2HDM Parameter Research Articles

First shot of the smoking gun: probing the electroweak phase transition in the 2HDM with novel searches for A → ZH in ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} and ννbb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ u \ u b\\overline{b} $$\\end{document} final states

Recently the ATLAS collaboration has reported the first results of searches for heavy scalar resonances decaying into a Z boson and a lighter new scalar resonance, where the Z boson decays leptonically and the lighter scalar decays into a top-quark pair, giving rise to ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} final states. This had previously been identified as a smoking-gun signature at the LHC for a first-order electroweak phase transition (FOEWPT) within the framework of two Higgs doublet models (2HDMs). In addition, ATLAS also presented new limits where the Z boson decays into pairs of neutrinos and the lighter scalar resonance into bottom-quark pairs, giving rise to the ννbb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ u \ u b\\overline{b} $$\\end{document} final state. We analyze the impact of these new searches on the 2HDM parameter space, with emphasis on their capability to probe currently allowed 2HDM regions featuring a strong FOEWPT. We also study the complementarity of these new searches with other LHC probes that could target the FOEWPT region of the 2HDM. Remarkably, the ATLAS search in the ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} final state shows a local 2.85 σ excess (for masses of about 650 GeV and 450 GeV for the heavy and light resonance) in the 2HDM parameter region that would yield a FOEWPT in the early universe, which could constitute the first experimental hint of baryogenesis at the electroweak scale. We analyze the implications of this excess, and discuss the detectability prospects for the associated gravitational wave signal from the FOEWPT. Furthermore, we project the sensitivity reach of the ℓ+ℓ−tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\ell}^{+}{\\ell}^{-}t\\overline{t} $$\\end{document} signature for the upcoming runs of the LHC. Finally, we introduce the python package thdmTools, a state-of-art tool for the exploration of the 2HDM.

Sharpening the A → Z(*)h signature of the Type-II 2HDM at the LHC through advanced Machine Learning

The A → Z(*)h decay signature has been highlighted as possibly being the first testable probe of the Standard Model (SM) Higgs boson discovered in 2012 (h) interacting with Higgs companion states, such as those existing in a 2-Higgs Doublet Model (2HDM), chiefly, a CP-odd one (A). The production mechanism of the latter at the Large Hadron Collider (LHC) takes place via bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ b\\overline{b} $$\\end{document}-annihilation and/or gg-fusion, depending on the 2HDM parameters, in turn dictated by the Yukawa structure of this Beyond the SM (BSM) scenario. Among the possible incarnations of the 2HDM, we test here the so-called Type-II, for a twofold reason. On the one hand, it intriguingly offers two very distinct parameter regions compliant with the SM-like Higgs measurements, i.e., where the so-called ‘SM limit’ of the 2HDM can be achieved. On the other hand, in both configurations, the AZh coupling is generally small, hence the signal is strongly polluted by backgrounds, so that the exploitation of Machine Learning (ML) techniques becomes extremely useful. In this paper, we show that the application of advanced ML implementations can be decisive in establishing such a signal. This is true for all distinctive kinematical configurations involving the A → Z(*)h decay, i.e., below threshold (mA < mZ + mh), at its maximum (mZ + mh < mA < 2mt) and near the onset of tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document} pair production (mA ≈ 2mt), for which we propose Benchmark Points (BPs) for future phenomenological analyses.

Chasing the two-Higgs doublet model in the di-Higgs boson production

We investigate the di-Higgs production at the Large Hadron Collider in the two-Higgs doublet model (2HDM). In particular, we study the production of an extra neutral Higgs boson $\phi$ in association with the Standard Model (SM) Higgs boson $h$ in the Higgs alignment limit. We analyze two scenarios where the additional Higgs $\phi$ is CP-even or -odd state with a large top-Yukawa interaction. The leading contribution of this production comes from the top-quark loop-induced gluon-fusion channel $gg \to h\phi$. The measurement of the $h\phi$ production can probe the quartic couplings in the Higgs potential as well as the top-Yukawa couplings. Imposing both theoretical constraints (from the perturbative unitarity and the vacuum stability bounds) and experimental bounds (from the SM Higgs and flavor physics measurements) on the 2HDM parameter space, we calculate the production cross-section of $gg \to h\phi$. Furthermore, we scrutinize these processes in the parameter spaces where the CMS di-tau and di-photon excesses around 100$\,$GeV, and/or the muon $g-2$ anomaly can be accommodated.

