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Otto Eberhardt

Publications and source records attributed to Otto Eberhardt.

At least 19 recordsLinked to original sources

Simple and statistically sound recommendations for analysing physical theories

Physical theories that depend on many parameters or are tested against data from many different experiments pose unique challenges to statistical inference. Many models in particle physics, astrophysics and cosmology fall into one or both of these categories. These issues are often sidestepped with statistically unsound ad hoc methods, involving intersection of parameter intervals estimated by multiple experiments, and random or grid sampling of model parameters. Whilst these methods are easy to apply, they exhibit pathologies even in low-dimensional parameter spaces, and quickly become problematic to use and interpret in higher dimensions. In this article we give clear guidance for going beyond these procedures, suggesting where possible simple methods for performing statistically sound inference, and recommendations of readily-available software tools and standards that can assist in doing so. Our aim is to provide any physicists lacking comprehensive statistical training with recommendations for reaching correct scientific conclusions, with only a modest increase in analysis burden. Our examples can be reproduced with the code publicly available at https://doi.org/10.5281/zenodo.4322283.

hep-ph

Constraints on coloured scalars from global fits

We consider a simple extension of the electroweak theory, incorporating one $SU(2)_L$ doublet of colour-octet scalars with Yukawa couplings satisfying the principle of minimal flavour violation. Using the HEPfit package, we perform a global fit to the available data, including all relevant theoretical constraints, and extract the current bounds on the model parameters. Coloured scalars with masses below 1.05 TeV are already excluded, provided they are not fermiophobic. The mass splittings among the different (charged and CP-even and CP-odd neutral) scalars are restricted to be smaller than 20 GeV. Moreover, for scalar masses smaller than 1.5 TeV, the Yukawa coupling of the coloured scalar multiplet to the top quark cannot exceed the one of the SM Higgs doublet by more than 80%. These conclusions are quite generic and apply in more general frameworks (without fine tunings). The theoretical requirements of perturbative unitarity and vacuum stability enforce relevant constraints on the quartic scalar potential parameters that are not yet experimentally tested.

hep-ph

Global fits in the Aligned Two-Higgs-Doublet model

We present the results of a global fit to the Aligned Two-Higgs Doublet Model, assuming that there are no new sources of CP violation beyond the quark mixing matrix. We use the most constraining flavour observables, electroweak precision measurements and the available data on Higgs signal strengths and collider searches for heavy scalars, together with the theoretical requirements of perturbativity and positivity of the scalar potential. The combination of all these constraints restricts the values of the scalar masses, the couplings of the scalar potential and the flavour-alignment parameters. The numerical fits have been performed using the open-source HEPfit package.

hep-ph

$\texttt{HEPfit}$: a Code for the Combination of Indirect and Direct Constraints on High Energy Physics Models

$\texttt{HEPfit}$ is a flexible open-source tool which, given the Standard Model or any of its extensions, allows to $\textit{i)}$ fit the model parameters to a given set of experimental observables; $\textit{ii)}$ obtain predictions for observables. $\texttt{HEPfit}$ can be used either in Monte Carlo mode, to perform a Bayesian Markov Chain Monte Carlo analysis of a given model, or as a library, to obtain predictions of observables for a given point in the parameter space of the model, allowing $\texttt{HEPfit}$ to be used in any statistical framework. In the present version, around a thousand observables have been implemented in the Standard Model and in several new physics scenarios. In this paper, we describe the general structure of the code as well as models and observables implemented in the current release.

hep-ph

New theoretical constraints on scalar color octet models

We study theoretical constraints on a model whose scalar sector contains one color octet and one or two color singlet $SU(2)_L$ doublets. Using the unitarity of the theory, we constrain the parameters of the scalar potential for the first time at next-to-leading order in perturbation theory. We also derive new conditions guaranteeing the stability of the potential. We use the HEPfit package to single out the viable parameter regions at the electroweak scale and test the stability of the renormalization group evolution up to the multi-TeV region. Additionally, we set upper limits on the scalar mass splittings. All results are given for both cases, with and without a second scalar color singlet.

