded as a tool that can guarantee direct observation of a Higgs boson, the remnant of the mechanism believed responsible for electroweak symmetry breaking and fermion mass generation, and the last unobserved element of the Standard Model (SM) of elementary particle physics. Furthermore, the LHC promises complete coverage of Higgs decay scenarios [1,2], including general MSSM parameterizations [1,3], and even invisible Higgs decays [4]. This capability has been greatly enhanced recently by the addition of the weak boson fusion (WBF) production channel to the search strategies [3,5,6]. While being extremely useful at the LHC, WBF has too low a rate and is too similar to background processes at the Fermilab Tevatron [7].Observation of a resonance in some expected decay channel is, however, only the beginning of Higgs physics. Continuing efforts will include the search for more than one Higgs boson, as predicted e.g. by two-Higgs doublet models, of which the MSSM [8,9] is a subset. At least as important is the detailed study of the properties of the Higgs-like resonance, not only at a future Linear Collider [10] but also at the LHC: determination of all the quantum numbers and couplings of the state. These include the gauge, Yukawa and self-couplings as well as the charge, color, spin, and CP quantum numbers. While charge and color identification is straightforward and a technique has been proposed for the gauge and Yukawa coupling determinations [11], the LHC has considerable difficulty in practice to determine the Higgs CP transformation properties for intermediate Higgs masses [12] via a weak boson coupling, and no technique has yet been proposed to identify the tensor structure of the Higgs-weak boson vertex in the intermediate mass range. The methods of Ref.[13] may be useful, but only for very light Higgs masses. Furthermore, this method does not involve the weak boson vertices at all.In this letter we propose a technique which achieves the CP measurement goal via a study of WBF events. WBF Higgs production, while not the largest cross section at the LHC, is useful because of its characteristic kinematical structure, involving two forward tagging jets and central Higgs decay products, which allows one to isolate the signal in a low background environment. The angular distribution of the two tagging jets carries unambiguous information on the CP properties of the Higgs couplings to weak bosons which is independent of the Higgs decay channel observed.As a theoretical framework we consider two possible ways to couple a spin zero field to two gauge bosons via hig...