Is the new CDF M measurement consistent with the two-Higgs doublet model?

Motivated by the new CDF measurement of the $W$ boson mass reported recently which clearly illustrates a large deviation compared to the Standard Model (SM) prediction. In the present paper, we study the Two-Higgs Doublet Model (2HDM) contributions to $M_W$ and its phenomenological implications in the case where the heavy CP-even $H$ is identified as the observed Higgs boson with a mass of $125$ GeV. Taking into account theoretical and all the available experimental constraints as well as the new CDF measurement, we demonstrate that the 2HDM parameter space can provide a large correction which predicts the W mass close to the new CDF $M_W$ measurement. It is found that $M_{H\pm}=M_A$ is excluded and the splitting of the charged Higgs boson with all other states is positive. We also discuss the consequence on the effective mixing angle $\sin^2\theta_{\text{eff}}$ as well as the phenomenological implications on the charged Higgs and CP-odd decays.

The trap in the early Universe: impact on the interplay between gravitational waves and LHC physics in the 2HDM

We analyze the thermal history of the 2HDM and determine the parameter regions featuring a first-order electroweak phase transition (FOEWPT) and also much less studied phenomena like high-temperature electroweak (EW) symmetry non-restoration and the possibility of vacuum trapping (i.e. the Universe remains trapped in an EW-symmetric vacuum throughout the cosmological evolution, despite at T = 0 the EW breaking vacuum is deeper). We show that the presence of vacuum trapping impedes a first-order EW phase transition in 2HDM parameter-space regions previously considered suitable for the realization of electroweak baryogenesis. Focusing then on the regions that do feature such a first-order transition, we show that the 2HDM parameter space that would yield a stochastic gravitational wave signal potentially detectable by the future LISA observatory is very contrived, and will be well probed by direct searches of 2HDM Higgs bosons at the HL-LHC, and (possibly) also via measurements of the self-coupling of the Higgs boson at 125 GeV. This has an important impact on the interplay between LISA and the LHC regarding the exploration of first-order phase transition scenarios in the 2HDM: the absence of new physics indications at the HL-LHC would severely limit the prospects of a detection by LISA. Finally, we demonstrate that as a consequence of the predicted enhancement of the self-coupling of the Higgs boson at 125 GeV the ILC would be able to probe the majority of the 2HDM parameter space yielding a FOEWPT through measurements of the self-coupling, with a large improvement in precision with respect to the HL-LHC.

Sensitivity and constraints to the 2HDM soft-breaking Z2 parameter m12

In this letter we study the specific sensitivity and constraints to the soft breaking Z2 parameter m12 of the Two Higgs Doublet Model (2HDM). m12 is considered here as an input parameter of the model together with the masses of the Higgs bosons, mh, mH, mA, mH±, the ratio of the two Higgs vacuum expectation values, tan⁡β, and cos⁡(β−α), with α and β the mixing angles of the CP-even and CP-odd Higgs sector, respectively. We explore in particular the sensitivity to m12 in the decays of the lightest Higgs boson h to two photons, assuming: 1) that h is the state observed at the LHC at ∼125GeV, and 2) the h couplings to the SM particles are SM-like, by going to the alignment limit, cos⁡(β−α)=0. The aim of this work is to demonstrate possible constraints on m12 from the present measurement of the di-photon signal strength, μγγ. These constraints are relevant in the region of the 2HDM parameter space that is allowed by all the other constraints, specifically, theoretical constraints as well as experimental constraints from LHC Higgs rate measurements, Higgs boson searches, flavor physics and precision observables. The exploration is performed in the four different Yukawa types of 2HDM, where the allowed region by all the other constraints depends on the model type. We also analyze the case that the Higgs-boson mass spectrum accommodates a possible new world average of mW including the recent CDF measurement.

