Measurements of Higgs boson production via gluon-gluon fusion and vector-boson fusion using $H ightarrow WW^st ightarrow llνllν$ decays in $pp$ collisions with the ATLAS detector and their effective field theory interpretations

Higgs boson production cross-sections via gluon-gluon fusion and vector-boson fusion in proton-proton collisions are measured in the $H\rightarrow WW^\ast \rightarrow \ellν\ellν$ decay channel. The Large Hadron Collider delivered proton-proton collisions at a centre-of-mass energy of $13,\textrm{TeV}$ between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of $140,\textrm{fb}^{-1}$. The total cross-sections for Higgs boson production by gluon-gluon fusion and vector-boson fusion times the $H\rightarrow WW^\ast$ branching ratio are measured to be $12.4^{+1.3}{-1.2},\textrm{pb}$ and $0.79^{+0.18}{-0.16},\textrm{pb}$, respectively, in agreement with the Standard Model predictions. Higgs boson production is further characterised through measurements of Simplified Template Cross-Sections in a total of fifteen kinematic fiducial regions. A new scheme of kinematic fiducial regions has been introduced to enhance the sensitivity to CP-violating effects in Higgs boson interactions. Both schemes are used to constrain CP-even and CP-odd dimension-six operators in the Standard Model effective field theory.

💡 Research Summary

This paper presents a comprehensive analysis by the ATLAS Collaboration at CERN, detailing precision measurements of Higgs boson production cross-sections using the full Run 2 dataset from the Large Hadron Collider (LHC). The study focuses on the decay channel where the Higgs boson decays into two W bosons, which subsequently decay into two charged leptons (electrons or muons) and neutrinos (H→WW*→ℓνℓν). The analyzed data comprises proton-proton collisions at a center-of-mass energy of 13 TeV, recorded between 2015 and 2018, corresponding to an integrated luminosity of 140 fb⁻¹.

The primary objectives are to measure the production cross-sections for the two dominant Higgs production mechanisms: gluon-gluon fusion (ggF) and vector-boson fusion (VBF), multiplied by the H→WW* branching ratio. The measured values are 12.4 +1.3 -1.2 pb for ggF and 0.79 +0.18 -0.16 pb for VBF, both in agreement with Standard Model (SM) predictions. The analysis introduces several improvements over the previous ATLAS Run 2 analysis, including the inclusion of same-flavor lepton final states, the use of deep neural networks (DNNs) for signal-background discrimination, refined estimation of backgrounds with misidentified leptons, and interpretations within the Standard Model Effective Field Theory (SMEFT) framework.

A central component of the analysis is the measurement of Simplified Template Cross-Sections (STXS) in fifteen exclusive kinematic fiducial regions. These regions are defined based on the number of jets, the transverse momentum of the Higgs boson (pTH), and, for events with at least two jets, the invariant mass of the two leading jets (mjj). This staged approach allows for a more granular test of the SM and enhances sensitivity to potential deviations arising from new physics.

Furthermore, the paper pioneers a novel differential measurement scheme designed specifically to probe CP-violating effects in Higgs boson interactions. For events with two or more jets, an additional variable sensitive to CP properties—the azimuthal angle difference between the two jets, ordered by their rapidity (Δφ±jj)—is used to subdivide the STXS categories. Signal strengths are measured in a total of twenty bins spanning several pTH and mjj categories, and the resulting asymmetries in the Δφ±jj distribution are used to constrain CP-odd effective operators.

The experimental results are interpreted within the SMEFT framework, which parameterizes potential effects of new physics at a high scale Λ in a model-independent way. Using the Warsaw basis in the top-flavor scheme, constraints are placed on the Wilson coefficients of sixteen dimension-six operators that modify Higgs boson production and decay. Separate interpretations are performed for CP-conserving and CP-violating scenarios, providing stringent limits on both CP-even and CP-odd contributions beyond the SM.

In summary, this work not only validates the SM predictions for Higgs boson production with unprecedented precision in the H→WW* channel but also establishes advanced methodologies for differential measurements and sets powerful, model-independent constraints on a broad spectrum of potential new physics phenomena through SMEFT interpretations. The results supersede the previous ATLAS publication and represent a significant step forward in the precision Higgs physics program.