arXiv subjects

Rui Santos

Publications and source records attributed to Rui Santos.

At least 91 records · Page 5Linked to original sources

arXiv2016

Gauge-independent Renormalization of the 2-Higgs-Doublet Model

The 2-Higgs-Doublet Model (2HDM) belongs to the simplest extensions of the Standard Model (SM) Higgs sector that are in accordance with theoretical and experimental constraints. In order to be able to properly investigate the experimental Higgs data and, in the long term to distinguish between possible models beyond the SM, precise predictions for the Higgs boson observables have to be made available on the theory side. This requires the inclusion of the higher order corrections. In this work, we investigate in detail the renormalization of the 2HDM, a pre-requisite for the computation of higher order corrections. We pay particular attention to the renormalization of the mixing angles $α$ and $β$, which diagonalize the Higgs mass matrices and which enter all Higgs observables. The implications of various renormalization schemes in next-to-leading order corrections to the sample processes $H^\pm \to W^\pm h/H$ and $H \to ZZ$ are investigated. Based on our findings, we will present a renormalization scheme that is at the same time process independent, gauge independent and numerically stable.

hep-ph↗

arXiv2016

Optimising Charged Higgs Boson Searches at the Large Hadron Collider Across $b\bar b W^\pm$ Final States

In the light of the most recent data from Higgs boson searches and analyses, we re-assess the scope of the Large Hadron Collider in accessing heavy charged Higgs boson signals in $b\bar b W^\pm$ final states, wherein the contributing channels can be $H^+\to t\bar b$, $hW^\pm, HW^\pm$ and $AW^\pm$. We consider a 2-Higgs Doublet Model Type-II and we assume as production mode $bg\to tH^-$ + c.c., the dominant one over the range $M_{H^\pm}\ge 480$ GeV, as dictated by $b\to sγ$ constraints. Prospects of detection are found to be significant for various Run 2 energy and luminosity options.

hep-ph↗

arXiv2015

Production of t$γ$, tZ and tH via Flavour Changing Neutral Currents

A UFO model describing general top quark Flavour Changing Neutral Currents is presented. We use it to study t$γ$, tH and tZ production via FCNCs anomalous couplings at the Large Hadron Collider, in particular how the distributions of physical observables depend on the anomalous couplings. A sensitivity study of the Large Hadron Collider experiments to tZ production via FCNC in its second stage of operation is also performed.

hep-ph↗

arXiv2015

Undoubtable signs of CP-violation in Higgs decays at the LHC run 2

With the discovery of the Higgs boson at the Large Hadron Collider the high energy physics community's attention has now turned to understanding the properties of the Higgs boson, together with the hope of finding more scalars during run 2. In this work we discuss scenarios where using a combination of three decays, involving the 125 GeV Higgs boson, the Z boson and at least one more scalar, an indisputable signal of CP-violation arises. We use a complex two-Higgs doublet model as a reference model and present some benchmark points that have passed all current experimental and theoretical constraints, and that have cross sections large enough to be probed during run 2.

hep-ph↗

arXiv2015

Large pseudo-scalar components in the C2HDM

We discuss the CP nature of the Yukawa couplings of the Higgs boson in the framework of a complex two Higgs doublet model (C2HDM). After analysing all data gathered during the Large Hadron Collider run 1, the measurement of the Higgs couplings to the remaining SM particles already restricts the parameter space of many extensions of the SM. However, there is still room for very large CP-odd Yukawa couplings to light quarks and leptons while the top-quark Yukawa coupling is already very constrained by current data. Although indirect measurements of electric dipole moments play a very important role in constraining the pseudo-scalar components of the Yukawa couplings, we argue that a direct measurement of the ratio of pseudoscalar to scalar couplings should be one of the top priorities for the LHC run 2.