Fine-tuning in the 2HDM

The Two-Higgs Doublet Model (2HDM) is one of the most popular and natural extensions of the Higgs sector; but it has two potential fine-tuning problems, related to the electroweak (EW) breaking and the requirement of alignment with the SM Higgs boson. We have quantified the fine-tunings obtaining analytical expressions, both in terms of the initial 2HDM parameters and the physical ones (masses, mixing angles, etc.). We also provide simple approximate expressions. We have taken into account that the fine-tunings are not independent and removed the “double counting” by projecting the variations of the alignment onto the constant-v^2 hypersurface. The EW and the alignment fine-tunings become severe in different, even opposite, regions of the parameter space, namely in the regimes of large and small extra-Higgs masses, respectively; emerging an intermediate region, 500~{textrm{GeV}} {mathop {{}_sim }limits ^{<}}{m_H, m_A, m_{H^pm }} {mathop {{}_sim }limits ^{<}}700~{textrm{GeV}}, where both are acceptably small. We also discuss a remarkable trend that is not obvious at first glance. Namely, for large tan beta both the EW and the alignment fine-tunings become mitigated. In consequence, the 2HDM becomes quite natural for tan beta ge {mathcal {O}} (10), even if m_H, m_A, m_{H^pm } are as large as 1500 GeV. We explain why this is not the case for the 2HDM stemming from supersymmetry. We have illustrated all these trends by numerically analyzing several representative scenarios.

LHC data interpretation within the 2HDM type II via a new analysis toolkit

We review the status of the 2-Higgs doublet model (2HDM) type-II, in the light of the current experimental results and various theoretical consistency conditions. Compared to the existing literature, in this paper, we apply for the first time a new method that can improve the standard procedure for setting bounds on the 2HDM parameter space, as no experimental evidence has been found so far. Our new numerical framework, called Magellan, and statistical techniques can be applied to any beyond the Standard Model (BSM) scenario. Here, we take as testing ground the 2HDM, particularly as it is physically interesting and moreover characterized by a far from trivial multidimensional parameter space where the effectiveness of the new methods can be proved. Magellan uses a Markov chain Monte Carlo technique for scanning the parameter space and leverages the use of data processing and visualisation methods, allowing the user to perform inference on the model in a complete and efficient way. The novelty of the proposed method is that the parameter space of any BSM theory can be projected onto any bidimensional plane while still retaining all underlying attributes of those points, so that it is possible to investigate the associations between the properties of the various lower dimensional subspaces of the complete parameter space. The Magellan's website interactive dashboards can be accessed via a public link. Through this website, the user can explore the full parameter space and exploit the phenomenological features of a BSM model with ease.

Parameter dependence and analysis of the 2HDM neutral Higgs boson pair production and decay at future lepton colliders

In this work, we present a study of the neutral Higgs bosons in the two Higgs doublet model (2HDM) in terms of their production processes and decay channels as a function of the model parameters. The analysis is performed for all four types of the 2HDM and the most promising processes and decay channels are identified for each type of the model. Several Higgs boson mass scenarios below and above the threshold of decay to gauge boson pair are introduced and the corresponding categories of final states are analyzed. The event analysis including collider beam spectrum and detector simulation shows that future lepton colliders have the potential to explore regions in the 2HDM parameter space, which have not yet been excluded by LHC, in a few weeks of operation. Final results are presented in terms of the signal distributions on top of the background and 95% CL exclusion and 5σ contours based on center of mass energy of s=500 GeV at the Compact Linear Collider (CLIC) or International Linear Collider (ILC).

Probing exotic charged Higgs decays in the Type-II 2HDM through top rich signal at a future 100 TeV pp collider

The exotic decay modes of non-Standard Model Higgs bosons are efficient in probing the hierarchical Two Higgs Doublet Models (2HDM). In particular, the decay mode H±→HW± serves as a powerful channel in searching for charged Higgses. In this paper, we analyze the reach for H±→HW±→ toverline{t}W at a 100 TeV pp collider, and show that it extends the reach of the previously studied ττW final states once above the top threshold. Top tagging technique is used, in combination with a boosted decision tree classifier. At the low tan β region, almost the entire hierarchical Type-II 2HDM parameter space can be probed via the combination of all exotic decay channels.

