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| CMS-PAS-EXO-25-001 | ||
| Search for Higgs compositeness effects in high-mass dilepton final states in proton-proton collisions at $ \sqrt{s} = $ 13 and 13.6 TeV | ||
| CMS Collaboration | ||
| 2026-07-06 | ||
| Abstract: A search for beyond the standard model effects in high-energy dielectron and dimuon final states with the CMS detector is performed, with a specific focus on Higgs compositeness. The search uses data collected at LHC at 13 and 13.6 TeV center-of-mass energies, for a total integrated luminosity of 207 fb$ ^{-1} $. These compositeness effects imply the presence of new massive boson resonances with couplings to difermions and dibosons. The analysis uses a reweighting procedure that is exact at next-to-leading order in QCD. Assuming new boson states with mass well above the TeV scale, an effective field theory approach provides stringent bounds on the oblique parameters $ {\cal W} $ and $ {\cal Y} $, which are zero in the standard model. One-dimensional fits, where only one of the oblique parameters is allowed to deviate from zero, provide the following limits: $ -3.9 \times 10^{-4} < {\cal Y} < 3.5 \times 10^{-4} $ and $ -2.1 \times 10^{-4} < {\cal W} < 1.6 \times 10^{-4} $ at 95% confidence level. More stringent compositeness constraints are derived under the assumption that a new Z$ ^\prime $ resonance is physically present inside or slightly above the visible mass spectrum. Various theoretical scenarios are considered in this context, including cases with large resonance decay width. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Run 2 (left) and Run 3 (right) reconstructed dilepton invariant mass distribution after selection for (upper) muons and (lower) electrons. The uncertainty bands from the simulation include statistical uncertainties and systematic effects that modify the overall differential shape but not the normalization. |
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Figure 1-a:
Run 2 (left) and Run 3 (right) reconstructed dilepton invariant mass distribution after selection for (upper) muons and (lower) electrons. The uncertainty bands from the simulation include statistical uncertainties and systematic effects that modify the overall differential shape but not the normalization. |
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Figure 1-b:
Run 2 (left) and Run 3 (right) reconstructed dilepton invariant mass distribution after selection for (upper) muons and (lower) electrons. The uncertainty bands from the simulation include statistical uncertainties and systematic effects that modify the overall differential shape but not the normalization. |
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Figure 1-c:
Run 2 (left) and Run 3 (right) reconstructed dilepton invariant mass distribution after selection for (upper) muons and (lower) electrons. The uncertainty bands from the simulation include statistical uncertainties and systematic effects that modify the overall differential shape but not the normalization. |
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Figure 1-d:
Run 2 (left) and Run 3 (right) reconstructed dilepton invariant mass distribution after selection for (upper) muons and (lower) electrons. The uncertainty bands from the simulation include statistical uncertainties and systematic effects that modify the overall differential shape but not the normalization. |
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Figure 2:
Simulated DY invariant mass times the sign of $ \cos\theta_{\rm CS} $ distributions for electrons in Run 2, after the application of selection criteria. The expectations in the SM and in several BSM scenarios, corresponding to different values of $ {\cal W} $ and $ {\cal Y} $, are shown. The uncertainty band from the simulation includes the estimated statistical uncertainties and systematic effects. |
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Figure 3:
Allowed regions in the 2-dimensional $ ({\cal W},{\cal Y} ) $ parameter plane, from a fit to the full data sample considered in the analysis. Both statistical and systematic uncertainties are considered in the fit. There is consistency with the SM at the 2 $ \sigma $ level. This deviation is largely driven by the presence of a dielectron event observed in 2022 data with an invariant mass above 5 TeV. |
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Figure 4:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions, for Run 2 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 4-a:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions, for Run 2 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 4-b:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions, for Run 2 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 5:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions for Run 3 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 5-a:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions for Run 3 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 5-b:
Two-dimensional EFT fit results superimposed on the observed data and SM predictions for Run 3 dimuons (upper plot) and dielectrons (lower plot). |