hep-ph↗

arXiv2015

Two-loop stability of singlet extensions of the SM with dark matter

We present a study of the high energy stability of a minimal complex singlet extension of the Standard Model with or without dark matter (CxSM). We start by obtaining the beta functions of the couplings of the theory from the effective potential and then perform the RGE evolution for the allowed parameter space of the model at the electroweak scale. We obtain the scale up to which the model survives and combine this information with all the LHC measurements as well as bounds from dark matter detection experiments as well as the relic density best measurement from Planck. We conclude that scenarios which solve both the dark matter and stability problems must contain a dark particle heavier than 50 GeV and a new visible state (mixing with the SM-like Higgs) heavier than 170 GeV.

hep-ph↗

arXiv2015

Two-loop stability of a complex singlet extended Standard Model

Motivated by the dark matter and the baryon asymmetry problems, we analyse a complex singlet extension of the Standard Model (SM) with a Z2 symmetry (which provides a dark matter candidate). After a detailed two-loop calculation of the renormalization group equations for the new scalar sector, we study the radiative stability of the model up to a high energy scale (with the constraint that the 126 GeV Higgs boson found at the LHC is in the spectrum) and find it requires the existence of a new scalar state mixing with the Higgs with a mass larger than 140 GeV. This bound is not very sensitive to the cut-off scale as long as the latter is larger than 10^10 GeV. We then include all experimental and observational constraints/measurements from collider data, dark matter direct detection experiments and from the Planck satellite and in addition force stability at least up to the GUT scale, to find that the lower bound is raised to about 170 GeV, while the dark matter particle must be heavier than about 50 GeV.

hep-ph↗

arXiv2015

Large pseudoscalar Yukawa couplings in the complex 2HDM

We start by presenting the current status of a complex flavour conserving two-Higgs doublet model. We will focus on some very interesting scenarios where unexpectedly the light Higgs couplings to leptons and to b-quarks can have a large pseudoscalar component with a vanishing scalar component. Predictions for the allowed parameter space at end of the next run with a total collected luminosity of $300 \, fb^{-1}$ and $3000 \, fb^{-1}$ are also discussed. These scenarios are not excluded by present data and most probably will survive the next LHC run. However, a measurement of the mixing angle $ϕ_τ$, between the scalar and pseudoscalar component of the 125 GeV Higgs, in the decay $h \to τ^+ τ^-$ will be able to probe many of these scenarios, even with low luminosity. Similarly, a measurement of $ϕ_t$ in the vertex $\bar t t h$ could help to constrain the low $\tan β$ region in the Type I model.

hep-ph↗

arXiv2014

The Wrong Sign limit in the 2HDM

A sign change in the Higgs couplings to fermions and massive gauge bosons is still allowed in the framework of two-Higgs doublet models (2HDM). In this work we discuss the possible sign changes in the Higgs couplings to fermions and gauge bosons, while reviewing the status of the 8-parameter CP-conserving 2HDM after the Large Hadron Collider 8 TeV run.

hep-ph↗

arXiv2014

The CP-conserving 2HDM after the 8 TeV run

We confront the most common CP-conserving 2HDM with the LHC data analysed so far while taking into account all previously available experimental data. A special allowed corner of the parameter space is analysed - the so-called wrong-sign scenario where the Higgs coupling to down-type quarks changes sign relative to the Standard Model while the coupling to the massive vector bosons does not.

hep-ph↗

arXiv2014

Wrong sign and symmetric limits and non-decoupling in 2HDMs

We analyse the possibility that, in two Higgs doublet models, one or more of the Higgs couplings to fermions or to gauge bosons change sign, relative to the respective Higgs Standard Model couplings. Possible sign changes in the coupling of a neutral scalar to charged ones are also discussed. These \textit{wrong signs} can have important physical consequences, manifesting themselves in Higgs production via gluon fusion or Higgs decay into two gluons or into two photons. We consider all possible wrong sign scenarios, and also the \textit{symmetric limit}, in all possible Yukawa implementations of the two Higgs doublet model, in two different possibilities: the observed Higgs boson is the lightest CP-even scalar, or the heaviest one. We also analyse thoroughly the impact of the currently available LHC data on such scenarios. With all 8 TeV data analysed, all wrong sign scenarios are allowed in all Yukawa types, even at the 1$σ$ level. However, we will show that B-physics constraints are crucial in excluding the possibility of wrong sign scenarios in the case where $\tan β$ is below 1. We will also discuss the future prospects for probing the wrong sign scenarios at the next LHC run. Finally we will present a scenario where the alignment limit could be excluded due to non-decoupling in the case where the heavy CP-even Higgs is the one discovered at the LHC.

