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Michael Spira

Publications and source records attributed to Michael Spira.

At least 37 records · Page 2Linked to original sources
arXiv2019

2HDECAY - A program for the Calculation of Electroweak One-Loop Corrections to Higgs Decays in the Two-Higgs-Doublet Model Including State-of-the-Art QCD Corrections

We present the program package 2HDECAY for the calculation of the partial decay widths and branching ratios of the Higgs bosons of a general CP-conserving 2-Higgs doublet model (2HDM). The tool includes the full electroweak one-loop corrections to all two-body on-shell Higgs decays in the 2HDM that are not loop-induced. It combines them with the state-of-the-art QCD corrections that are already implemented in the program HDECAY. For the renormalization of the electroweak sector an on-shell scheme is implemented for most of the renormalization parameters. Exceptions are the soft-$\mathbb{Z}_2$-breaking squared mass scale $m_{12}^2$, where an $\overline{\text{MS}}$ condition is applied, as well as the 2HDM mixing angles $α$ and $β$, for which several distinct renormalization schemes are implemented. The tool 2HDECAY can be used for phenomenological analyses of the branching ratios of Higgs decays in the 2HDM. Furthermore, the separate output of the electroweak contributions to the tree-level partial decay widths for several different renormalization schemes allows for an efficient analysis of the impact of the electroweak corrections and the remaining theoretical error due to missing higher-order corrections. The latest version of the program package 2HDECAY can be downloaded from the URL https://github.com/marcel-krause/2HDECAY .

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arXiv2019

The hMSSM approach for Higgs self-couplings revisited

We compare the decay of the heavy Higgs boson into two SM-like Higgs bosons, $H\to hh$, calculated in a Feynman-diagrammatic approach at the one-loop level based on the one hand on the full effective potential involving the top quark and stops in the Minimal Supersymmetric Standard Model (MSSM) accompanied by the matched Two-Higgs-Doublet Model (2HDM) as its low-energy limit and on the other hand on the hMSSM approximation. We identify missing contributions due to the top quark in the Higgs self-couplings of the hMSSM, that - when taken into account - lead to a good agreement between the hMSSM and a full MSSM calculation, at least in the limit of the Higgsino mass parameter $μ$ being small compared to the stop spectrum. We also thoroughly analyze momentum-dependent and kinetic corrections intrinsic to the Feynman-diagrammatic approach and the matching to the effective Lagrangian, respectively, for both our calculation in the MSSM and the hMSSM and for the latter suggest to include additional corrections from the top quark, which are independent of the unknown supersymmetric spectrum.

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arXiv2018

Higgs boson production at large transverse momentum within SMEFT: analytical results

We consider Higgs boson production through gluon fusion at large transverse momentum in hadronic collisions. We present the analytic expressions of the relevant one-loop QCD amplitudes including the effects of the complete set of dimension-six operators. The latter correspond to modifications of the top and bottom Yukawa couplings, to an effective point-like Higgs coupling to gluons and to the chromomagnetic operator of the top quark. The quantitative impact of the chromomagnetic operator is also studied. Our results confirm previous findings that the effect of the chromomagnetic operator at high $p_T$ can be large and should not be neglected.

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arXiv2018

Collider constraints on light pseudoscalars

We investigate the bounds on light pseudoscalars that arise from a variety of collider searches. Special attention is thereby devoted to the mass regions $[3, 5] \, {\rm GeV}$ and $[9,11] \, {\rm GeV}$, in which a meaningful theoretical description has to include estimates of non-perturbative effects such as the mixing of the pseudoscalar with QCD bound states. A compendium of formulas that allows to deal with the relevant corrections is provided. It should prove useful for the interpretation of future LHC searches for light CP-odd spin-0 states.

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arXiv2018

HDECAY: Twenty++ Years After

The program HDECAY determines the partial decay widths and branching ratios of the Higgs bosons within the Standard Model with three and four generations of fermions, including the case when the Higgs couplings are rescaled, a general two--Higgs doublet model where the Higgs sector is extended and incorporates five physical states and its most studied incarnation, the minimal supersymmetric Standard Model (MSSM). The program addresses all decay channels including the dominant higher-order effects such as radiative corrections and multi-body channels. Since the first launch of the program, more than twenty years ago, important aspects and new ingredients have been incorporated. In this update of the program description, some of the developments are summarized while others are discussed in some detail.