2HDM neutral scalars under the LHC

Two Higgs Doublet Models (2HDM) provide a simple framework for new physics models with an extended Higgs sector. The current LHC results, including both direct searches for additional non-Standard Model (SM) Higgs bosons, as well as precision measurements of the SM-like Higgs couplings, already provide strong constraints on the 2HDM parameter spaces. In this paper, we examine those constraints for the neutral scalars in the Type-I and Type-II 2HDM. In addition to the direct search channels with SM final states: H/A → foverline{f} , VV, Vh, hh, we study in particular the exotic decay channels of H/A → AZ/HZ once there is a mass hierarchy between the non-SM Higgses. We found that H/A → AZ/H Z channel has unique sensitivity to the alignment limit region which remains unconstrained by conventional searches and Higgs precision measurements. This mode also extends the reach at intermediate tβ for heavy mA that are not covered by the other direct searches.

Exotic Higgs decays in Type-II 2HDMs at the LHC and future 100 TeV hadron colliders

The exotic decay modes of non-Standard Model (SM) Higgses in models with extended Higgs sectors have the potential to serve as powerful search channels to explore the space of Two-Higgs Doublet Models (2HDMs). Once kinematically allowed, heavy Higgses could decay into pairs of light non-SM Higgses, or a non-SM Higgs and a SM gauge boson, with branching fractions that quickly dominate those of the conventional decay modes to SM particles. In this study, we focus on the prospects of probing Type-II 2HDMs at the LHC and a future 100 TeV pp collider via exotic decay channels. We study the three prominent exotic decay channels: A → HZ, A → H±W∓ and H± → HW±, and find that a 100-TeV pp collider can probe most of the region of the Type-II 2HDM parameter space that survives current theoretical and experimental constraints with sizable exotic decay branching fraction through these channels, making them complementary to the conventional decay channels for heavy non-SM Higgses.

Future prospects of mass-degenerate Higgs bosons in the CP -conserving two-Higgs-doublet model

The scenario of two mass-degenerate Higgs bosons within the general two-Higgs-doublet model (2HDM) is revisited. We focus on the global picture when two $CP$-even Higgs bosons of $h$ and $H$ are nearly mass-degenerate. A global fit to the signal strength of the 125 GeV Higgs measured at the LHC is performed. Based on the best-fit result of the 2HDM mixing angles $(\alpha,\beta)$, theoretical constraints, charged and $CP$-odd Higgs boson direct search constraints and the electroweak precision constraints are imposed to the 2HDM parameter space. We present the signal predictions of the $(4b\,, 2b\,2\gamma)$ channels for the benchmark models at the LHC 14 TeV runs. We also study the direct Higgs boson pair productions at the LHC, and the Z-associated Higgs boson pair production search at the ILC 500 GeV runs, as well as the indirect probes at the CEPC 250 GeV run. We find that the mass-degenerate Higgs boson scenario in the Type-II 2HDM can be fully probed by these future experimental searches.

Higher dimensional operators in 2HDM

We present a complete (non-redundant) basis of CP- and flavour-conserving six-dimensional operators in a two Higgs doublet model (2HDM). We include {mathbb{Z}}_2 -violating operators as well. In such a 2HDM effective field theory (2HDMEFT), we estimate how constraining the 2HDM parameter space from experiments can get disturbed due to these operators. Our basis is motivated by the strongly interacting light Higgs (SILH) basis used in the standard model effective field theory (SMEFT). We find out bounds on combinations of Wilson coefficients of such operators from precision observables, signal strengths of Higgs decaying into vector bosons etc. In 2HDMEFT, the 2HDM parameter space can play a significant role while deriving such constraints, by leading to reduced or even enhanced effects compared to SMEFT in certain processes. We also comment on the implications of the SILH suppressions in such considerations.

Natural Alignment in the Two Higgs Doublet Model

As the LHC Higgs data persistently suggest the couplings of the observed 125 GeV Higgs boson to be consistent with the Standard Model (SM) expectations, any extended Higgs sector must lead to the so-called SM alignment limit, where one of the Higgs bosons behaves exactly like that of the SM. In the context of the Two Higgs Doublet Model (2HDM), this alignment is often associated with either decoupling of the heavy Higgs sector or accidental cancellations in the 2HDM potential. We present a novel symmetry justification for ‘natural’ alignment without necessarily decoupling or fine-tuning. We show that there exist only three different symmetry realizations of the natural alignment scenario in 2HDM. We identify the 2HDM parameter space satisfying the natural alignment condition up to the Planck scale. We also analyze new collider signals for the heavy Higgs sector in the natural alignment limit, which dominantly lead to third-generation quarks in the final state and can serve as a useful observational tool during the Run-II phase of the LHC.

