Standard Model Extensions at the LHC

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Vacuum structure of the Z(2) symmetric Georgi-Machacek model

Publication . Azevedo, Duarte; Ferreira, Pedro Miguel; Logan, Heather E.; Santos, Rui

We 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.

2021-03-23Journal article

Open access

Impact of SM parameters and of the vacua of the Higgs potential in gravitational waves detection

Publication . Freitas, Felipe F.; Lourenço, Gabriel; Morais, António P.; Nunes, André; Olívia, João; Pasechnik, Roman; Santos, Rui; Viana, João

In this work we discuss two different phases of a complex singlet extension of the Standard Model (SM) together with an extension that also includes new fermion fields, in particular, a Majoron model equipped with an inverse seesaw mechanism. All considered scenarios contain a global U(1) symmetry and allow for first-order phase transitions while only two of them are strong enough to favour the detection of primordial gravitational waves (GWs) in planned experiments such as LISA. In particular, this is shown to be possible in the singlet extension with a non vanishing real VEV at zero temperature and also in the model with extra fermions. In the singlet extension with no additional fermions, the detection of GWs strongly depends on the U(1) symmetry breaking pattern of the scalar potential at zero temperature. We study for the first time the impact of the precision in the determination of the SM parameters on the strength of the GWs spectrum. It turns out that the variation of the SM parameters such as the Higgs boson mass and top quark Yukawa coupling in their allowed experimental ranges has a notable impact on GWs detectability prospect

2022-03Journal article

Open access

CP-violation, asymmetries and interferences in tt¯ϕ

Publication . Azevedo, Duarte; Capucha, Rodrigo; Onofre, António; Santos, Rui

In 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.

2022-09-30Journal article

Open access

Impact of electroweak group representation in models for B and g − 2 anomalies from dark loops

Publication . Capucha, Rodrigo; Huang, Da; Lopes, Tomás; Santos, Rui

We discuss two models that are part of a class providing a common explanation for lepton flavor universality violation in b→sl+l− decays, the dark matter (DM) problem and the muon (g−2) anomaly. The B meson decays and the muon (g−2) anomalies are explained by additional one-loop diagrams with DM candidates. The models have one extra fermion field and two extra scalar fields relative to the Standard Model. The SU(3) quantum numbers are fixed by the interaction with the Standard Model fermions in a new Yukawa Lagrangian that connects the dark and the visible sectors. We compare two models, one where the fermion is a singlet and the scalars are doublets under SU(2)L and another one where the fermion is a doublet and the scalars are singlets under SU(2)L. We conclude that both models can explain all of the previously mentioned issues imultaneously, while satisfying all other flavor and DM constraints. However, there are crucial differences between how the DM constraints affect the two models leading to a noticeable difference in the allowed DM mass range.

2022-11Journal article

Open access

Direct detection of pseudo-Nambu-Goldstone dark matter in a two Higgs doublet plus singlet extension of the SM

Publication . Biekötter, Thomas; Gabriel, Pedro; Romacho, María Olalla Olea; Santos, Rui

We calculate the leading radiative corrections to the dark-matter-nucleon scattering in the pseudo-Nambu-Goldstone dark matter model augmented with a second Higgs doublet (S2HDM). In this model, the cross sections for the scattering of the darkmatter on nuclei vanishes at tree-level in the limit of zero momentum-transfer due to a U(1) symmetry. However, this symmetry is softly broken in order to give a mass to the dark-matter particle. As a consequence, non-vanishing scattering cross sections arise at the loop level. We find that the current cross-section limits from dark-matter direct-detection experiments can hardly constrain the parameter space of the S2HDM. However, the loop-corrected predictions for the scattering cross sections can be well within the reach of future direct-detection experiments. As a consequence, future phenomenological analyses of the S2HDM should take into account cross-section predictions beyond tree-level and the experimental constraints from dark-matter direct-detection experiments.

2022-10Journal article

Open access