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- Scalar mass dependence of angular variables in t(t)over-bar phi productionPublication . Azevedo, Duarte; Capucha, Rodrigo; Onofre, Antonio; Santos, RuiIn this paper we explore CP discrimination in the associated production of top-quark pairs (t (t) over bar) with a generic scalar boson (phi) at the LHC. We probe the CP-sensitivity of several observables for a varying scalar boson mass and CP-number, either CP-even (phi = H) or CP-odd (phi= A), using dileptonic final states of the t (t) over bar phi system, with phi -> (b) over bar. We show that CP-searches are virtually impossible for phi boson masses above a few hundred GeV in this channel. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5 aMC@NLO and reconstructed using a kinematic fit. The most sensitive CP-observables are used to compute Confidence Levels (CLs), as a function of luminosity, for the exclusion of different signal hypotheses with scalar and pseudoscalar boson masses that range from m(phi) = 40 GeV up to 200 GeV. We finalize by analysing the impact of a measurement (or limit) of the CP-violating angle in the parameter space of a complex two-Higgs doublet model known as the C2HDM.
- Electroweak corrections in a pseudo Nambu-Goldstone Dark Matter model revisitedPublication . Glaus, Seraina; Muehlleitner, Margarete; Mueller, Jonas; Patel, Shruti; Roemer, Tizian; Santos, RuiHaving so far only indirect evidence for the existence of Dark Matter a plethora of experiments aims at direct detection of Dark Matter through the scattering of Dark Matter particles off atomic nuclei. For the correct interpretation and identification of the underlying nature of the Dark Matter constituents higher-order corrections to the cross section of Dark Matter-nucleon scattering are important, in particular in models where the tree-level cross section is negligibly small. In this work we revisit the electroweak corrections to the dark matter-nucleon scattering cross section in a model with a pseudo Nambu-Goldstone boson as the Dark Matter candidate. Two calculations that already exist in the literature, apply different approaches resulting in different final results for the cross section in some regions of the parameter space leading us to redo the calculation and analyse the two approaches to clarify the situation. We furthermore update the experimental constraints and examine the regions of the parameter space where the cross section is above the neutrino floor but which can only be probed in the far future.
- Vacuum structure of the Z(2) symmetric Georgi-Machacek modelPublication . Azevedo, Duarte; Ferreira, Pedro Miguel; Logan, Heather E.; Santos, RuiWe discuss the vacuum structure of a version of the Georgi-Machecek model with an exact Z(2) symmetry acting on the triplet fields. Besides the usual custodial-symmetric model, with rho = 1 at tree-level, a model with a dark matter candidate is also viable. The other phases of the model lead to electric charge breaking, a wrong pattern of electroweak symmetry breaking or to rho not equal 1 at tree-level. We derive conditions to have an absolute minimum in each of the two viable phases, the custodial and the dark matter phases.
- Light Higgs searches in t(t)over-bar phi production at the LHCPublication . Azevedo, Duarte; Capucha, Rodrigo; Gouveia, Emanuel; Onofre, Antonio; Santos, RuiIn this paper we propose a new reconstruction method to explore the low mass region in the associated production of top-quark pairs (t (t) over bar) with a generic scalar boson (phi) at the LHC. The new method of mass reconstruction shows an improved resolution of at least a factor of two in the low mass region when compared to previous methods, without the loss of sensitivity of previous analyses. It turns out that it also leads to an improvement of the mass reconstruction of the 125 GeV Higgs for the same production process. We use an effective Lagrangian to describe a scalar with a generic Yukawa coupling to the top quarks. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5_aMC@NLO and reconstructed using a kinematic fit. The use of CP-sensitive variables allows then to maximize the distinction between CP-even and CP-odd components of the Yukawa couplings. Confidence Levels (CLs) for the exclusion of phi bosons with mixed CP (both CP-even and CP-odd components) were determined as a function of the top Yukawa couplings to the phi boson. The mass range analysed starts slightly above the Upsilon mass up to 40 GeV, although the analysis can be used for an arbitrary mass. If no new light scalar is found, exclusion limits at 95% CL for the absolute value of the CP-even and CP-odd Yukawa are derived. Finally, we analyse how these limits constrain the parameter space of the complex two-Higgs doublet model (C2HDM).
- The dark phases of the N2HDMPublication . Engeln, Isabell; Ferreira, Pedro Miguel; Muehlleitner, Milada Margarete; Santos, Rui; Wittbrodt, JonasWe discuss the dark phases of the Next-to-2-Higgs Doublet model. The model is an extension of the Standard Model with an extra doublet and an extra singlet that has four distinct CP-conserving phases, three of which provide dark matter candidates. We discuss in detail the vacuum structure of the different phases and the issue of stability at tree-level of each phase. Taking into account the most relevant experimental and theoretical constraints, we found that there are combinations of measurements at the Large Hadron Collider that could single out a specific phase. The measurement of h(125) -> gamma gamma together with the discovery of a new scalar with specific rates to tau (+)tau (-) or gamma gamma could exclude some phases and point to a specific phase.
- CP violating hW(+)W(-) coupling in the Standard Model and beyondPublication . Huang, Da; Morais, António P.; Santos, RuiInspired by the recent development in determining the property of the observed Higgs boson, we explore the CP-violating (CPV) -cCPVhW+mu nu Wmu nu-/upsilon coupling in the Standard Model (SM) and beyond, where W-+/- mu nu and W+/- mu nu denote the W-boson field strength and its dual. To begin with, we show that the leading-order SM contribution to this CPV vertex appears at two-loop level. By summing over the quark flavor indices in the two loop integrals analytically, we can estimate the order of the corresponding Wilson coefficient to be cCPVSMsimilar to O10-23, which is obviously too small to be probed at the LHC and planned future colliders. Then we investigate this CPV hW(+)W(-) interaction in two Beyond the Standard Model benchmark models: the left-right model and the complex 2-Higgs doublet model (C2HDM). Unlike what happens for the SM, the dominant contributions in both models arise at the one-loop level, and the corresponding Wilson coefficient can be as large as of O(10(-9)) in the former model and of O(10(-3)) for the latter. In light of such a large CPV effect in the hW(+)W(-) coupling, we also give the formulae for the leading one-loop contribution to the related CPV hZZ effective operator in the complex 2-Higgs doublet model. The order of magnitude of the Wilson coefficients in the C2HDM may be within reach of the high-luminosity LHC or planned future colliders.