Prospects for 2HDM charged Higgs searches

We discuss the prospects for charged Higgs boson searches at the LHC, within the two-Higgs-doublet models (2HDM). The 2HDM is generally less constrained than the corresponding sector of the MSSM, but there are still severe theoretical and experimental constraints that already exclude significant regions of the naive parameter space. Explicit searches in the H+ → τ+ν and H+ → channels are further restricting parts of the 2HDM parameter space.

High scale flavor alignment in two-Higgs doublet models and its phenomenology

The most general two-Higgs doublet model (2HDM) includes potentially large sources of flavor changing neutral currents (FCNCs) that must be suppressed in order to achieve a phenomenologically viable model. The flavor alignment ansatz postulates that all Yukawa coupling matrices are diagonal when expressed in the basis of mass-eigenstate fermion fields, in which case tree-level Higgs-mediated FCNCs are eliminated. In this work, we explore models with the flavor alignment condition imposed at a very high energy scale, which results in the generation of Higgs-mediated FCNCs via renormalization group running from the high energy scale to the electroweak scale. Using the current experimental bounds on flavor changing observables, constraints are derived on the aligned 2HDM parameter space. In the favored parameter region, we analyze the implications for Higgs boson phenomenology.

Constraints on two-Higgs-doublet model parameters in light of rare B decays

We established the allowed parameters of two-Higgs doublet model(2HDM) from flavor physics observables, precisely from rare $B$ meson decays. In our analysis most formidable constraints on the 2HDM parameters arise from the branching ratio of rare radiative $B$ meson decay i.e., $B\to X_{s}\gamma$. However, the constraints arising from the branching ratio of $B_{s}\to\mu^{+}\mu^{-}$ decay in $m_{H^{\pm}}-\tan\beta$ plane give $m_{H^{\pm}} >$ 80 GeV for the value of $\tan\beta\sim 2$, that is in agreement with large electron-positron collider (LEP) data. Furthermore, we also investigate the bounds on the $CP$-even $m_{H}$ and $CP$-odd $m_{A^{0}}$ Higgs boson not only from above mentioned physical observables, but also from the zero crossing of the forward-backward asymmetry of $B\to K^{\ast}\mu^{+}\mu^{-}$ decay. Therefore, these bounds on parameters of the 2HDM will provides a fertile ground to test the 2HDM at current and future $B$-physics experiments.

Strong first order electroweak phase transition in the CP-conserving 2HDM revisited

The discovery of the Higgs boson by the LHC experiments ATLAS and CMS has marked a milestone for particle physics. Yet, there are still many open questions that cannot be answered within the Standard Model (SM). For example, the generation of the observed matter-antimatter asymmetry in the universe through baryogenesis can only be explained qualitatively in the SM. A simple extension of the SM compatible with the current theoretical and experimental constraints is given by the 2-Higgs-Doublet Model (2HDM) where a second Higgs doublet is added to the Higgs sector. We investigate the possibility of a strong first order electroweak phase transition in the CP-conserving 2HDM type I and type II where either of the CP-even Higgs bosons is identified with the SM-like Higgs boson. The renormalisation that we apply on the loop-corrected Higgs potential allows us to efficiently scan the 2HDM parameter space and simultaneously take into account all relevant theoretical and up-to-date experimental constraints. The 2HDM parameter regions found to be compatible with the applied constraints and a strong electroweak phase transition are analysed systematically. Our results show that there is a strong interplay between the requirement of a strong phase transition and collider phenomenology with testable implications for searches at the LHC.

Anatomy of exotic Higgs decays in 2HDM

Large mass splittings between new scalars in two-Higgs-doublet models (2HDM) open a key avenue to search for these new states via exotic heavy Higgs decays. We discuss in detail the different search channels for these new scalars at the LHC in the presence of a sizable mass splitting, i.e. a hierarchical 2HDM scenario, taking into account the theoretical and experimental constraints. We provide benchmark planes to exploit the complementarity among these searches, analyzing their potential to probe the hierarchical 2HDM parameter space during LHC Run 2.