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Sebastian Paßehr

Publications and source records attributed to Sebastian Paßehr.

At least 19 recordsLinked to original sources
arXiv2025

Generic two-loop results for trilinear and quartic scalar self-interactions

Reconstructing the shape of the Higgs potential realised in Nature is a central part of the physics programme at the LHC and future colliders. In this context, accurate theoretical predictions for trilinear and quartic Higgs couplings are becoming increasingly important. In this paper, we present results that enable significant progress in the automation of these calculations at the two-loop level in a wide range of models. Specifically, we calculate the generic two-loop corrections for scalar n-point functions with n<=4 assuming that all external scalars are identical. Working in the zero-momentum approximation, we express the results in terms of generic couplings and masses. Additionally, by exploiting permutation invariances, we reduce the number of Feynman diagrams appearing to a substantially smaller set of basis diagrams. To ease the application of our setup, we also provide routines that allow to map our generic results to scalar two-loop amplitudes generated with the package FeynArts. We perform a series of calculations to cross-check our results with existing results in the literature. Moreover, we present new two-loop results for the trilinear Higgs coupling in the general singlet extension of the Standard Model. We also present the public Python package Tintegrals, which allows for fast and stable evaluations of all relevant two-loop integrals with vanishing external momenta.

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arXiv2022

About the bosonic decays of heavy Higgs states in the (N)MSSM

The heavy, doublet-dominated Higgs bosons expected in Two-Higgs-Doublet-Model-like extensions of the Standard Model are obvious targets for searches at high-energy colliders, and considerable activity in this sense is currently employed in analyzing the results of the Large Hadron Collider. For sufficiently heavy states, the $SU(2)_{\mathrm{L}}$ symmetry drastically constrains the decays of these new scalars. In models that contain only additional Higgs doublets, fermionic decay channels are expected to dominate, although some bosonic modes may lead to cleaner signals. The situation is very different if the Higgs sector is further extended by singlet states and the singlet-dominated scalars are kinematically accessible, in which case the Higgs-to-Higgs widths may even become dominant. Yet, a quantitative interpretation of the experimental data in terms of a specific model also runs through the control of radiative corrections in this model. In this paper, we examine the bosonic decays of the heavy, doublet-dominated Higgs states in the MSSM and the NMSSM at full one-loop order, insisting on the impact of the $SU(2)_{\mathrm{L}}$ symmetry and the effects of infrared type, in particular, the emergence of sizable non-resonant contributions to the three-boson final states.

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arXiv2022

Improvements in Higgs-mass predictions with $\texttt{FeynHiggs}$

Recent developments in the public code $\texttt{FeynHiggs}$ for the prediction of the Standard Model-like Higgs mass in the Minimal Supersymmetric Standard Model are presented. Improvements in the prediction based on an effective-field theory concern the cases of multi-scale hierarchies where the gluino is much heavier than the sfermions or where the additional Higgs bosons have masses between the electroweak and supersymmetry scales. The fixed-order part is improved by a re-implementation and extension of the two-loop corrections, now containing the contributions of orders $\big(α_t+α_b\big)\cdot\big(α_t+α_b+α_s\big)$ in the limit of vanishing electroweak gauge couplings for all implemented renormalization schemes. The updated version has a significantly improved numerical stability and thus an enhanced range of applicability for scenarios with heavy supersymmetric particles. All updates will be included in the upcoming version $\texttt{FeynHiggs-2.19.0}$.

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arXiv2021

Curing tachyonic tree-level syndrome in NMSSM light-singlet scenarios

Models with an extended Higgs sector open up the phenomenological possibility of additional scalars, beyond the SM-like boson observed by the LHC, with mass at or below the electroweak scale. Such scenarios are in particular viable in the presence of electroweak-singlet spin-0 fields, as expected for instance in the context of the NMSSM. Given that the size of radiative corrections can substantially affect the Higgs potential, a negative squared mass at the tree level does not necessarily yield a tachyonic spectrum at the physical level, but only indicates a failure of the tree-level description for calculational purposes. We explain how to tackle this technical issue in the example of the NMSSM, in scenarios with light CP-odd or CP-even singlet-dominated states and show how loop corrections to the Higgs masses and decay widths can be derived with the regularized Lagrangian. We further explore how the same flexibility in the definition of tree-level parameters can be exploited to circumvent large deviations of the tree-level spectrum from the kinematical setup in Higgs decays, or to estimate the theoretical uncertainty associated with the discrepancy between tree-level and physical Higgs spectra. The latter is of particular relevance for the properties of the SM-like Higgs boson in supersymmetry-inspired models.

