[Paper Review] Combined CDF and D0 Upper Limits on Standard Model Higgs Boson Production with up to 8.2 fb$^{-1}$ of Data

B. Trocmé|arXiv (Cornell University)|Jul 1, 2010

Particle physics theoretical and experimental studies10 references51 citations

TL;DR

This paper presents combined upper limits on Standard Model Higgs boson production from CDF and D0 experiments at Fermilab's Tevatron, using up to 6.7 fb⁻¹ of data. By integrating 129 exclusive analysis channels across multiple decay modes (bb, WW, ττ, γγ), and applying advanced multivariate techniques and updated theoretical inputs, the study sets the most sensitive limits to date, excluding a Higgs boson in the 158–175 GeV/c² mass range at 95% confidence level.

ABSTRACT

We combine results from CDF and D0 on direct searches for the standard model (SM) Higgs boson H in ppbar collisions at the Fermilab Tevatron at sqrt(s)=1.96 TeV. Compared to the previous Tevatron Higgs search combination more data have been added, additional new channels have been incorporated, and some previously used channels have been reanalyzed to gain sensitivity. We use the latest parton distribution functions and gg to H theoretical cross sections when comparing our limits to the SM predictions. With up to 5.9 fb-1 of data analyzed at CDF, and up to 6.7 fb-1 at D0, the 95% C.L. upper limits on Higgs boson production are factors of 1.56 and 0.68 the values of the SM cross section for a Higgs boson mass of m_H=115 GeV/c^2 and 165~GeVc^2. We exclude, at the 95% C.L., a new and larger region at high mass between 158<m_H<175 GeV/c^2.

Motivation & Objective

• To improve the sensitivity of the Tevatron's search for the Standard Model Higgs boson by combining results from multiple decay channels and analysis techniques.
• To update and extend previous Higgs search combinations with more data, new channels, and refined analysis methods.
• To set the most stringent upper limits on Higgs boson production cross sections across a wide mass range using the latest theoretical and experimental inputs.
• To exclude a new, previously unexplored region of high Higgs mass (158–175 GeV/c²) at 95% confidence level.
• To enhance the robustness of the search by incorporating sensitivity-weighted luminosities and advanced multivariate discriminants across 129 mutually exclusive final states.

Proposed method

• Combines results from 129 mutually exclusive analysis sub-channels (56 CDF, 73 D0) across multiple Higgs decay modes: H→bb, WW, ττ, γγ, and VH associated production.
• Uses sensitivity-weighted averaging of integrated luminosities (5.8 fb⁻¹ low mass, 6.0 fb⁻¹ high mass) to reflect each channel’s contribution to the final limit.
• Applies multivariate techniques—random forests, boosted decision trees, Bayesian neural networks, and matrix element methods—to enhance signal-background separation.
• Incorporates updated parton distribution functions and theoretical cross sections for gg→H and other production modes to improve comparison with SM predictions.
• Performs a unified limit-setting procedure using profile likelihood ratio tests, with systematic uncertainties propagated through detailed error budgeting.
• Uses separate, orthogonal analysis categories for each decay mode and channel to avoid double-counting and ensure statistical independence.

Experimental results

Research questions

• RQ1 What is the most stringent upper limit on SM Higgs boson production at the Tevatron, given up to 6.7 fb⁻¹ of data?
• RQ2 Can the inclusion of new decay channels and improved analysis techniques significantly enhance the sensitivity of the Higgs search?
• RQ3 Is there a previously unexplored mass region for the Higgs boson that can now be excluded with higher confidence?
• RQ4 How do updated theoretical inputs (PDFs, cross sections) affect the interpretation of the observed limits?
• RQ5 To what extent do systematic uncertainties—especially in jet energy, b-tagging, and trigger efficiencies—affect the final exclusion limits?

Key findings

• At a Higgs boson mass of 115 GeV/c², the 95% confidence level upper limit on the production cross section is 1.56 times the SM prediction.
• At 165 GeV/c², the 95% CL upper limit is 0.68 times the SM cross section, indicating significant sensitivity improvement.
• A new, previously unexcluded region between 158 and 175 GeV/c² is excluded at 95% confidence level, expanding the reach of the Tevatron Higgs search.
• The inclusion of new sub-channels—such as the low dilepton mass channel in H→WW and updated ee/µµ/µµtrk/eeICR channels—increased signal acceptance by 10% at mH = 120 GeV.
• Systematic uncertainties were carefully evaluated and propagated, with the largest contributions coming from jet energy scale, PDFs, and lepton identification efficiencies.
• The combination achieved higher sensitivity than previous Tevatron Higgs search combinations due to improved analysis techniques and expanded data coverage.