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Search for boosted vector boson scattering $W^{\pm}W^{\pm}H$ production in final states with same-sign leptons and $b$ -jets in proton-proton collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector

Using the full Run 2 dataset of 140 fb $^{-1}$ collected by the ATLAS detector at $\sqrt{s}=13$ TeV, this paper presents a search for boosted $W^{\pm}W^{\pm}Hjj$ production in final states with same-sign leptons and $b$ -jets, finding no significant deviation from the Standard Model and setting 95% confidence level limits on the quartic $WWHH$ coupling modifier $\kappa_{WW}$ within the range of approximately $-1.9$ to $3.9$.

Original authors: ATLAS Collaboration

Published 2026-09-23

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CC BY 4.0

Original authors: ATLAS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is held together by a set of invisible rules that dictate how particles interact and how forces behave. For decades, physicists have been testing these rules, known as the Standard Model, to see if they hold up under extreme conditions. One of the most important discoveries in this field was the Higgs boson, a particle that gives mass to other fundamental particles. While scientists have confirmed the existence of the Higgs boson, they are still trying to understand exactly how it behaves when it interacts with other particles, particularly the carriers of the weak nuclear force, known as W bosons. The Standard Model predicts a very specific way these particles should interact, but if the rules are slightly different, it could mean there is new, unknown physics waiting to be discovered. Understanding these interactions is crucial because it helps explain why the universe has the structure it does and whether there are hidden layers of reality beyond what we currently know.

In a recent study, researchers at the Large Hadron Collider used the ATLAS detector to look for a rare and specific event: a Higgs boson created at the same time as two W bosons that carry the same electric charge. This process is incredibly difficult to spot because it happens very infrequently and is easily drowned out by more common particle collisions. To find it, the team analyzed a massive amount of data collected over several years, corresponding to an integrated luminosity of 140 fb⁻¹. They focused on a specific signature where the two W bosons decay into charged particles called leptons, which appear as electrons or muons, while the Higgs boson breaks apart into a pair of bottom quarks. Because the Higgs boson in these events is moving at extremely high speeds, the two bottom quarks it produces are squashed together so tightly that they look like a single, large jet of particles rather than two separate ones.

The researchers built a sophisticated search strategy to separate these rare signals from the background noise of ordinary particle interactions. They used advanced computer algorithms to identify the unique pattern of two same-sign leptons, missing energy carried away by invisible neutrinos, and that single, large jet of bottom quarks. They also looked for two other jets that fly off in opposite directions, a hallmark of the specific type of collision that produces this trio of particles. By comparing the actual data from the collisions against detailed computer simulations of what should happen if the Standard Model is correct, the team could determine if anything unusual was occurring. They found that the data matched the Standard Model predictions perfectly, with no significant excess of events that would suggest new physics.

Because no new physics was found, the researchers used their results to set strict boundaries on how much the interaction between the Higgs boson and the W bosons could deviate from the expected rules. They calculated that any deviation must fall within a specific range, effectively ruling out many extreme possibilities that other theories had proposed. This study marks the first time the ATLAS experiment has searched for this particular combination of particles, providing a new and independent way to test the fundamental forces of nature. While the results confirm our current understanding of the universe, they also tighten the constraints on where scientists should look next, ensuring that future searches for new physics are guided by the most precise measurements possible.

Technical Summary: Search for Boosted Vector Boson Scattering $W^\pm W^\pm H$ Production

Problem and Motivation
The discovery of the Higgs boson initiated a program to measure its properties, yet fundamental interactions within the Higgs sector remain weakly constrained. Specifically, the quartic interaction between two vector bosons and two Higgs bosons ( $g_{VVHH}$ ) is a critical probe of the electroweak symmetry breaking mechanism. In the Standard Model (SM), this coupling is fixed by gauge structure and related to the trilinear coupling. Deviations from the SM expectation, parameterized by the coupling modifier $\kappa_{VV}$ (denoted as $\kappa_{WW}$ in this analysis), would signal physics beyond the Standard Model (BSM). Such deviations typically disrupt the cancellations ensuring perturbative unitarity in vector boson scattering (VBS) amplitudes, leading to enhanced cross-sections at high energies. While previous constraints on $\kappa_{VV}$ have been derived from Higgs pair production ( $HH$ ) in VBF and recent inclusive $VVH$ searches, this paper presents the first ATLAS study of the specific electroweak production of a Higgs boson in association with a same-sign $W$ boson pair ( $W^\pm W^\pm H jj$ ). This channel offers a direct probe of the quartic $WWHH$ interaction.

