Browsing by Author "Azevedo, Duarte"
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- Benchmarking di-Higgs production in various extended Higgs sector modelsPublication . Abouabid, Hamza; Arhrib, Abdesslam; Azevedo, Duarte; Falaki, Jaouad El; Ferreira, Pedro Miguel; Muhlleitner, Milada; Santos, RuiWe present a comprehensive study on Higgs pair production in various archetypical extended Higgs sectors such as the real and the complex 2-Higgs-Doublet Model, the 2-Higgs-Doublet Model augmented by a real singlet field and the Next-to-Minimal Supersymmetric extension of the Standard Model. We take into account all relevant theoretical and experimental constraints, in particular the experimental limits on non-resonant and resonant Higgs pair production. We present the allowed cross sections for Standard Model (SM)-like Higgs pair production and the ranges of the SM-like Yukawa and trilinear Higgs self-coupling that are still compatible with the applied constraints. Furthermore, we give results for the pair production of a SM-like with a non-SM-like Higgs boson and for the production of a pair of non-SM-like Higgs bosons. We find that di-Higgs production in the models under investigation can exceed the SM rate substantially, not only in the non-resonance region but also due to resonant enhancement. We give several benchmarks with interesting features such as large cross sections, the possibility to test CP violation, Higgs-to-Higgs cascade decays or di-Higgs production beating single Higgs production. In all of our benchmark points, the next-to-leading order QCD corrections are included in the large top-mass limit. For these points, we found that, depending on the model and the Higgs pair final state, the corrections increase the leading order cross section by a factor of 1.79 to 2.24. We also discuss the relation between the description of Higgs pair production in an effective field theory approach and in the specific models investigated here
- CP in the darkPublication . Azevedo, Duarte; Ferreira, Pedro Miguel; Muhlleitner, Milada; Patel, Shruti; Santos, Rui; Wittbrodt, JonasWe build a model containing two scalar doublets and a scalar singlet with a specific discrete symmetry. After spontaneous symmetry breaking, the model has Standard Model-like phenomenology, as well as a hidden scalar sector which provides a viable dark matter candidate. We show that CP violation in the scalar sector occurs exclusively in the hidden sector, and consider possible experimental signatures of this CP violation. In particular, we study contribution to anomalous gauge couplings from the hidden scalars.
- CP-violation, asymmetries and interferences in tt¯ϕPublication . Azevedo, Duarte; Capucha, Rodrigo; Onofre, António; Santos, RuiIn this paper, we use the associated production of top-quark pairs (tt¯) with a generic scalar boson (ϕ) at the LHC (pp → tt¯ϕ) to explore the sensitivity of a large set of observables to the sign of the CP mixing angle (α), present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to mϕ = 125 GeV (the Standard Model Higgs boson mass) and its coupling to top-quarks is varied such that α = 0°, 22.5°, 45.0°, 67.5°, 90.0°, 135.0° and 180.0°. Dileptonic final states of the tt¯ϕ system are used (pp → bℓ+νℓb¯¯ℓ−ν¯¯¯ℓbb¯¯), where the scalar boson is expected to decay according to ϕ → bb¯¯. A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model (SM) background and signal events generated with MadGraph5_aMC@NLO, in turn reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the ϕ boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar (σ0+) and pure pseudo-scalar (σ0−) production cross section values, it is unlikely the tt¯ϕ channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the btt¯ϕ2 and btt¯ϕ4 variables, exclusion limits at 95% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to κ∼ ∈ [-0.698,+0.698] and |κ| ∈ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.
- 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).
- Models with extended Higgs sectors at future e(+)e(-) collidersPublication . Azevedo, Duarte; Ferreira, Pedro Miguel; Muehlleitner, Milada Margarete; Santos, Rui; Wittbrodt, JonasWe discuss the phenomenology of several beyond the Standard Model (SM) extensions that include extended Higgs sectors. The models discussed are the SM extended by a complex singlet field, the 2-Higgs-doublet model with a CP-conserving and a CP-violating scalar sector, the singlet extension of the 2-Higgs-doublet model, and the next-to-minimal supersymmetric SM extension. All the above models have at least three neutral scalars, with one being the 125 GeV Higgs boson. This common feature allows us to compare the production and decay rates of the other two scalars and therefore to compare their behavior at future electron-positron colliders. Using predictions on the expected precision of the 125 GeV Higgs boson couplings at these colliders we are able to obtain the allowed admixtures of either a singlet or a pseudoscalar to the observed 125 GeV scalar. Therefore, even if no new scalar is found, the expected precision at future electron-positron colliders, such as CLIC, will certainly contribute to a clearer picture of the nature of the discovered Riggs boson.
