The link between the electroweak gauge boson masses and the Fermi constant via the muon lifetime measurement is instrumental for constraining and eventually pinning down new physics. We consider the simplest extension of the Standard Model with an additional real scalar SU (2) L ⊗ U (1) Y singlet and compute the electroweak precision parameter ∆r, along with the corresponding theoretical prediction for the Wboson mass. When confronted with the experimental W-boson mass measurement, our predictions impose limits on the singlet model parameter space. We identify regions, especially in the mass range which is accessible by the LHC, where these correspond to the most stringent experimental constraints that are currently available.

At the LHC, an effective theory of the Higgs sector allows us to analyze kinematic distributions in addition to inclusive rates, although there is no clear hierarchy of scales. We systematically analyze how well dimension-6 operators describe LHC observables in comparison to the full theory, and in a range where the LHC will be sensitive. The key question is how the breakdown of the dimension-6 description affects Higgs measurements during the upcoming LHC run for weakly interacting models. We cover modified Higgs sectors with a singlet and doublet extension, new top partners, and a vector triplet. First, weakly interacting models only generate small relevant subsets of dimension-6 operators. Second, the dimension-6 description tends to be justified at the LHC. Scanning over model parameters, significant discrepancies can nevertheless arise; their main source is the matching procedure in the absence of a well-defined hierarchy of scales. While these issues require vigilance, they should not present a major problem for future LHC analyses.

Abstract:We study the production of Higgs boson pairs via gluon fusion at the LHC in the Two-Higgs-Doublet Model. We present predictions at NLO accuracy in QCD, matched to parton showers through the MC@NLO method. A dedicated reweighting technique is used to improve the NLO calculation upon the infinite top-mass limit. We perform our calculation within the MadGraph5 aMC@NLO framework, along with the 2HDM implementation based on the NLOCT package. The inclusion of the NLO corrections leads to large K-factors and significantly reduced theoretical uncertainties. We examine the seven 2HDM Higgs pair combinations using a number of representative 2HDM scenarios. We show how the model-specific features modify the Higgs pair total rates and distribution shapes, leading to trademark signatures of an extended Higgs sector.

Scalar color octets are generic signals for new physics at LHC energies. We examine their pair production at the LHC to next-to-leading order QCD. This computation serves as another test of the fully automized MadGolem framework. We find large NLO production rates and sizeable quantum effects which depend on the sgluon mass. The shift in the sgluon distributions is mild and in good agreement with a multi-jet merging calculation.

Abstract:We study the decay of a heavy Higgs boson into a light Higgs pair at one loop in the singlet extension of the Standard Model. To this purpose, we construct several renormalization schemes for the extended Higgs sector of the model. We apply these schemes to calculate the heavy-to-light Higgs decay width Γ H→hh at next-to-leading order electroweak accuracy, and demonstrate that certain prescriptions lead to gauge-dependent results. We comprehensively examine how the NLO predictions depend on the relevant singlet model parameters, with emphasis on the trademark behavior of the quantum effects, and how these change under different renormalization schemes and a variable renormalization scale. Once all present constraints on the model are included, we find mild NLO corrections, typically of few percent, and with small theoretical uncertainties.