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Rui Santos

Publications and source records attributed to Rui Santos.

At least 109 records · Page 6Linked to original sources
arXiv2013

Metastability bounds on the two Higgs doublet model

In the two Higgs doublet model, there is the possibility that the vacuum where the universe resides in is metastable. We present the tree-level bounds on the scalar potential parameters which have to be obeyed to prevent that situation. Analytical expressions for those bounds are shown for the most used potential, that with a softly broken $Z_2$ symmetry. The impact of those bounds on the model's phenomenology is discussed in detail, as well as the importance of the current Large Hadron Collider results in determining whether the vacuum we live in is or is not stable. We demonstrate how the vacuum stability bounds can be obtained for the most generic CP-conserving potential, and provide a simple method to implement them.

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arXiv2013

Evading death by vacuum

In the Standard Model, the Higgs potential allows only one minimum at tree-level. But the open possibility that there might be two scalar doublets enriches the vacuum structure, allowing for the risk that we might now be in a metastable state, which we dub the panic vacuum. Current experiments at the LHC are probing the Higgs particle predicted as a result of the spontaneous symmetry breaking. Remarkably, in the two Higgs model with a softly broken U(1) symmetry, the LHC experiments already preclude panic vacuum solutions.

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arXiv2013

Mass-degenerate Higgs bosons at 125 GeV in the Two-Higgs-Doublet Model

The analysis of the Higgs boson data by the ATLAS and CMS Collaborations appears to exhibit an excess of h --> gammaγevents above the Standard Model (SM) expectations; whereas no significant excess is observed in h --> ZZ* --> {four lepton} events, albeit with large statistical uncertainty due to the small data sample. These results (assuming they persist with further data) could be explained by a pair of nearly mass-degenerate scalars, one of which is a SM-like Higgs boson and the other is a scalar with suppressed couplings to W+W- and ZZ. In the two Higgs doublet model, the observed γγand ZZ* --> {four lepton} data can be reproduced by an approximately degenerate CP-even (h) and CP-odd (A) Higgs boson for values of \sin(β-α) near unity and 0.7 < \tanβ< 1. An enhanced γγsignal can also arise in cases where m_h ~ m_H, m_H ~ m_A, or m_h ~ m_H ~ m_A. Since the ZZ* --> {four lepton} signal derives primarily from a SM-like Higgs boson whereas the γγsignal receives contributions from two (or more) nearly mass-degenerate states, one would expect a slightly different invariant mass peak in the ZZ* --> {four lepton} and γγchannels. The phenomenological consequences of such models can be tested with additional Higgs data that will be collected at the LHC in the near future.

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arXiv2013

Charged Higgs boson production in single top mode at the LHC

The main production mode for a light charged Higgs boson at the LHC is $pp \to t \bar t$, with one the top-quarks decaying to a charged Higgs and a $b$-quark. However, single top production also gives rise to final states with charged Higgs bosons. In this work we analyse how the two processes compare at the LHC@14TeV. We will be working in the framework of the two-Higgs double model, considering both a CP-conserving and a CP-violating version of the model. We conclude that the single top mode could help to constrain the parameter space in several versions of the model. We also discuss the role of other complementary production processes in future searches at the LHC.

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arXiv2013

ScannerS: Constraining the phase diagram of a complex scalar singlet at the LHC

We present the first version of a new tool to scan the parameter space of generic scalar potentials, ScannerS. The main goal of ScannerS is to help distinguish between different patterns of symmetry breaking for each scalar potential. In this work we use it to investigate the possibility of excluding regions of the phase diagram of several versions of a complex singlet extension of the Standard Model, with future LHC results. We find that if another scalar is found, one can exclude a phase with a dark matter candidate in definite regions of the parameter space, while predicting whether a third scalar to be found must be lighter or heavier. The first version of the code is publicly available and contains various generic core routines for tree level vacuum stability analysis, as well as implementations of collider bounds, dark matter constraints, electroweak precision constraints and tree level unitarity.