hep-ph

Global fits in the Georgi-Machacek model

Off the beaten track of scalar singlet and doublet extensions of the Standard Model, triplets combine an interesting LHC phenomenology with an explanation for neutrino masses. The Georgi-Machacek model falls into this category, but it has never been fully explored in a global fit. We use the {\texttt{HEPfit}} package to combine recent experimental Higgs data with theoretical constraints and obtain strong limits on the mixing angles and mass differences between the heavy new scalars as well as their decay widths. We also find that the current signal strength measurements allow for a Higgs to vector boson coupling with an opposite sign to the Standard Model, but this possibility can be ruled out by the lack of direct evidence for heavy Higgs states. For these hypothetical particles, we identify the dominant decay channels and extract bounds on their branching ratios from the global fit, which can be used to single out the decay patterns relevant for the experimental searches.

hep-ph

Current status of Two-Higgs-Doublet models with a softly broken $\mathbb{Z}_2$ symmetry

One of the most popular extensions of the Standard Model is the Two-Higgs-Doublet model (2HDM), in which a second Higgs doublet is added to the conventional Standard Model particle content. 2HDM's with a softly broken $\mathbb{Z}_2$ symmetry avoid flavour-changing neutral Higgs currents at tree-level. They also comprise the Higgs sector of the Minimal Supersymmetric Standard Model. Current Higgs observables put strong constraints on these models. We present global fits to these data and combine them with information from theoretical bounds, electroweak precision observables and the most important flavour constraints, using the open-source package HEPfit. The resulting limits on the 2HDM parameters like mixing angles, the heavy Higgs masses as well as the allowed Higgs mass splittings will be discussed.

hep-ph

Current and future constraints on Higgs couplings in the nonlinear Effective Theory

We perform a Bayesian statistical analysis of the constraints on the nonlinear Effective Theory given by the Higgs electroweak chiral Lagrangian. We obtain bounds on the effective coefficients entering in Higgs observables at the leading order, using all available Higgs-boson signal strengths from the LHC runs 1 and 2. Using a prior dependence study of the solutions, we discuss the results within the context of natural-sized Wilson coefficients. We further study the expected sensitivities to the different Wilson coefficients at various possible future colliders. Finally, we interpret our results in terms of some minimal composite Higgs models.

hep-ph

Update of Global Two-Higgs-Doublet Model Fits

We perform global fits of Two-Higgs-Doublet models with a softly broken $\mathbb{Z}_2$ symmetry to recent results from the LHC detectors CMS and ATLAS, that is signal strengths and direct search limits obtained at $\sqrt{s} = 8$ TeV and $\sqrt{s}=13$ TeV. We combine all available ATLAS and CMS constraints with the other relevant theoretical and experimental bounds and present the latest limits on the model parameters. We obtain that deviations from the so-called alignment limit $β-α=π/2$ cannot be larger than $0.03$ in type I and have to be smaller than $0.02$ in the remaining three types. For the latter, we also observe lower limits on the heavy Higgs masses in the global fit. The splittings between these masses cannot exceed $200$ GeV in the types I and X and $130$ GeV in the types II and Y. Finally, we find that the decay widths of the heavy Higgs particles cannot be larger than $7\%$ of their masses if they are lighter than $1.5$ TeV.

hep-ph

Two-Higgs-doublet model fits with HEPfit

The Two-Higgs-doublet model (2HDM) is one of the most studied extensions of the Standard Model. Like the other popular "New Physics" models, it gets more and more constrained by recent experimental progress, especially by the LHC data. For the 2HDM types I and II with a softly broken $\mathbb{Z}_2$ symmetry, I present updated results of global analyses obtained with the open-source HEPfit code, emphasizing the impact of the pre-EPS-HEP 2017 LHC data. Furthermore, I discuss the status of implementation into HEPfit of 2HDM's beyond the conventional $\mathbb{Z}_2$ symmetric types.

hep-ph

Next-to-leading order unitarity fits in Two-Higgs-Doublet models with soft $\mathbb{Z}_2$ breaking