hep-ph↗

arXiv2014

Probing wrong-sign Yukawa couplings at the LHC and a future linear collider

We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, $b$-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future $e^+ e^-$ linear collider could determine the sign of the Higgs coupling to $b$-quark pairs. Discrimination is possible for two reasons. First, the interference between the $b$-quark and the $t$-quark loop contributions to the $ggh$ coupling changes sign. Second, the charged-Higgs loop contribution to the $γγh$ coupling is large and fairly constant up to the largest charged-Higgs mass allowed by tree-level unitarity bounds when the $b$-quark Yukawa coupling has the opposite sign from that of the SM (the change in sign of the interference terms between the $b$-quark loop and the $W$ and $t$ loops having negligible impact).

hep-ph↗

arXiv2013

Limits on strong FCNC top couplings at the LHC

The best limit on the strong FCNC anomalous couplings was obtained using the direct top production process at the Large Hadron Collider by the ATLAS collaboration. We perform a similar analysis but using a next-to-leading order generator, MEtop. We then show how the limits could be improved if the FCNC single top process $pp \to tj$ would be included as signal. Finally we discuss a slightly modified analysis with an extra hard jet in the final state.

hep-ph↗

arXiv2013

Measuring the charged Higgs mass and distinguishing between models with top-quark observables

We study the process of single-top production in association with a charged Higgs boson at the LHC and discuss how top-quark polarization and an azimuthal asymmetry $A_φ$ of the charged lepton from top decay can be used in some extensions of the Standard Model to determine or constrain the charged Higgs boson mass. We also discuss some scenarios where these variables can be used to distinguish between different models.

hep-ph↗

arXiv2013

2HDM confronting LHC data

Almost all data collected at the LHC during the 7 and 8 TeV runs has now been analysed by the ATLAS and CMS collaborations. Its consistency with the Standard Model (SM) predictions has cornered the CP-conserving two-Higgs doublet model (2HDM) into the SM limit, sin (β-α) = 1. However, there are still allowed regions of the 2HDM parameter space away from this limit. In this work we discuss how the 2HDM is performing in view of the LHC data together with the remaining available experimental and theoretical constraints.

hep-ph↗

arXiv2013

Tree-level metastability bounds in two-Higgs doublet models

The two Higgs doublet model has a rich vacuum structure, including the possibility of existence of two Standard Model-like minima at tree-level. It is therefore possible that the universe's vacuum is metastable, and a deeper minimum exists. We present the analytical conditions one must demand of the potential's parameters to prevent that possibility, and analyse what the current LHC data tells us about the eventual existence of that second minimum.

hep-ph↗

arXiv2013

2HDM at the LHC - the story so far

We confront the most common version of the CP-conserving 2HDM with LHC data, taking into account all previously available experimental data. We also discuss the scenario where the 125 GeV Higgs discovered at the LHC is the lightest neutral scalar of a particular CP-violating 2HDM. In this scenario we focus on what data can already tell us about the amount of mixing between CP-even and CP-odd states.

hep-ph↗

arXiv2013

MEtop - a top FCNC event generator

We present a new Monte Carlo generator for Direct top and Single top production via flavour-changing neutral currents (FCNC). This new tool calculates the cross section and generates events with Next-to-Leading order precision for the Direct top process and Leading-Order precision for all other FCNC single top processes. A set of independent dimension six FCNC operators has been implemented - including four-fermion operators - where at least one top-quark is present in the interaction.

hep-ph↗