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arXiv2012

Charged Higgs discovery potential in the single top mode in 2HDMs

We discuss the discovery potential of a charged Higgs boson in the single top mode. The models discussed include a CP-conserving and a CP-violating version of the softly broken $Z_2$ symmetric 2HDM potential. We conclude that the single top mode could help to constrain the ($m_{H^\pm}$, $\tan β$) plane in several versions of 2HDMs.

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arXiv2012

Charged Higgs bosons in single top production at the LHC

We show that a light charged Higgs boson signal via $τ^\pmν$ decay can be established at the Large Hadron Collider (LHC) also in the case of single top production. This process complements searches for the same signal in the case of charged Higgs bosons emerging from $t \bar t$ production. The models accessible include the Minimal Supersymmetric Standard Model (MSSM) as well a variety of 2-Higgs Doublet Models (2HDMs). High energies and luminosities are however required, thereby restricting interest on this mode to the case of the LHC running at 14 TeV with design configuration.

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arXiv2012

MEtop - a generator for single top production via FCNC interactions

We present a generator for single top quark production via flavour-changing neutral currents. The MEtop event generator allows for Next-to-Leading-Order direct top production $pp \to t$ and Leading-Order production of several other single top processes. A few packages with definite sets of dimension six operators are available. We discuss how to improve the bounds on the effective operators and how well new physics can be probed with each set of independent dimension six operators.

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arXiv2012

Probing the scalar-pseudoscalar mixing in the 125 GeV Higgs particle with current data

LHC has found hints for a Higgs particle of 125 GeV. We investigate the possibility that such a particle is a mixture of scalar and pseudoscalar states. For definiteness, we concentrate on a two Higgs doublet model with explicit CP violation and soft Z2 violation. Including all Higgs production mechanisms, we determine the current constraints obtained by comparing h -> gamma gamma with h -> V V*, and comment on the information which can be gained by measurements of h -> b \bar{b}. We find bounds |s_2| < 0.83 at one sigma, where |s_2|=0 (|s_2|=1) corresponds to a pure scalar (pure pseudoscalar) state.

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arXiv2012

Two Higgs Doublet Model in light of the Standard Model H -> tau^+ tau^- search at the LHC

We study the implications of the searches based on H -> tau^+ tau^- by the ATLAS and CMS collaborations on the parameter space of the two Higgs doublet model (2HDM). In the 2HDM, the scalars can decay into a tau pair with a branching ratio larger than the SM one, leading to constraints on the 2HDM parameter space. We have also discussed the implications for the 2HDM of the recent di-muon search by the ATLAS collaboration for a CP-odd scalar in the mass range 4-12 GeV.

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arXiv2012

New physics contributions to the forward-backward asymmetry at the Tevatron

The Tevatron has measured a discrepancy relative to the Standard Model prediction in the forward-backward asymmetry in top quark pair production. This asymmetry grows with the rapidity difference of the two top quarks. It also increases with the invariant mass of the tt pair, reaching, for high invariant masses, 3.4 standard deviations above the Next to Leading Order prediction for the charge asymmetry of QCD. However, perfect agreement between experiment and the Standard Model was found in both total and differential cross section of top quark pair production. As this result could be a sign of new physics we have parametrized this new physics in terms of a complete set of dimension six operators involving the top quark. We have then used a Markov Chain Monte Carlo approach in order to find the best set of parameters that fits the data, using all available data regarding top quark pair production at the Tevatron. We have found that just a very small number of operators are able to fit the data better than the Standard Model.

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arXiv2011

Could the LHC two-photon signal correspond to the heavier scalar in two-Higgs-doublet models?

LHC has reported tantalizing hints for a Higgs boson of mass 125 GeV decaying into two photons. We focus on two-Higgs-doublet Models, and study the interesting possibility that the heavier scalar (H) has been seen, with the lightest scalar (h) having thus far escaped detection. Non-observation of h at LEP severely constrains the parameter-space of two-Higgs-doublet models. We analyze cases where the decay H --> h h is kinematically allowed, and cases where it is not, in the context of type I, type II, lepton-specific, and flipped models.