We fit the next-to-leading order unitarity conditions to the Two-Higgs-Doublet model with a softly broken $\mathbb{Z}_2$ symmetry. In doing so, we alleviate the existing uncertainty on how to treat higher order corrections to quartic couplings of its Higgs potential. A simplified approach to implementing the next-to-leading order unitarity conditions is presented. These new bounds are then combined with all other relevant constraints, including the complete set of LHC Run I data. The upper $95\%$ bounds we find are $4.2$ on the absolute values of the quartic couplings, and $235$ GeV ($100$ GeV) for the mass degeneracies between the heavy Higgs particles in the type I (type II) scenario. In type II, we exclude an unbroken $\mathbb{Z}_2$ symmetry with a probability of $95\%$. All fits are performed using the open-source code HEPfit.

hep-ph

Fitting the two-loop renormalized Two-Higgs-Doublet model

We present global fits to the Two-Higgs-Doublet model, assuming a softly broken $Z_2$ symmetry of the types I and II and CP conservation in the scalar potential. We show how much the parameter space is constrained by the combination of the relevant theoretical and experimental inputs, including the LHC data after the first run and interpreting the 125 GeV boson as the light CP even Higgs. Using the next-to-leading order renormalization group equations, we address the questions of vacuum stability and the hierarchy problem in the context of the mentioned Two-Higgs-Doublet models.

hep-ph

Global fits of the two-loop renormalized Two-Higgs-Doublet model with soft $Z_2$ breaking

We determine the next-to-leading order renormalization group equations for the Two-Higgs-Doublet model with a softly broken $Z_2$ symmetry and CP conservation in the scalar potential. We use them to identify the parameter regions which are stable up to the Planck scale and find that in this case the quartic couplings of the Higgs potential cannot be larger than 1 in magnitude and that the absolute values of the S-matrix eigenvalues cannot exceed 2.5 at the electroweak symmetry breaking scale. Interpreting the 125 GeV resonance as the light CP-even Higgs eigenstate, we combine stability constraints, electroweak precision and flavour observables with the latest ATLAS and CMS data on Higgs signal strengths and heavy Higgs searches in global parameter fits to all four types of $Z_2$ symmetry. We quantify the maximal deviations from the alignment limit and find that in type II and Y the mass of the heavy CP-even (CP-odd) scalar cannot be smaller than 340 GeV (360 GeV). Also, we pinpoint the physical parameter regions compatible with a stable scalar potential up to the Planck scale. Motivated by the question how natural a Higgs mass of 125 GeV can be in the context of a Two-Higgs-Doublet model, we also address the hierarchy problem and find that the Two-Higgs-Doublet model does not offer a perturbative solution to it beyond 5 TeV.

hep-ph

Benchmarks for Higgs Pair Production and Heavy Higgs Searches in the Two-Higgs-Doublet Model of Type II

The search for additional Higgs particles and the exact measurements of Higgs (self-) couplings is a major goal of future collider experiments. In this paper we investigate the possible sizes of new physics signals in these searches in the context of the $CP$-conserving two-Higgs doublet model of type II. Using current constraints from flavour, electroweak precision, and Higgs signal strength data, we determine the allowed sizes of the triple Higgs couplings and the branching fractions of the heavy Higgs bosons into lighter Higgs bosons. Identifying the observed Higgs resonance with the light $CP$-even 2HDM Higgs boson $h$, we find that the $hhh$ coupling cannot exceed its SM value, but can be reduced by a factor of $0.56$ at the 2 $σ$ level. The branching fractions of the heavy neutral Higgs bosons $H$ and $A$ into two-fermion or two-vector-boson final states can be reduced by factors of $0.4$ and $0.01$, respectively, if decays into lighter Higgs boson are possible and if the mass of the decaying Higgs is below the $t\bar t$ threshold. To facilitate future studies of collider signatures in 2HDM scenarios with large triple Higgs couplings or decay modes of the heavy Higgs bosons not covered by the SM Higgs searches we provide a set of benchmark points which exhibit these features and agree with all current constraints. We also discuss the effect of the heavy Higgs bosons on the $gg\to hh$ cross section at a 14 TeV LHC for some of these benchmarks. For $m_H$ below the $hh$ threshold we see a reduction of the SM $gg\to hh$ cross section due to destructive interference, but for $m_H$ above the $hh$ threshold current constraints allow enhancement factors above 50. An enhancement factor of 6 is still possible in scenarios in which the heavy Higgs particles would not be discovered by standard searches after 300 fb${}^{-1}$ of data.