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arXiv2017

Effective Field Theory for Higgs properties parametrisation: the transverse momentum spectrum case

After the Higgs boson discovery, LHC can be used as a precision machine to explore its properties. Indeed, in case new resonances will not be found, the only access to New Physics would be via measuring small deviations from the SM predictions. A consistent approach is provided by a bottom-up Effective Field Theory, with dimension six operators built of Standard Model fields (SMEFT). We discuss how this approach works in case of the transverse momentum spectrum of the Higgs particle. In our calculation we augmented the Standard Model with three additional operators describing modifications of the top and bottom Yukawa couplings, and a point-like Higgs coupling to gluons. Based on recently presented resummed transverse-momentum spectra including these operators at NLL+NLO accuracy,which we recently presented in Ref [1], in this note we show the extension of the calculation to NNLL+NNLO. We find that such modifications, while affecting the total rate within the current uncertainties, can lead to significant distortions of the spectrum. Additionally, through significant reduction of the scale uncertainty we improve the sensitivity to the new operators.

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arXiv2016

Modeling BSM effects on the Higgs transverse-momentum spectrum in an EFT approach

We consider the transverse-momentum distribution of a Higgs boson produced through gluon fusion in hadron collisions. At small transverse momenta, the large logarithmic terms are resummed up to next-to-leading-logarithmic (NLL) accuracy. The resummed computation is consistently matched to the next-to-leading-order (NLO) result valid at large transverse momenta. The ensuing Standard Model prediction is supplemented by possible new-physics effects parametrised through three dimension-six operators related to the modification of the top and bottom Yukawa couplings, and to the inclusion of a point-like Higgs-gluon coupling, respectively. We present resummed transverse-momentum spectra including the effect of these operators at NLL+NLO accuracy and study their impact on the shape of the distribution. We find that such modifications, while affecting the total rate within the current uncertainties, can lead to significant distortions of the spectrum. The proper parametrization of such effects becomes increasingly important for experimental analyses in Run II of the LHC.

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arXiv2016

Effective Multi-Higgs Couplings to Gluons

Standard-Model Higgs bosons are dominantly produced via the gluon-fusion mechanism $gg \to H$ at the LHC, i.e. in a loop-mediated process with top loops providing the dominant contribution. For the measured Higgs boson mass of $\sim 125$ GeV the limit of heavy top quarks provides a reliable approximation as long as the relative QCD corrections are scaled with the full mass-dependent LO cross section. In this limit the Higgs coupling to gluons can be described by an effective Lagrangian. The same approach can also be applied to the coupling of more than one Higgs boson to gluons. We will derive the effective Lagrangian for multi-Higgs couplings to gluons up to N$^4$LO thus extending previous results for more than one Higgs boson. Moreover we discuss gluonic Higgs couplings up to NNLO, if several heavy quarks contribute.

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arXiv2016

Signs of Composite Higgs Pair Production at Next-to-Leading Order

In composite Higgs models the Higgs boson arises as a pseudo-Goldstone boson from a strongly-interacting sector. Fermion mass generation is possible through partial compositeness accompanied by the appearance of new heavy fermionic resonances. The Higgs couplings to the Standard Model (SM) particles and between the Higgs bosons themselves are modified with respect to the SM. Higgs pair production is sensitive to the trilinear Higgs self-coupling but also to anomalous couplings like the novel 2-Higgs-2-fermion coupling emerging in composite Higgs models. The QCD corrections to SM Higgs boson pair production are known to be large. In this paper we compute, in the limit of heavy loop particle masses, the next-to-leading order (NLO) QCD corrections to Higgs pair production in composite Higgs models without and with new heavy fermions. The relative QCD corrections are found to be almost insensitive both to the compositeness of the Higgs boson and to the details of the heavy fermion spectrum, since the leading order cross section dominantly factorizes. With the obtained results we investigate the question if, taking into account Higgs coupling constraints, new physics could first be seen in Higgs pair production. We find this to be the case in the high-luminosity option of the LHC for composite Higgs models with heavy fermions. We also investigate the invariant mass distributions at NLO QCD. While they are sensitive to the Higgs non-linearities and hence anomalous couplings, the influence of the heavy fermions is much less pronounced.

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arXiv2015

BSM effects on the Higgs transverse-momentum spectrum in an EFT approach

Effective Field Theories offer a consistent bottom-up approach to parametrise small deviations from Standard Model predictions. In this work we report on the application of the Effective Field Theory to shed light on effects from high-scale physics beyond the Standard Model on the Higgs transverse-momentum spectrum. The Standard Model prediction for the transverse-momentum distribution in Higgs boson production through gluon fusion is augmented by three new dimension-six operators, implying the modification of the top and bottom Yukawa couplings, and the inclusion of a point-like Higgs-gluon coupling. We present resummed transverse-momentum spectra including these operators at NLO+NLL accuracy and study their effects on the shape of the distribution. The proper parametrization of such effects becomes increasingly important for experimental analyses in Run II of the LHC.