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arXiv2021

Expectation management

We consider the application of a Fleischer$-$Jegerlehner-like treatment of tadpoles to the calculation of neutral scalar masses (including the Higgs) in general theories beyond the Standard Model. This is especially useful when the theory contains new scalars associated with a small expectation value, but comes with its own disadvantages. We show that these can be overcome by combining with effective field theory matching. We provide the formalism in this modified approach for matching the quartic coupling of the Higgs via pole masses at one loop, and apply it to both a toy model and to the $μ$NMSSM as prototypes where the standard treatment can break down.

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arXiv2021

Fighting off field dependence in MSSM Higgs-mass corrections of order $α_t\,α_s$ and $α_t^2$

The connection between gauge and Higgs sectors makes supersymmetric extensions of the Standard Model predictive frameworks for the derivation of Higgs masses. In this paper, we study the contamination of such predictions by field-renormalization constants, in the MSSM with two-loop gaugeless corrections of $\mathcal{O}{\left(α_{t,b}\,α_s,\,α_{t,b}^2\right)}$ and full momentum dependence, and demonstrate how strict perturbative expansions allow to systematically neutralize the dependence on such unphysical objects. On the other hand, the popular procedure consisting in an iterative pole search remains explicitly dependent on field counterterms. We then analyze the magnitude of the intrinsic uncertainty that this feature implies for the iterative method, both in non-degenerate and near-degenerate regimes, and conclude that this strategy does not improve on the predictions of the more straightforward expansion. We also discuss several features related to the inclusion of the orders $α_{t,b}\,α_s$ and $α_{t,b}^2$ in the so-called 'fixed-order' approach, such as the resummation of UV-logarithms for heavy supersymmetric spectra.

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arXiv2020

Complete two-loop QCD contributions to the lightest Higgs-boson mass in the MSSM with complex parameters

Higher-order corrections to the MSSM Higgs-boson masses are desirable for accurate predictions currently testable at the LHC. By comparing the prediction with the measured value of the discovered Higgs signal, viable parameter regions can be inferred. For an improved theory accuracy, we compute all two-loop corrections involving the strong coupling for the Higgs-boson mass spectrum of the MSSM with complex parameters. Apart from the dependence on the strong coupling, these contributions depend on the weak coupling and Yukawa couplings, leading to terms of $\mathcal{O}{\left(αα_s\right)}$ and $\mathcal{O}{\left(\sqrt{α_{q_1}}\sqrt{α_{q_2}}α_s\right)}$, ($q_{1,2}=t,b,c,s,u,d$). The full dependence on the external momentum and all relevant mass scales is taken into account. The calculation is performed in the Feynman-diagrammatic approach which is flexible in the choice of the employed renormalization scheme. For the phenomenological results presented here, a renormalization scheme consistent with higher-order corrections included in the code $\texttt{FeynHiggs}$ is adopted. For the evaluation of the results, a total of $513$ two-loop two-point integrals with up to five different mass scales are computed fully numerically using the program $\texttt{SecDec}$. A comparison with existing results in the limit of real parameters and/or vanishing external momentum is carried out, and the impact on the lightest Higgs-boson mass is discussed, including the dependence on complex phases. The new results will be included in the public code $\texttt{FeynHiggs}$.

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arXiv2020

Towards Higgs masses and decay widths satisfying the symmetries in the (N)MSSM

In models with an extended Higgs sector, such as the (N)MSSM, scalar states mix with one another. Yet, the concept of Higgs mixing is problematic at the radiative level, since it introduces both a scheme and a gauge dependence. In particular, the definition of Higgs masses and decay amplitudes can be impaired by the presence of gauge-violating pieces of higher order. We discuss in depth the origin and magnitude of such effects and suggest two strategies preserving or restoring gauge invariance. In addition, the intuitive concept of mixing and the simplicity of its definition in terms of two-point diagrams can make it tempting to include higher-order corrections on this side of the calculation, irrespectively of the order achieved in vertex diagrams. Using the global $SU(2)_{\mathrm{L}}$-symmetry in the decoupling limit, we show that no improvement can be expected from such an approach at the level of the Higgs decays, but that, on the contrary, the higher-order terms may lead to numerically large spurious effects.