- One-loop contribution to dark-matter-nucleon scattering in the pseudo-scalar dark matter modelPublication . Azevedo, Duarte; Duch, Mateusz; Grzadkowski, Bohdan; Huang, Da; Iglicki, Michal; Santos, RuiRecent dark matter (DM) direct searches place very stringent constraints on the possible DM candidates proposed in extensions of the Standard Model. There are however models where these constraints are avoided. One of the simplest and most striking examples comes from a straightforward Higgs-portal pseudo-scalar DM model featured with a softly broken U(1) symmetry. In this model the tree-level DM-nucleon scattering cross section vanishes in the limit of zero momentum transfer. It has also been argued that the leading-order DM-nucleon cross section appears at the one-loop level. In this work we have calculated the exact cross section in the zero momentum transfer at the leading order i.e., at the one-loop level of perturbative expansion. We have concluded that, in agreement with expectations, the amplitude for the scattering process is UV finite and approaches zero in the limit of vanishing DM masses. Moreover, we made clear that the finite DM velocity correction at tree level is subdominant with respect to the one-loop contribution. Based on the analytic formulae, our numerical studies show that, for a typical choice of model parameters, the DM nuclear recoiling cross section is well below OO(10−50 cm2), which indicates that the DM direct detection signal in this model naturally avoids present strong experimental limits on the cross section.
- One-loop corrections to the Higgs boson invisible decay in the dark doublet phase of the N2HDMPublication . Azevedo, Duarte; Gabriel, Pedro; Muehlleitner, Milada Margarete; Sakurai, Kodai; Santos, RuiThe Higgs invisible decay width may soon become a powerful tool to probe extensions of the Standard Model with dark matter candidates at the Large Hadron Collider. In this work, we calculate the next-to-leading order (NLO) electroweak corrections to the 125 GeV Higgs decay width into two dark matter particles. The model is the next-to-minimal 2-Higgs-doublet model (N2HDM) in the dark doublet phase, that is, only one doublet and the singlet acquire vacuum expectation values. We show that the present measurement of the Higgs invisible branching ratio, BR(H -> invisible < 0.11), does not lead to constraints on the parameter space of the model at leading order. This is due to the very precise measurements of the Higgs couplings but could change in the near future. Furthermore, if NLO corrections are required not to be unphysically large, no limits on the parameter space can be extracted from the NLO results.
- 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.
- Search for an invisible scalar in tt¯ final states at the LHCPublication . Azevedo, Duarte; Capucha, Rodrigo; Chaves, Pedro; Martins, João Bravo; Onofre, A.; Santos, RuiWe use the current tt¯ experimental analysis to look for Dark Matter (DM) particles hidden in the final state. We present a phenomenological study where we successfully perform the reconstruction of a tt¯ system in the presence of a scalar mediator Y 0, that couples to both Standard Model (SM) and to DM particles. We use a MadGraph5_aMC@NLO simplified DM model, where signal samples of pp → tt¯ Y 0 are generated at the Large Hadron Collider (LHC) with both Charge-Parity (CP) -even and CP-odd couplings of Y 0 to the top quarks. Different mass scales for the Y 0 mediator are considered, from the low mass region (~ 0 GeV) to masses close to the Higgs boson mass (125 GeV). The dileptonic final states of the tt¯ system were used in our analysis. The reconstruction of the tt¯ system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic tt¯ search at the LHC are considered. Furthermore, CP angular observables were used to probe the CP-nature of the coupling between the mediator and top-quarks, which allowed to set confidence level (CL) limits for those Yukawa couplings as a function of the mediator mass.
- Signal versus background interference in H+ → t¯b signals for MSSM benchmark scenariosPublication . Arhrib, Abdesslam; Azevedo, Duarte; Benbrik, Rachid; Harouiz, Hicham; Moretti, Stefano; Patrick, Riley; Santos, RuiIn this paper, we investigate sizeable interference effects between a heavy charged Higgs boson signal produced dominantly via gg -> tbH- (+ c.c.) followed by the decay H--> bt (+ c.c.) and the irreducible background given by pp -> ttbb topologies at the Large Hadron Collider (LHC). We show that it may be possible that such effects could spoil current H-+/- searches where signal and background are normally treated separately. The reason for this is that a heavy charged Higgs boson can have a large total width, in turn enabling such interferences, altogether leading to potentially very significant alterations, both at the inclusive and exclusive level, of the yield induced by the signal alone. This therefore implies that currently established LHC searches for such wide charged Higgs bosons might require modifications. We show such effects quantitatively using two different benchmark configurations of the minimal realisation of Supersymmetry, wherein such H-+/- states naturally exist. However, on the basis of the limited computing resources available, we are unable to always bring the statistical error down to a level where all such interference effects are unequivocal, so that we advocate dedicated experimental analyses to confirm this with higher statistics data samples.