Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC $pp$ collision data at $\sqrt{s}=$ 7 and 8 TeV (1606.02266v2)

Abstract: Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a $W$ or a $Z$ boson or a pair of top quarks, and of the six decay modes $H \to ZZ, WW$, $\gamma\gamma, \tau\tau, bb$, and $\mu\mu$. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton--proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb$^{-1}$ at $\sqrt{s}=7$ TeV and 20 fb$^{-1}$ at $\sqrt{s} = 8$ TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 $\pm$ 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the $H \to \tau\tau$ decay of $5.4$ and $5.5$ standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.

• The paper delivers the most precise measurements of Higgs boson production and decay, reporting a combined signal yield of 1.09 ± 0.11 relative to the Standard Model.
• It employs dual parameterizations to analyze cross sections, branching fractions, and coupling ratios across multiple production modes and decay channels using 5 fb⁻¹ (7 TeV) and 20 fb⁻¹ (8 TeV) data.
• It establishes strong evidence for ttH and VBF processes with significances of 4.4σ and above 5σ respectively, while constraining non-SM decay branching fractions to below 34%.

Overview of the Combined ATLAS and CMS Higgs Boson Analysis

The paper presents a comprehensive analysis of the Higgs boson production and decay rates using data from the LHC recorded by the ATLAS and CMS experiments. This joint analysis provides a more precise measurement of the Higgs boson's properties by leveraging the combined datasets from both experiments at center-of-mass energies of 7 TeV and 8 TeV. The research focuses on evaluating the production processes and decay modes of the Higgs boson and assessing its couplings to Standard Model (SM) particles.

Methodology

The analysis is centered around five Higgs production processes: gluon fusion (ggF), vector boson fusion (VBF), and associated production with a W or Z boson (VH) or top quarks (ttH). The Higgs decays considered are to ZZ, WW, γγ, ττ, bb̅, and μμ. The data from 2011 and 2012, with integrated luminosities of approximately 5 fb⁻¹ at 7 TeV and 20 fb⁻¹ at 8 TeV, enable a robust examination of these interactions.

To achieve precise estimations, the paper employs two primary parameterizations: one detailing cross sections and branching fractions, and another focusing on ratios of coupling modifiers. These differing approaches allow for a nuanced understanding of potential deviations from the SM predictions and accommodate various theoretical uncertainties.

Results

The combined Higgs boson signal yield relative to the SM prediction is measured to be $1.09 \pm 0.11$. This indicates good agreement with the SM, with a combined statistical significance above 5σ for both the VBF production process and the H→ττ decay mode. Moreover, under the assumption of no additional beyond the Standard Model (BSM) contributions, the analysis places a 95% confidence level upper limit on the Higgs boson's branching fraction into non-SM particles at 34%.

Strong evidence is found for the ttH production mode, where the significance reaches 4.4σ, exceeding the 2σ expectation. This highlights the correlated excess of events in specific top quark-associated final states, which deserve further investigation.

Implications and Future Work

This combined analysis from ATLAS and CMS provides the most precise and comprehensive set of measurements of Higgs boson properties to date, offering a substantial enhancement over previous individual results. The findings confirm the SM predictions for Higgs boson properties, but also push the boundaries toward detecting potential new physics.

As the LHC continues to operate at higher energies and collects more data, future analyses will undoubtedly refine these measurements, further testing the robustness of the SM and exploring the frontiers of particle physics. Enhanced precision in coupling measurements, especially for couplings with second-generation fermions like muons, remains a key objective. The potential discovery of discrepancies would be a strong indicator of BSM physics, providing directions for theoretical developments and experimental searches.

In summary, this paper represents a significant step in cementing our understanding of the Higgs boson—a pillar of the SM—and sets a benchmark for future explorations into the quantum field of particle physics.