Methodology
The analysis utilizes the full Run 2 proton–proton collision dataset recorded by the ATLAS detector at $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 140 fb $^{-1}$ . The search targets the final state where both $W$ bosons decay leptonically ( $W^\pm \to \ell^\pm \nu$ ), yielding two same-sign charged leptons ( $e$ or $\mu$ ), missing transverse momentum ( $E_T^{miss}$ ), and a Higgs boson candidate.

To maximize sensitivity to BSM effects, the analysis focuses on the "boosted" regime where the Higgs boson has high transverse momentum, causing its decay products ( $b\bar{b}$ ) to be highly collimated. Consequently, the Higgs boson is reconstructed as a single large-radius ( $R=1.0$ ) jet ( $J$ ), rather than as two resolved small-radius jets. The VBS topology is characterized by two forward jets ($jj$).

Key reconstruction and selection steps include:

Lepton Selection: Two same-sign leptons with $p_T > 27$ GeV (leading) and $p_T > 15$ GeV (subleading), satisfying tight identification and isolation criteria.
Higgs Candidate: A large-radius jet with $250 < p_T < 3000$ GeV, $|\eta| < 2.0$ , and mass $90 < m_J < 150$ GeV. It is tagged as originating from $H \to b\bar{b}$ using the GN2X transformer-based neural network.
VBS Tagging: A recurrent neural network (RNN) tagger distinguishes VBS-like topologies from non-VBS backgrounds using the four-momenta of small-radius jets, requiring a classifier score $> 0.2$ .
Background Estimation: Dominant backgrounds include $t\bar{t}V$ (primarily $t\bar{t}W$ ), $t\bar{t}H$ , and electroweak diboson production. Non-prompt lepton backgrounds are estimated using a data-driven fake-factor method in a resolved control region. Charge-misidentification backgrounds are modeled using simulation corrected by data-driven scale factors.
Statistical Analysis: A binned maximum-likelihood fit is performed simultaneously in the signal region and control regions using the Higgs candidate $p_T$ distribution as the discriminating variable. Systematic uncertainties are treated as nuisance parameters.

Key Contributions

First ATLAS Search: This work constitutes the first ATLAS search for electroweak $W^\pm W^\pm H jj$ production.
Direct Constraints: It provides the first direct ATLAS constraints on the individual $\kappa_{WW}$ coupling modifier, complementing previous constraints derived from Higgs pair production and inclusive $VVH$ searches.
Boosted Topology: The analysis demonstrates the utility of the boosted $H \to b\bar{b}$ reconstruction technique for probing quartic gauge-Higgs interactions, where deviations from the SM predict harder kinematic spectra.

Results
No significant excess over the Standard Model prediction was observed in the data. Consequently, 95% confidence level (CL) upper limits were set on the $W^\pm W^\pm H jj$ production cross-section times branching ratio as a function of $\kappa_{WW}$ .

Exclusion Limits: The observed (expected) 95% CL limits exclude values of $\kappa_{WW}$ $κ_{W W}$ outside the interval:
- Observed: $-2.1 < \kappa_{WW} < 4.1$
- Expected: $-1.9 < \kappa_{WW} < 3.9$
Sensitivity Drivers: The analysis sensitivity is primarily limited by the statistical precision of the data sample. Among systematic uncertainties, the estimation of the non-prompt lepton background and flavor-tagging calibration (GN2X) are the dominant contributors.

Significance
The paper claims that this analysis establishes electroweak $W^\pm W^\pm H jj$ production as a complementary probe of the electroweak symmetry-breaking sector. By setting direct constraints on the quartic $WWHH$ interaction, it demonstrates the potential of electroweak $VVH jj$ topologies to probe quartic Higgs–gauge boson interactions, offering a distinct perspective from Higgs pair production measurements. The results are consistent with the Standard Model, providing a baseline for future high-luminosity studies.

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