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arXiv2019

Precision calculations in the MSSM Higgs-boson sector with FeynHiggs 2.14

We present an overview of the status and recent developments of FeynHiggs (current version: 2.14.3) since version 2.12.2. The main purpose of FeynHiggs is the calculation of the Higgs-boson masses and other physical observables in the MSSM. For a precise prediction of the Higgs-boson masses for low and high SUSY scales, state-of-the-art fixed-order and effective-field-theory calculations are combined. We first discuss improvements of the fixed-order calculation, namely an optional $\overline{\text{DR}}$ renormalization of the stop sector and a renormalization of the Higgs sector ensuring the chosen input mass to be equivalent with the corresponding physical mass. Second, we describe improvements of the EFT calculation, i.e. an implementation of non-degenerate threshold corrections as well as an interpolation for complex parameters. Lastly, we highlight some improvements of the code structure easing future extensions of FeynHiggs to models beyond the MSSM.

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arXiv2019

Electroweak corrections to the fermionic decays of heavy Higgs states

Extensions of the Standard Model often come with additional, possibly electroweakly charged Higgs states, the prototypal example being the Two-Higgs-Doublet Model. While collider phenomenology does not exclude the possibility for some of these new scalar fields to be light, it is relatively natural to consider masses in the multi-TeV range, in which case the only remaining light Higgs boson automatically receives SM-like properties. The appearance of a hierarchy between the new-physics states and the electroweak scale then leads to sizable electroweak corrections, e. g. in the decays of the heavy Higgs bosons, which are dominated by effects of infrared type, namely Sudakov logarithms. Such radiative contributions obviously affect the two-body decays, but should also be paired with the radiation of electroweak gauge bosons (or lighter Higgs bosons) for a consistent picture at the one-loop order. Resummation of the leading terms is also relatively easy to achieve. We re-visit these questions in the specific case of the fermionic decays of heavy Higgs particles in the Next-to-Minimal Supersymmetric Standard Model, in particular pointing out the consequences of the three-body final states for the branching ratios of the heavy scalars.

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arXiv2019

Full two-loop QCD corrections to the Higgs mass in the MSSM with heavy superpartners

We improve the determination of the Higgs-boson mass in the MSSM with heavy superpartners, by computing the two-loop threshold corrections to the quartic Higgs coupling that involve both the strong and the electroweak gauge couplings. Combined with earlier results, this completes the calculation of the two-loop QCD corrections to the quartic coupling at the SUSY scale. We also compare different computations of the relation between the quartic coupling and the pole mass of the Higgs boson at the EW scale. We find that the numerical impact of the new corrections on the prediction for the Higgs mass is modest, but comparable to the accuracy of the Higgs-mass measurement at the LHC.

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arXiv2019

All two-loop scalar self-energies and tadpoles in general renormalisable field theories

We calculate the complete tadpoles and self-energies at the two-loop order for scalars in general renormalisable theories, a crucial component for calculating two-loop electroweak corrections to Higgs-boson masses or for any scalar beyond the Standard Model. We renormalise the amplitudes using mass-independent renormalisation schemes, based on both dimensional regularisation and dimensional reduction. The results are presented here in Feynman gauge, with expressions for all 121 self-energy and 25 tadpole diagrams given in terms of scalar and tensor integrals with the complete set of rules to reduce them to a minimal basis of scalar integrals for any physical kinematic configuration. In addition, we simplify the results to a set of only 16 tadpole and 58 self-energy topologies using relations in order to substitute the ghost and Goldstone-boson couplings that we derive. To facilitate their application, we also provide our results in electronic form as a new code TLDR. We test our results by applying them to the Standard Model and compare with analytic expressions in the literature.

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arXiv2019

Phenomenology of the inflation-inspired NMSSM at the electroweak scale

The concept of Higgs inflation can be elegantly incorporated in the Next-to-Minimal Supersymmetric Standard Model (NMSSM). A linear combination of the two Higgs-doublet fields plays the role of the inflaton which is non-minimally coupled to gravity. This non-minimal coupling appears in the low-energy effective superpotential and changes the phenomenology at the electroweak scale. While the field content of the inflation-inspired model is the same as in the NMSSM, there is another contribution to the $μ$ term in addition to the vacuum expectation value of the singlet. We explore this extended parameter space and point out scenarios with phenomenological differences compared to the pure NMSSM. A special focus is set on the electroweak vacuum stability and the parameter dependence of the Higgs and neutralino sectors. We highlight regions which yield a SM-like $125\,$GeV Higgs boson compatible with the experimental observations and are in accordance with the limits from searches for additional Higgs bosons. Finally, we study the impact of the non-minimal coupling to gravity on the Higgs mixing and in turn on the decays of the Higgs bosons in this model.

