Source-linked AI summary
HDECAY: Twenty++ Years After
Abdelhak Djouadi, Jan Kalinowski, Margarete Muehlleitner, Michael Spira
TL;DR
HDECAY addresses the need for comprehensive Higgs-decay predictions across the Standard Model and several extensions, where collider interpretations depend on decay channels and theoretical scenarios. The updated program incorporates broad decay-channel coverage, higher-order corrections, rescaled couplings, and MSSM developments. It extends the treatment of radiative and supersymmetric effects while retaining a stated theoretical uncertainty for SM4 H → γZ because electroweak corrections are unknown.
Problem
Higgs collider searches require decay predictions across the Standard Model and extensions, including scenarios with varied Higgs couplings and additional fermions or supersymmetric states.
Method
HDECAY computes Higgs partial widths and branching ratios using kinematically allowed decay channels, higher-order corrections, rescaled-coupling options, and MSSM radiative and supersymmetric-particle treatments.
Results
The update incorporates new theoretical calculations and improvements, including NLO and NNLO MSSM corrections and the resulting significant impact on loop-induced and tree-level decays in the fourth-generation option.
Takeaways & Limitations
HDECAY provides a flexible framework for comparing Higgs decay predictions with measurements across the SM, rescaled-coupling scenarios, and extended Higgs sectors.
Takeaways & Limitations
The SM4 H → γZ mode is treated only at leading order, and unknown electroweak corrections imply a sizeable theoretical uncertainty because of large leading-order W–fermion-loop cancellations.
Abstract
from arXiv · showhide
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.
1. Introduction
HDECAY provides flexible Higgs-decay predictions across the Standard Model and extensions, incorporating kinematically allowed channels and important higher-order effects. Its updates include multi-body, off-shell, QCD, MSSM radiative, and supersymmetric-particle decay treatments.
- Program scope: HDECAY addresses Higgs-decay phenomenology needed for collider searches across the SM, two-Higgs-doublet models, MSSM, and fourth-generation variants.Search strategies depend on the available Higgs decay channels, collider setup, and theoretical scenario.
- Decay channels: All kinematically allowed channels with branching ratios above 10^-4 are included, including loop-mediated, three-body, MSSM cascade, and supersymmetric decays.The program also includes below-threshold three- and four-body decays.
- Radiative corrections: Relevant higher-order QCD corrections are incorporated for quark-pair decays and loop-mediated decays into gluons.The implementation targets dominant corrections affecting important Higgs decay modes.
- MSSM developments: MSSM radiative corrections use effective-potential results with full stop/sbottom mixing and renormalization-group-improved Higgs masses and couplings.This incorporates relevant NLO and NNLO corrections.
- MSSM developments: MSSM decays into neutralinos, charginos, sleptons, and squarks are calculated when kinematically allowed, including mixing in the stop, sbottom, and stau sectors.Supersymmetric particles also contribute to loop-mediated γγ, Zγ, and gg decays.
2. The major updates and extensions of the program
HDECAY has been progressively extended with higher-order corrections, new decay modes, external interfaces, and broader Higgs-sector scenarios across the SM, MSSM, SM4, 2HDM, and hMSSM.
- Interfaces and usability: Interfaces and input options were broadened through proper SLHA support, FeynHiggsFast and SUSPECT links, expanded input files, and switchable electroweak corrections.SLHA output can be reused as input by other programs and by HDECAY itself.
- New physical scenarios and channels: HDECAY expanded its phenomenological coverage through gravitino and neutralino or chargino decays, charged-Higgs channels, fourth-generation fermions, and full CKM mixing in charged-Higgs and top decays.The SM4 option produces significant effects in loop-induced and tree-level decays because heavy fourth-generation fermions have large Yukawa couplings.
- MSSM developments: The program added MSSM radiative corrections, resummed ∆b and ∆s Yukawa effects, improved squark treatments, and two-loop Higgs masses, couplings, and self-interactions.These developments include arbitrary stop and sbottom mixing, mass splitting, and NNLO extensions of the ∆b and ∆s terms.
- Higher-order corrections: HDECAY incorporated increasingly complete QCD and electroweak corrections for Higgs and top decays, including mass-dependent NLO and NNLO effects.Updates cover quark, squark, gluon, photon, fermion, gauge-boson, and top-decay channels.
- Extended Higgs sectors: The hMSSM provides a good approximation with two parameters, but low-tan β regions require very large SUSY scales and caution about MSSM consistency.The approach captures low tan β values previously overlooked under SUSY scales of order 1 TeV, at the cost of fine-tuning.
3. The input file
The hdecay.in input file specifies model choices, physical parameters, and switches controlling decay treatments and output. It supports Standard Model, fourth-generation, 2HDM, MSSM, and hMSSM configurations, including selectable higher-order, off-shell, supersymmetric, and coupling-rescaling options.
- Decay options: Decay switches control off-shell channels, double off-shell W and Z pair decays, running versus pole Higgs masses, and inclusion of supersymmetric decays and loops.ON-SHELL, ON-SH-WZ, IPOLE, and OFF-SUSY provide these choices.
- Output and particle content: Additional flags determine whether chargino, neutralino, and sfermion decays are summed or printed individually, the number of light flavors in gluonic decays, and which MSSM Higgs branching ratios are calculated.IGOLD and related inputs separately control gravitino and gaugino decays.
- Input parameters: Coupling-rescaling inputs independently modify Higgs couplings to gauge bosons, fermions, and point-like gamma-gamma and gluon operators, with electroweak corrections optionally applied to all rescalings.The file also lists Standard Model masses, widths, couplings, and fourth-generation parameters.
- Model selection: The input supports four 2HDM types—type I, type II, lepton-specific, and flipped—with parameters specified through masses or lambda_i.The example uses TYPE = 2 and PARAM = 1, with tan(beta), mixing, and Higgs-mass inputs.
4. The output files
The output examples show how HDECAY reports branching ratios and total widths for SM, 2HDM, and MSSM benchmark scenarios. They illustrate dominant fermionic decays alongside Higgs, gauge-boson, and supersymmetric final states.
- Standard Model: For the 125 GeV SM Higgs, the output files list branching ratios into fermion pairs, gluons, photons, Zγ, and massive gauge bosons, plus the total width.The numerical output corresponds to MHSM = 125 GeV and the final entry in br.sm2 is the total width in GeV.
- 2HDM: In the 2HDM example, a heavy CP-even Higgs above 350 GeV dominantly decays into a top-quark pair, while decays into lighter Higgs pairs remain small.The output files separate fermionic, gauge-boson, and Higgs-boson final states.
- 2HDM: The 2HDM pseudoscalar likewise dominantly decays into a top-quark pair, while the ZH channel contributes about 10%.The charged-Higgs outputs include fermionic and Higgs–gauge-boson final states, with the latter potentially significant in the HW channel.
- 2HDM: For the 2HDM SM-like Higgs at mh = 125.09 GeV, the h boson behaves SM-like, and the top quark decays entirely into Wb final states.The corresponding output files provide the SM-like Higgs branching-ratio channels and width.
- MSSM: In the MSSM benchmark with tan β = 30, b¯b dominates over t¯t, while kinematically allowed chargino and neutralino decays have branching ratios of about 30% and 20%.The light-Higgs-pair decay is very small and the AA mode is far off-shell and tiny.
- MSSM: In the MSSM h scenario, chargino-neutralino decays exceed 50% and dominate, whereas SUSY-particle decays of the light CP-even Higgs are otherwise kinematically closed in the decoupling-limit example.The light Higgs mass in this scenario is mh = 122.644 GeV.