[Paper Review] LHC Higgs Boson searches

W. J. Murray|arXiv (Cornell University)|Jan 22, 2012

Particle physics theoretical and experimental studies10 references3 citations

TL;DR

This paper summarizes the ATLAS and CMS collaborations' searches for the Standard Model Higgs boson using 1 fb⁻¹ of LHC data, employing multiple decay channels and production modes. It reports exclusions of the Higgs boson at 95% confidence level in mass ranges 155–190 GeV and 295–450 GeV (ATLAS) and 149–206 GeV, 270–290 GeV, and 300–400 GeV (CMS), with a notable 2–3σ excess near 120–150 GeV possibly indicating a low-mass Higgs or statistical fluctuation.

ABSTRACT

A summary of the Higgs boson searches by the ATLAS and CMS collabrations using 1 f b-1 of LHC data is presented, concentrating on the Standard Model Higgs boson. Both experiments have the sensitivity to exclude at 95% CL a Standard Model Higgs boson in most of the Higgs boson mass region between about 130 GeV and 400 GeV. The observed data allow the exclusion of a Higgs Boson of mass 155 GeV to 190 GeV and 295 GeV to 450 GeV (ATLAS) and 149 GeV to 206 GeV and 300 GeV to 440 GeV (CMS). The lower limits are not as constraining as might be expected due to an excess in both experiments of order 2-3σ which could be related to a low mass Higgs boson or to a statistical fluctuation.

Motivation & Objective

• To test the existence of the Standard Model Higgs boson using 1 fb⁻¹ of LHC data.
• To evaluate the sensitivity of ATLAS and CMS experiments to Higgs boson production across a wide mass range.
• To identify and interpret anomalies, such as a 2–3σ excess in the low-mass region, possibly indicating a Higgs signal or statistical fluctuation.
• To compare the performance of different search channels—γγ, WW, ZZ, bb̄, ττ—across experiments and mass regions.
• To provide a historical snapshot of Higgs search methodologies and results prior to later data updates.

Proposed method

• Combines multiple decay channels (γγ, WW, ZZ, bb̄, ττ) to enhance sensitivity to the Higgs boson across different mass regions.
• Uses multivariate analysis techniques in CMS’s WW→ℓνℓν and ZZ→ℓℓℓℓ searches, improving sensitivity compared to ATLAS’s cut-based approach.
• Analyzes three main Higgs production modes: gluon fusion, vector boson fusion (VBF), and associated production with W/Z or top quarks.
• Applies 95% confidence level (CL) limits to exclude Higgs boson signal strengths across the mass spectrum.
• Performs separate and combined analyses by ATLAS and CMS, with observed and expected exclusion limits compared across experiments.
• Evaluates significance of observed excesses using p-values under the background-only hypothesis, without look-elsewhere effect correction.

Experimental results

Research questions

• RQ1 What mass ranges of the Standard Model Higgs boson are excluded by ATLAS and CMS using 1 fb⁻¹ of LHC data at 95% CL?
• RQ2 Why are the lower mass exclusion limits (149–155 GeV) higher than expected, and what does the observed 2–3σ excess in the low-mass region suggest?
• RQ3 How do the sensitivities of ATLAS and CMS compare across different Higgs decay channels and mass regions?
• RQ4 What role do multivariate analysis techniques play in improving sensitivity, particularly in the WW→ℓνℓν channel?
• RQ5 To what extent do observed excesses at 128 GeV, 145 GeV, and 246 GeV deviate from background-only expectations?

Key findings

• ATLAS excludes a Standard Model Higgs boson in the mass ranges 155–190 GeV and 295–450 GeV at 95% confidence level.
• CMS excludes the Higgs boson in the ranges 149–206 GeV, 270–290 GeV, and 300–400 GeV at 95% CL.
• A persistent 2–3σ excess is observed in both ATLAS and CMS data near 120–150 GeV, primarily driven by WW→ℓνℓν and γγ channels, suggesting a possible Higgs signal or statistical fluctuation.
• CMS achieves higher sensitivity than ATLAS around 160 GeV and 250 GeV due to optimized multivariate analysis in WW and ZZ searches.
• ATLAS shows greater sensitivity above 340 GeV, primarily due to the ZZ→ℓℓνν channel.
• The observed excesses at 246 GeV (ATLAS) and 580 GeV (ATLAS) are not strongly supported by CMS data, indicating possible local fluctuations.