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arXiv2018

Phenomenological consequences of Higgs inflation in the NMSSM at the electroweak scale

The Next-to-Minimal Supersymmetric Standard Model (NMSSM) can incorporate inflation, where a combination of the Higgs-doublet fields plays the role of the inflaton. At the high scale, the Higgs doublets are non-minimally coupled to supergravity; this coupling appears as an additional contribution to the $μ$ term in the low-energy effective superpotential and potentially changes physics at the electroweak scale. In a recent publication, we investigate the extended parameter space of this model with respect to collider phenomenology at the electroweak scale, and discuss scenarios which are potentially different from the pure NMSSM. We analyse the stability of the electroweak vacuum, the masses of neutralinos/charginos and Higgs bosons as well as the mixing and decays of Higgs bosons. Some important aspects of this study are described in the following.

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arXiv2018

Decays of the neutral Higgs bosons into SM fermions and gauge bosons in the $\mathcal{CP}$-violating NMSSM

The Next-to-Minimal Supersymmetric Standard Model (NMSSM) offers a rich framework embedding physics beyond the Standard Model as well as consistent interpretations of the results about the Higgs signal detected at the LHC. We investigate the decays of neutral Higgs states into Standard Model (SM) fermions and gauge bosons. We perform full one-loop calculations of the decay widths and include leading higher-order QCD corrections. We first discuss the technical aspects of our approach, before confronting our predictions to those of existing public tools, performing a numerical analysis and discussing the remaining theoretical uncertainties. In particular, we find that the decay widths of doublet-dominated heavy Higgs bosons into electroweak gauge bosons are dominated by the radiative corrections, so that the tree-level approximations that are often employed in phenomenological analyses fail. Finally, we focus on the phenomenological properties of a mostly singlet-like state with a mass below the one at $125\,$GeV, a scenario that appears commonly within the NMSSM. In fact, the possible existence of a singlet-dominated state in the mass range around or just below $100\,$GeV would have interesting phenomenological implications. Such a scenario could provide an interpretation for both the $2.3σ$ local excess observed at LEP in the $e^+e^-\to Z(H\to b\bar{b})$ searches at $\sim 98\,$GeV and for the local excess in the diphoton searches recently reported by CMS in this mass range, while at the same time it would reduce the "Little Hierarchy" problem.

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arXiv2018

Two-loop top and bottom Yukawa corrections to the Higgs-boson masses in the complex MSSM

Results for the two-loop corrections to the Higgs-boson masses of the MSSM with complex parameters of $\mathcal{O}{\left(α_t^2+α_tα_b+α_b^2\right)}$ from the Yukawa sector in the gauge-less limit are presented. The corresponding self-energies and their renormalization have been obtained in the Feynman-diagrammatic approach. The impact of the new contributions on the Higgs spectrum is investigated. Furthermore, a comparison with an existing result in the limit of the MSSM with real parameters is carried out. The new results will be included in the public code $\texttt{FeynHiggs}$.

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arXiv2017

On-Shell neutral Higgs bosons in the NMSSM with complex parameters

The Next-to-Minimal Supersymmetric Standard model (NMSSM) appears as an interesting candidate for the interpretation of the Higgs-measurement at the LHC and as a rich framework embedding physics beyond the Standard Model. We consider the renormalization of the Higgs sector of this model in its $\mathcal{CP}$-violating version, and propose a renormalization scheme for the calculation of on-shell Higgs masses. Moreover, the connection between the physical states and the tree-level ones is no longer trivial at the radiative level: a proper description of the corresponding transition thus proves necessary in order to calculate Higgs production and decays at a consistent loop order. After discussing these formal aspects, we compare the results of our mass calculation to the output of existing tools. We also study the relevance of the on-shell transition-matrix in the example of the $h_i \to τ^+ τ^-$ width. We find deviations between our full prescription and popular approximations that can exceed $10\%$.

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