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arXiv2015

Higgs Boson Production via Gluon Fusion: Soft-Gluon Resummation including Mass Effects

We analyze soft and collinear gluon resummation effects at the N$^3$LL level for Standard Model Higgs boson production via gluon fusion $gg\to H$ and the neutral scalar and pseudoscalar Higgs bosons of the minimal supersymmetric extension at the N$^3$LL and NNLL level, respectively. We introduce refinements in the treatment of quark mass effects and subleading collinear gluon effects within the resummation. Soft and collinear gluon resummation effects amount to up to about 5% beyond the fixed-order results for scalar and pseudoscalar Higgs boson production.

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arXiv2015

Testing the Higgs Boson Coupling to Gluons

We study the possibility to separate in gluon fusion loop-induced Higgs boson production from point-like production. The Higgs boson is reconstructed in the Hgg final state at very large transverse momentum. Using the Higgs boson yields (normalized to the overall rate) and the shape of the Higgs boson pt distribution the two hypotheses can be separated with 2 standard deviations with an integrated luminosity of about 500 fb^-1. The largest experimental uncertainty affecting this estimate is the background event yield. The theoretical uncertainties from missing top mass effects are large, but can be decreased with dedicated calculations.

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arXiv2015

Improved cross-section predictions for heavy charged Higgs boson production at the LHC

In most extensions of the Standard Model, heavy charged Higgs bosons at the LHC are dominantly produced in association with heavy quarks. An up-to-date determination of the next-to-leading order total cross section in a type-II two-Higgs-doublet model is presented, including a thorough estimate of the theoretical uncertainties due to missing higher-order corrections, parton distribution functions and physical input parameters. Predictions in the four- and five-flavour schemes are compared and reconciled through a recently proposed scale-setting prescription. A four- and five-flavour scheme matched prediction is provided for the interpretation of current and future experimental searches for heavy charged Higgs bosons at the LHC.

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arXiv2015

NLO QCD Corrections to Higgs Pair Production including Dimension-6 Operators

New Physics that becomes relevant at some high scale $Λ$ beyond the experimental reach, can be described in the effective theory approach by adding higher-dimensional operators to the Standard Model (SM) Lagrangian. In Higgs pair production through gluon fusion, which gives access to the trilinear Higgs self-coupling, this leads not only to modifications of the SM couplings but also induces novel couplings not present in the SM. For a proper prediction of the cross section, higher order QCD corrections that are important for this process, have to be taken into account. The various higher-dimensional contributions are affected differently by the QCD corrections. In this paper, we provide the next-to-leading order (NLO) QCD corrections to Higgs pair production including dimension-6 operators in the limit of large top quark masses. Depending on the dimension-6 coefficients entering the Lagrangian, the new operators affect the relative NLO QCD corrections by several per cent, while modifying the cross section by up to an order of magnitude.

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arXiv2014

Squark Production and Decay matched with Parton Showers at NLO

Extending previous work on the predictions for the production of supersymmetric (SUSY) particles at the LHC, we present the fully differential calculation of the next-to-leading order (NLO) SUSY-QCD corrections to the production of squark and squark-antisquark pairs of the first two generations. The NLO cross sections are combined with the subsequent decay of the final state (anti)squarks into the lightest neutralino and (anti)quark at NLO SUSY-QCD. No assumptions on the squark masses are made, and the various subchannels are taken into account independently. In order to obtain realistic predictions for differential distributions the fixed-order calculations have to be combined with parton showers. Making use of the Powheg method we have implemented our results in the Powheg-Box framework and interfaced the NLO calculation with the parton shower Monte Carlo programs Pythia6 and Herwig++. The code is publicly available and can be downloaded from the Powheg-Box webpage. The impact of the NLO corrections on the differential distributions is studied and parton shower effects are investigated for different benchmark scenarios.

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arXiv2014

Neutral MSSM Higgs-boson production with heavy quarks: NLO supersymmetric QCD corrections

Within the minimal supersymmetric extension of the Standard Model (MSSM) the associated production of neutral Higgs bosons with top and bottom quarks belongs to the most important Higgs-boson production processes at the LHC. At large values of tan(beta), in particular, bottom--Higgs associated production constitutes the dominant production channel within the MSSM. We have calculated the next-to-leading-order supersymmetric QCD corrections to neutral Higgs production through the parton processes q qbar, gg -> t tbar / b bbar + h/H/A and present results for the total cross sections. The genuine SUSY-QCD corrections are of moderate size for small tan(beta), but can be sizable for large tan(beta). In the latter case the bulk of these corrections can be absorbed into effective bottom Yukawa couplings.

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arXiv2014

Effective Lagrangian for a light Higgs-like scalar

We reconsider the effective Lagrangian that describes a light Higgs-like boson and better clarify a few issues which were not exhaustively addressed in the previous literature. In particular we highlight the strategy to determine whether the dynamics responsible for the electroweak symmetry breaking is weakly or strongly interacting. We also discuss how the effective Lagrangian can be implemented into automatic tools for the calculation of Higgs decay rates and production cross sections.

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