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Search for Higgs boson compositeness effects in high-mass dilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 and 13.6 TeV

Using 207 fb $^{-1}$ of proton-proton collision data at 13 and 13.6 TeV, the CMS collaboration performed a search for Higgs boson compositeness in high-mass dilepton final states, establishing stringent constraints on the oblique parameters $\mathcal{W}$ and $\mathcal{Y}$ and setting new limits on TeV-scale compositeness scenarios involving broad $Z'$ resonances.

Original authors: CMS Collaboration

Published 2026-10-06

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CC BY 4.0

Original authors: CMS 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 built on a set of rules known as the standard model, a framework that successfully describes how the smallest particles interact and how the forces of nature hold them together. For decades, this model has been the most reliable map physicists have, yet it leaves a significant question unanswered: why is the Higgs boson, the particle responsible for giving other particles mass, so light? According to the math, the Higgs should be much heavier, bombarded by energy corrections that would naturally push its mass up to enormous scales. The fact that it remains light suggests that our current map might be incomplete, hinting at new, hidden layers of reality just beyond our current view. One compelling idea is that the Higgs boson is not a fundamental, indivisible particle, but rather a composite object, much like a proton is made of smaller quarks. If this were true, the Higgs would be held together by a new, powerful force, and this structure would leave subtle fingerprints in high-energy collisions.

Scientists at the Large Hadron Collider in Switzerland have been hunting for these fingerprints by smashing protons together at record-breaking speeds. In a recent study, the CMS collaboration analyzed a massive collection of data from these collisions, looking specifically at the debris that flies out when protons collide. They focused on a rare outcome where the collision produces a pair of high-energy electrons or muons, which are heavy cousins of the electron. By examining the mass of these pairs, the researchers could test whether the Higgs boson behaves as a simple, fundamental particle or if it shows signs of being a composite structure. The study covered a vast range of energies, utilizing data equivalent to 207 inverse femtobarns of luminosity, which represents an enormous amount of collision events recorded at energies of 13 and 13.6 tera-electronvolts.

The researchers approached the problem by looking for distortions in the expected pattern of these particle pairs. If the Higgs were composite, the interactions between particles would not follow the smooth curve predicted by the standard model; instead, the data would show unexpected bumps or shifts, particularly at very high masses. To find these subtle deviations, the team compared their real-world observations against incredibly precise computer simulations. These simulations accounted for the complex behavior of particles as they interact, including the effects of the strong nuclear force and the electromagnetic force. The team also carefully corrected for the "noise" of the experiment, such as the overlapping collisions that occur when multiple proton bunches cross paths, ensuring that any signal they found was genuine and not an artifact of the detector or the data processing.

After sifting through the data, the researchers found that the observed distribution of particle masses matched the predictions of the standard model with remarkable precision. There were no significant deviations, no strange bumps, and no signs of the distortions that would indicate the Higgs boson is made of smaller parts. This result allowed the team to set strict limits on the possibility of compositeness. They determined that if the Higgs is indeed composite, the energy scale at which this new structure becomes visible must be far higher than the energies currently reachable by the collider. Specifically, they placed tight constraints on two theoretical parameters that describe how new physics might alter particle interactions, finding that these parameters must be extremely close to zero. In the language of the study, the data rules out many scenarios where the Higgs would show composite behavior at the tera-electronvolt scale.

The investigation also looked for the presence of new, heavy particles that might act as carriers of this new force, often called Z-prime bosons. The team searched for these particles not just as sharp, narrow peaks in the data, but also as broad, wide resonances that might be harder to spot. They explored scenarios where these new particles could decay into other heavy particles, such as pairs of W or Z bosons, which would create a wide spread of energy rather than a single point. Even with these broader search strategies, no evidence for such new particles was found. The study also combined its findings with previous results from searches involving neutrinos and missing energy to create an even more comprehensive picture. The combined analysis confirmed that the standard model remains robust, and any new physics related to the structure of the Higgs boson must be hiding at energy levels well beyond the current reach of our machines.

This work does not mean the search is over; rather, it pushes the boundary of what we know further out. The absence of a signal is a powerful result in itself, telling theorists that if new particles or forces exist to solve the mystery of the Higgs mass, they are likely much heavier or more elusive than previously hoped. The study provides a clear, data-driven foundation for future experiments, guiding the next generation of particle accelerators toward the energies where these hidden secrets might finally be revealed. For now, the Higgs boson remains a mystery in its own right, a fundamental particle that continues to behave exactly as the standard model predicts, defying the expectation that it should be built from something smaller.

