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Recent Developments in HiggsBounds and a Preview of HiggsSignals
Philip Bechtle, Oliver Brein, Sven Heinemeyer, Oscar Stål, Tim Stefaniak, Georg Weiglein, Karina Williams
TL;DR
HiggsBounds and HiggsSignals address how arbitrary Higgs-sector predictions can be tested against collider exclusions and Higgs signal measurements while respecting search assumptions. The paper explains the SM-likeness procedure, previews updated tools, and illustrates HiggsSignals with an MSSM fit to the LHC Higgs signal near 126 GeV. The example finds a good-fit point with χ2/ndf = 29.3/32.
Problem
Arbitrary Higgs sectors need collider constraints that account for whether searches based on combined Standard Model signal topologies are applicable.
Method
The paper describes HiggsBounds's SM-likeness test and previews HiggsBounds-4.0.0 and HiggsSignals, which tests predictions against signal-rate and mass measurements with correlated uncertainties and unresolved signal superpositions.
Results
The MSSM illustration finds a best-fit point at (mA, tanβ) = (101.0 GeV, 6.0) with χ2/ndf = 29.3/32.
Takeaways & Limitations
SM weights improve HiggsBounds's applicability to arbitrary Higgs sectors, while HiggsSignals supports global fits and generic Higgs coupling determinations.
Takeaways & Limitations
The illustrated exclusion analysis uses limits from the LHC 7 TeV run; newer 8 TeV limits are more constraining.
Abstract
from arXiv · showhide
We report on recent developments in the public computer code HiggsBounds, which confronts arbitrary Higgs sector predictions with 95% C.L. exclusion limits from Higgs searches at the LEP, Tevatron and LHC experiments. We discuss in detail the performance of the Standard Model (SM) likeness test as implemented in the latest version HiggsBounds-3.8.0, whose outcome decides whether a search for a SM Higgs boson can be applied to a model beyond the SM. Furthermore, we give a preview of features in the upcoming version HiggsBounds-4.0.0 and the new program HiggsSignals, which performs a chi-squared test of Higgs sector predictions against the signal rate and mass measurements from Higgs boson analyses at the Tevatron and LHC. This is illustrated with an example where the heavier CP-even Higgs boson of the Minimal Supersymmetric Standard Model (MSSM) is considered as an explanation of the LHC Higgs signal at ~126 GeV.
1. Introduction to HiggsBounds
HiggsBounds confronts arbitrary Higgs-sector predictions with collider exclusion limits while preserving their 95% C.L. interpretation through a defined analysis-selection procedure. The upcoming HiggsBounds-4.0.0 extends the code with recent 8 TeV LHC results.
- HiggsBounds framework: Collider searches provide either model-independent limits for single signal topologies or combined cross-section limits based on Standard Model assumptions.The latter combine several production and decay modes to maximize discovery potential.
- HiggsBounds framework: HiggsBounds confronts arbitrary neutral and charged Higgs-sector predictions with 95% C.L. exclusion limits from LEP, Tevatron, and LHC searches.Its inputs include Higgs masses, total widths, branching ratios, production cross sections, and relevant top-quark branching ratios.
- Statistical procedure: To preserve the 95% C.L. interpretation, HiggsBounds first selects the most sensitive available analysis and then tests the model point against that analysis's observed limit only.Sensitivity is determined using the best expected exclusion limit for the particular model parameter point.
- Developments: HiggsBounds-4.0.0 was planned to include the latest 8 TeV LHC results, extending the then-current 7 TeV coverage of HiggsBounds-3.8.0.HiggsBounds-3.8.0 was released in May 2012.
2. Performance of the Standard Model likeness test
The SM-likeness test determines whether combined SM Higgs searches can be applied to beyond-the-SM models by comparing topology-specific and total signal strengths with SM-weighted contributions. In the ATLAS H →γγ example, SM weights explain why dominant channels control the test while subdominant channels may deviate more.
- Test rationale: The SM-likeness test checks whether a model satisfies the assumptions of searches combining multiple signal topologies under the SM hypothesis.The test is necessary because efficiencies for the separate topologies are rarely publicly available.
- Test construction: For each production-and-decay topology, HiggsBounds defines an individual signal strength modifier c_i and an SM weight ω_i representing its relative SM contribution to the total rate.The total modifier is approximated by µ = ∑i ω_i c_i when model and SM topology proportions are similar.
- Test construction: The test requires the maximal weighted deviation of individual modifiers from the total modifier to remain below ε = 2%.The paper characterizes ε = 2% as a conservative choice because rate-prediction uncertainties are generally larger.
- Example setup: In the 125 GeV ATLAS H →γγ example, the SM weights at 7 TeV are approximately 87.7%, 6.8%, 3.2%, 1.8%, and 0.5% for ggF, VBF, HW, HZ, and Ht¯t.The example modifies the squared effective Higgs couplings to gluons or vector bosons while keeping other effective couplings, including Hγγ, at SM values.
- Example results: Because ggF carries approximately 87.7% of the SM weight, the total signal strength µ follows c(ggf), and the test fails at g2_Hgg = 0.835 and 1.225 because of the ggF topology.Larger deviations in lower-weight VBF, HW, HZ, and Ht¯t channels can still be tolerated.
- Example results: Changing g2_HVV affects VBF, HW, and HZ modifiers but only slightly changes µ because those channels have small SM weights; their deviation from µ can nevertheless cause test failure.Including SM weights allows subdominant signal topologies to deviate further from µ than dominant ones.
- Impact: Introducing SM weights in HiggsBounds-3.8.0 widened the applicability of SM Higgs search results to arbitrary Higgs sectors and significantly improved the code's performance.
3. Prospects and new developments
The paper previews HiggsSignals and HiggsBounds-4.0.0 while testing whether the MSSM’s heavier CP-even Higgs can explain the approximately 126 GeV LHC signal. In the studied scenario, the best fit is good but remains testable by Higgs searches, especially with newer data.
- New programs: HiggsSignals performs a χ2 test against collider signal-rate and mass measurements, incorporating correlated systematic uncertainties and unresolved Higgs-signal superpositions.It is intended for global fits beyond the Standard Model and generic Higgs coupling determinations.
- MSSM interpretation: The MSSM illustration scans the (mA, tanβ) plane for fixed parameters MSUSY = 1 TeV, |Xt| = 2.4 TeV, µ = 1 TeV, M1 = 100 GeV, and M2 = 200 GeV.The scenario also fixes M3 = 800 GeV.
- MSSM interpretation: The likelihood map combines HiggsSignals signal-rate and mass measurements with a LEP χ2 value and displays HiggsBounds-3.8.0 exclusions.The LEP χ2 information was planned for inclusion in HiggsBounds-4.0.0.
- MSSM interpretation: ATLAS combined SM channels and the CMS h,H,A →ττ search constrain tanβ ≳8, while the ATLAS charged-Higgs search excludes regions at low mA and tanβ.The charged-Higgs rate in top decays is fairly close to the ATLAS exclusion limit.
- MSSM interpretation: The best-fit point is (mA, tanβ) = (101.0 GeV, 6.0), with χ2/ndf = 29.3/32 and masses mH± = 126 GeV and mh = 92.3 GeV.The light CP-even Higgs escapes LEP limits because of its reduced coupling to Z bosons.
- Prospects: The applied limits use LHC 7 TeV-era information, while newer 8 TeV exclusion limits are even more constraining.Consequently, additional charged-Higgs data could probe this interpretation.