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arXiv2011

Implications of the LHC two-photon signal for two-Higgs-doublet models

We study the implications for Two Higgs Doublet Models of the recent announcement at the LHC giving a tantalizing hint for a Higgs boson of mass 125 GeV decaying into two photons. We require that the experimental result be within a factor of two of the theoretical Standard Model prediction, and analyze the type I and type II models as well as the lepton-specific and flipped models, subject to this requirement. It is assumed that there is no new physics other than two Higgs doublets. In all of the models, we display the allowed region of parameter space taking the recent LHC announcement at face value, and we analyze the $W^+W^-$, $ZZ$, $\bar{b}b$ and $τ^+τ^-$ expectations in these allowed regions. Throughout the entire range of parameter space allowed by the $γγ$ constraint, the number of events for Higgs decays into $WW$, $ZZ$ and $b \bar b$ are not changed from the Standard Model by more than a factor of two. In contrast, in the Lepton Specific model, decays to $τ^+ τ^- $ are very sensitive across the entire $γγ$-allowed region.

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arXiv2011

Light Charged Higgs bosons at the LHC in 2HDMs

We present a discussion of light charged Higgs boson searches at the Large Hadron Collider (LHC) in CP-conserving 2-Higgs Doublet Models (2HDMs). Taking into account all available experimental and theoretical constraints we review all possible processes that would allow for a detection of such a particle with a mass below the top quark mass. Two different types of processes are analysed: one that depends only on tan beta and on the charged Higgs boson mass because it involves only the charged Higgs boson Yukawa couplings; the other that depends on almost all model parameters, mainly due to the presence of Higgs self-couplings. We discuss the regions of parameter space of 2HDMs that can be covered be each type of process and define some guidelines for experimental searches at the LHC.

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arXiv2010

top FCNC physics at a Linear Collider after the LHC

Flavour changing neutral currents are absent at tree level in the Standard Model (SM) and are highly suppressed at higher orders due the Glashow-Iliopoulos-Maiani mechanism. Hence, any evidence of such currents related to the top quark will definitely signal new physics beyond the SM. In this work we will discuss how an electron-positron collider can contribute to the field after the Large Hadron Collider (LHC) has collected at least 100 fb^{-1} of integrated luminosity operating at sqrt{s} = 14 TeV.

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arXiv2010

Higgs boson phenomenology in tau^+ tau^- final states at the LHC

We perform a detailed parton level study on the feasibility of the detection of a Higgs boson in the gluon fusion process $pp (gg+gq) \to h + jet \to τ^+ τ^- + jet$ at the Large Hadron Collider (LHC). The obtained results are applied to a few chosen Beyond the Standard Model (BSM) scenarios where the branching ratio of a Higgs boson decaying into a $τ^+ τ^-$ pair is enhanced as compared to the Standard Model (SM) case. We present the parameter space of the BSM scenarios that can be observed at the LHC and conclude that some regions of the parameter space can be probed with just a few $fb^{-1}$ of integrated luminosity. Noticeably, our results are presented in a form which potentially allows their application to any generic model giving rise to a $pp (gg+gq) \to h + jet \to τ^+ τ^- + jet$ signature.

hep-ph↗
arXiv2010

BSM Higgs searches in the gluon fusion process pp -> h +jet -> tau^+ tau^- + jet at the LHC

The mass of the Standard Model (SM) Higgs boson was constrained by the Large Electron-Positron (LEP) collider to be above 114.4 GeV. Simple extensions of the scalar sector like adding singlets and/or doublets allow the lightest Higgs to be much lighter via a reduction in its coupling to gauge bosons. Such a light Higgs could have evaded the LEP searches but could be detected at the Large Hadron Collider (LHC) in the process pp -> h j -> tau^+ tau^- j, where j is a resolved jet. We conclude that in some Beyond the Standard Model (BSM) extensions of the scalar sector, a very light Higgs state could be detected at the 14 TeV LHC with early data.

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