Measurements of the Higgs boson production and decay rates and coupling strengths using $pp$ collision data at $\sqrt{s}=7$ and $8$ TeV in the ATLAS experiment

TL;DR

<3-5 sentence high-level summary>: The ATLAS Collaboration performs a comprehensive combination of Higgs-boson production and decay measurements using 7 and 8 TeV LHC data to test Standard Model predictions for a 125.36 GeV Higgs. Through exclusive categorization of multiple final states (γγ, ZZ*, WW*, Zγ, bb, ττ, μμ) and production modes (ggF, VBF, VH, ttH), the analysis derives a global signal strength μ ≈ 1.18 and constrains coupling-strength scale factors κ within various benchmark models, including off-shell and width scenarios. The results show overall SM compatibility, with strong evidence for gluon fusion, significant evidence for vector-boson fusion, and no compelling indication of beyond-SM contributions in loops or decays within current precision. The study also tests custodial symmetry and fermion-vs-boson coupling relations, and provides model-dependent bounds on the Higgs total width and invisible decays, highlighting the robustness of the SM Higgs sector against a broad class of new-physics scenarios.

Abstract

Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the $H\toγγ,\, ZZ^*,\, WW^*,\, Zγ,\, b\bar{b},\, ττ$ and $μμ$ decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 $\mathrm{fb}^{-1}$ at $\sqrt{s}=7$ TeV and 20.3 $\mathrm{fb}^{-1}$ at $\sqrt{s}=8$ TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is $1.18^{+0.15}_{-0.14}$. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.

Figures (27)

• Figure 1: Summary of the signal-strength measurements, as published, from individual analyses that are inputs to the combinations. The Higgs boson mass column indicates the $m_H$ value at which the result is quoted. The overall signal strength of each analysis (black) is the combined result of the measurements for different production processes (blue) assuming SM values for their cross-section ratios. The error bars represent $\pm 1\sigma$ total uncertainties, combining statistical and systematic contributions. The green shaded bands indicate the uncertainty on the overall signal strength obtained by each analysis. The combined signal strength of the $H\to\gamma\gamma$ analysis also includes the $ttH$ contribution which is listed separately under $ttH$ production.
• Figure 2: The observed signal strengths and uncertainties for different Higgs boson decay channels and their combination for $m_H=125.36$ GeV. Higgs boson signals corresponding to the same decay channel are combined together for all analyses, assuming SM values for the cross-section ratios of different production processes. The best-fit values are shown by the solid vertical lines. The total $\pm1\sigma$ uncertainties are indicated by green shaded bands, with the individual contributions from the statistical uncertainty (top), the total (experimental and theoretical) systematic uncertainty (middle), and the signal theoretical uncertainty (bottom) on the signal strength shown as horizontal error bars.
• Figure 3: The best-fit signal-strength values of different production modes determined from the combined fit to the $\sqrt{s}=7$ and 8 TeV data. Higgs boson signals corresponding to the same production process but from different decay channels are combined together, assuming SM values for the ratios of the branching ratios of different Higgs boson decay channels. The inner and outer error bars correspond to 68% CL and 95% CL intervals. Total uncertainties combining statistical, experimental and theoretical systematic uncertainties are shown.
• Figure 4: Likelihood contours in the $(\mu^f_{\mathrm{ggF}+ttH}, \mu^f_{\mathrm{VBF}+VH})$ plane for a Higgs boson mass $m_H=125.36$ measured separately for $H\to WW^*,\, ZZ^*,\, b\bar{b},\, \gamma\gamma$ and $\tau\tau$ decays. SM values are assumed for the relative contributions between ggF and $ttH$ and between VBF and $VH$ production. The straight lower portions of the $H\to\gamma\gamma$ and $H\to ZZ^*\to 4\ell$ contours are due to the small numbers of events in these channels and the requirement of a positive probability density function. The best-fit values to the data (+) and the $68\%$ (full) and $95\%$ (dashed) CL contours are indicated, as well as the SM expectation ($\star$).
• Figure 5: The ratios of cross sections for the vector-boson- and fermion-mediated processes relative to their SM values at $m_H=125.36$ GeV, measured in the individual Higgs boson decay final states and their combination, $R_{\rm Combined}$ (see text). The inner and outer error bars represent 68% CL and 95% CL intervals, combining statistical and systematic uncertainties. These measurements are independent of Higgs boson decay branching ratios.