hep-ph

Fitting the Two-Higgs-Doublet model of type II

We present the current status of the Two-Higgs-Doublet model of type II. Taking into account all available relevant information, we exclude at $95$% CL sizeable deviations of the so-called alignment limit, in which all couplings of the light CP-even Higgs boson $h$ are Standard-Model-like. While we can set a lower limit of $240$ GeV on the mass of the pseudoscalar Higgs boson at $95$% CL, the mass of the heavy CP-even Higgs boson $H$ can be even lighter than $200$ GeV. The strong constraints on the model parameters also set limits on the triple Higgs couplings: the $hhh$ coupling in the Two-Higgs-Doublet model of type II cannot be larger than in the Standard Model, while the $hhH$ coupling can maximally be $2.5$ times the size of the Standard Model $hhh$ coupling, assuming an $H$ mass below $1$ TeV. The selection of benchmark scenarios which maximize specific effects within the allowed regions for further collider studies is illustrated for the $H$ branching fraction to fermions and gauge bosons. As an example, we calculate the cross section of $gg\to hh$ for four benchmark points and show that a resonant $H$ could enhance it by almost a factor of $70$ at a centre-of-mass energy of $14$ TeV.

hep-ph

Extra doublets - Global analyses of Standard Model extensions in the fermionic or scalar sector

I present comprehensive analyses of the viable parameter regions of the Standard Model of particle physics and two of its extensions: the Standard Model with four fermion generations and the Two-Higgs-Doublets model of type II. Applying frequentist methods, I have performed global fits which show that the Standard Model is compatible with electroweak precision data and the measured Higgs signal strengths. The Standard Model with four generations is excluded by the experimental results with a p-value corresponding to more than five standard deviations. In the Two-Higgs-Doublets model of type II, the fit prefers the decoupling limit where it mimics the Standard Model; however, neutral heavy Higgs boson masses below 200 GeV are still allowed.

hep-ph

Status of the two-Higgs-doublet model of type II

We determine the allowed parameter space of the CP-conserving two-Higgs-doublet model (2HDM) of type II with a softly broken Z_2 symmetry. Our analysis includes theoretical constraints from vacuum stability and perturbativity as well as experimental constraints from signal strengths of the 126 GeV Higgs boson, the non-observation of additional Higgs resonances and electroweak precision and flavour observables. If the 126 GeV resonance is interpreted as the light CP-even Higgs boson of the 2HDM our analysis shows that scenarios where the couplings of this boson deviate substantially from those of the SM Higgs boson are disfavoured at one standard deviation and completely excluded for small values of tan(beta). We also discuss bounds on the masses of the heavy 2HDM Higgs bosons and their implications for the possible decay modes of these particles. We find that the region in which both non-standard neutral Higgs bosons are simultaneously lighter than 300 GeV is excluded at two standard deviations.

hep-ph

Impact of a Higgs boson at a mass of 126 GeV on the standard model with three and four fermion generations

We perform a comprehensive statistical analysis of the standard model (SM) with three and four generations using the latest Higgs search results from LHC and Tevatron, the electroweak precision observables (EWPOs) measured at LEP and SLD and the latest determinations of M_W, m_t and alpha_s. For the three-generation case we analyse the tensions in the electroweak fit by removing individual observables from the fit and comparing their predicted values with the measured ones. In particular, we discuss the impact of the Higgs search results on the deviations of the EWPOs from their best-fit values. Our indirect prediction of the top mass is m_t=175.7(+3.0)(-2.2) GeV at 68.3% CL, in good agreement with the direct measurement. We also plot the preferred area in the M_W - m_t plane. The best-fit Higgs mass is 126.0 GeV. For the case of the SM with a perturbative sequential fourth fermion generation (SM4) we discuss the deviations of the Higgs signal strengths from their best-fit values. The H -> gamma gamma signal strength now disagrees with its best-fit SM4 value at more than 4 sigma. We perform a likelihood-ratio test to compare the SM and SM4 and show that the SM4 is excluded at 5.3 sigma. Without the Tevatron data on H -> b bbar the significance drops to 4.8 sigma.

hep-ph