Technical Summary: Search for Higgs Boson Compositeness Effects in High-Mass Dilepton Final States

Problem and Motivation
The Standard Model (SM) of particle physics faces the "hierarchy problem," where the Higgs boson mass receives quadratically divergent radiative corrections, suggesting an unnatural lightness unless new physics exists near the electroweak scale. One theoretical resolution posits that the Higgs boson is a composite particle arising from new strong interactions, such as in the Strongly Interacting Light Higgs (SILH) scenario. Such compositeness would manifest as distortions in high-invariant-mass dilepton spectra and the potential presence of new massive vector boson resonances (e.g., $Z'$ ). While previous searches have constrained the electroweak oblique parameters $S$ and $T$ at LEP, and the parameter $W$ in $\ell\nu$ final states at the LHC, the parameters $W$ and $Y$ remain less constrained in high-mass dilepton ( $\ell^+\ell^-$ ) channels. This analysis aims to probe these parameters to test the hypothesis of Higgs compositeness and search for deviations from the SM in the high-mass Drell–Yan (DY) production regime.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at center-of-mass energies of $\sqrt{s} = 13$ TeV and 13.6 TeV, corresponding to a total integrated luminosity of 207 fb $^{-1}$ . The search focuses on high-mass dielectron ($ee$) and dimuon ( $\mu\mu$ ) final states.

Event Selection: Events are selected requiring two high-transverse-momentum ( $p_T$ ) leptons. For electrons, a transverse energy $E_T > 100$ GeV is required, with at least one in the barrel region. For muons, $p_T > 53$ GeV is required. Events must have an invariant mass $m_{\ell\ell} > 200$ GeV. Backgrounds from top quark production are suppressed by vetoing events with identified $b$ -jets (using the DeepJet algorithm), while diboson backgrounds are estimated via simulation.
Signal Modeling and Reweighting: Instead of generating dedicated Monte Carlo samples for every hypothesis, the analysis employs a reweighting technique. SM DY samples generated at Next-to-Leading Order (NLO) in QCD using POWHEG are reweighted event-by-event to simulate non-zero values of the oblique parameters $W$ and $Y$ . This approach preserves NLO QCD precision while allowing for the exploration of the effective field theory (EFT) parameter space. The reweighting factors account for the interference between SM and BSM amplitudes, including dependencies on the Collins–Soper angle and quark/lepton chiralities.
Fitting Strategy: A binned likelihood fit is performed on the signed invariant mass distribution ( $m_{\ell\ell} \times \text{sign}(\cos\theta_{CS})$ ) in the range of $\pm 6$ TeV. The fit includes a free rate parameter to normalize the total yield, ensuring the sensitivity is driven by the shape distortions of the mass spectrum rather than absolute event counts. Systematic uncertainties, including Parton Distribution Function (PDF) variations, scale uncertainties, and detector effects (energy/momentum scales and resolutions), are incorporated as nuisance parameters.

Key Contributions

First Exploration of Large Widths: The paper extends the search for $Z'$ resonances in compositeness scenarios to include cases with large relative decay widths (up to 40%), which can occur if the resonance decays significantly into diboson ( $WW, ZH$ ) final states.
Combined Run 2 and Run 3 Analysis: The study combines data from LHC Run 2 (13 TeV) and Run 3 (13.6 TeV), significantly increasing the statistical power and extending the kinematic reach compared to previous single-run analyses.
Complementarity to $\ell\nu$ Searches: By focusing on $\ell^+\ell^-$ final states, this analysis provides sensitivity to both $W$ and $Y$ parameters, complementing previous CMS results in the $\ell\nu$ channel which were primarily sensitive to $W$ .

Results

EFT Constraints: No significant deviations from the SM were observed in the high-mass tails. One-dimensional fits, where only one parameter is allowed to deviate from zero, yielded the following 95% confidence level (CL) limits:
- $-3.9 \times 10^{-4} < Y < 3.5 \times 10^{-4}$
- $-2.1 \times 10^{-4} < W < 1.6 \times 10^{-4}$
Two-Dimensional Fit: A two-dimensional fit in the $(W, Y)$ plane showed a slight departure from the SM at the two-standard-deviation level (95.7% CL), driven largely by a single dielectron candidate with an invariant mass of 5.2 TeV observed in 2022 data. Removing this event reduces the deviation to the one-standard-deviation level.
Universal Scenarios: Assuming positivity constraints ( $W, Y > 0$ ) relevant to SILH models, the analysis sets 95% Bayesian credible intervals of $W < 7.8 \times 10^{-5}$ and $Y < 7.6 \times 10^{-4}$ .
Resonance Limits: Constraints on the compositeness scale ( $m^* g^*$ ) were derived for various universal scenarios (Y-universal, Little Higgs, etc.), with lower bounds ranging from 1.2 TeV to 10.1 TeV depending on the specific model and Wilson coefficients. Limits were also set for $Z'$ resonances with masses up to 10 TeV and large decay widths.

Significance
The paper establishes the most stringent limits to date on the oblique parameters $W$ and $Y$ in the context of Higgs boson compositeness, utilizing the full high-mass dilepton dataset available from the CMS experiment. By combining high-precision NLO simulations with a robust reweighting strategy, the analysis effectively probes the high-energy tail of the DY spectrum where BSM effects are expected to accumulate. The results strongly constrain new physics scenarios predicting non-zero values for these parameters, particularly those involving universal couplings. Furthermore, the exploration of large-width resonance scenarios provides a more comprehensive test of compositeness models that predict significant branching fractions into diboson channels, a regime previously less explored in non-resonant searches. The analysis demonstrates that while a minor tension with the SM exists (driven by a single high-mass event), the overall data remains consistent with the Standard Model, placing tight bounds on the scale of potential Higgs compositeness.

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