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Figure 6:
Allowed region in the $ ({\cal W},{\cal Y} ) $ parameter plane, using the full data sample, when the $ {\cal W} $ constraint, $ {\cal W} = (- $ 1.2 $ \pm $ 0.6 $) 10^{-4} $ is included in the fit. The SM is consistent with the observations at the 2 $ \sigma $ level. The figure also shows the region constrained by LEP electroweak measurements [42]. |
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Figure 7:
Allowed regions as a function of $ {\cal W} $ and $ {\cal Y} $ under a universality assumption, which requires that both $ {\cal W} $ and $ {\cal Y} $ are positive. The likelihood is derived from the analysis of the full Run 2+2022+2023 data sample of dielectrons and dimuons, considering both statistical and systematic uncertainties. The green and yellow areas correspond to BSM to the 68% and 95% Bayesian credible intervals (CrI) [8], under the positivity constraint and a flat prior assumption for the parameters. |
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Figure 8:
Excluded region in the $ (m^*,g^*) $ plane in a Y-universal scenario [44], assuming the presence of a neutral gauge boson $ \mathrm{Z}^\prime $ of mass $ m^* $. Decays into WW and ZH final states are assumed to increase the width of the new resonance, which is explicitly defined as a function of the strong coupling constant $ g^* $ of the SILH model (see text). These bounds are compared with those deduced from a non-resonant, EFT fit and from the current CMS global fits of the Higgs signal strength modifier for $ c_{H}= $ 1 ($ g^* < 1.4 m^*[ \text{Te\hspace{-.08em}V}] $ at 95% CL) [49]. |
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Figure 9:
Excluded regions in the $ (m^*,g^*) $ plane in a Little Higgs scenario with custodial symmetry [46], assuming the presence of a neutral gauge boson $ \mathrm{Z}^\prime $ of mass $ m^* $. Decays into WW and ZH final states are assumed to increase the width of the new resonance, which is explicitly defined as a function of the strong coupling constant $ g^* $ of the SILH model (see text). These bounds are compared with those deduced from a non-resonant, EFT fit and from the current CMS global fits of the Higgs signal strength modifier for $ c_{H}= $ 0.5 ($ g^* < 2 m^*[ \text{Te\hspace{-.08em}V}] $ at 95% CL) [49]. |
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Figure 10:
Allowed regions in the 2-dimensional $ ({\cal W},{\cal Y} ) $ parameter plane, assuming a nearby new physics energy scale. The presence of neutral gauge boson resonances of 6 TeV (left) and 10 TeV (right) is assumed, with a medium (10%) or large (40%) relative decay width, due to the potential presence of a significant decay rate into WW or ZH final states. Both statistical and systematic uncertainties are considered in the fit. |
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Figure 10-a:
Allowed regions in the 2-dimensional $ ({\cal W},{\cal Y} ) $ parameter plane, assuming a nearby new physics energy scale. The presence of neutral gauge boson resonances of 6 TeV (left) and 10 TeV (right) is assumed, with a medium (10%) or large (40%) relative decay width, due to the potential presence of a significant decay rate into WW or ZH final states. Both statistical and systematic uncertainties are considered in the fit. |
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Figure 10-b:
Allowed regions in the 2-dimensional $ ({\cal W},{\cal Y} ) $ parameter plane, assuming a nearby new physics energy scale. The presence of neutral gauge boson resonances of 6 TeV (left) and 10 TeV (right) is assumed, with a medium (10%) or large (40%) relative decay width, due to the potential presence of a significant decay rate into WW or ZH final states. Both statistical and systematic uncertainties are considered in the fit. |
| Tables | |
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Table 1:
Systematic uncertainty sources considered in the analysis that have a non-negligible impact on the shape of the reconstructed invariant mass distribution in the 0.3 $ < M < $ 6 TeV mass interval. Their typical size, which was used as input for the likelihood fits, is also shown. |
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Table 2:
Confidence intervals (CI) of the one-dimensional fits to $ {\cal Y} $ and $ {\cal W} $. The value of the non-fitted parameter is fixed to be zero during minimization. The expected boundaries at the 95% CL under the SM assumption are also given. |
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Table 3:
EFT constraints on the oblique parameters $ {\cal W} $ and $ {\cal Y} $ for several universal scenarios: Y-universal [Y-none], Little Higgs [45,5], Little Higgs with custodial symmetry [46,5], and holographic composite Higgs with equal bulk gauge couplings [47,5]). The quoted bounds on $ m^*g^* $ correspond to the 95% Bayesian credible intervals [8]. |
| Summary |
| Using the CMS detector, a search for new physics effects due to Higgs compositeness in high-energy dielectron and dimuon final states was performed, using LHC data at center-of-mass energies of $13$ and $13.6$ TeV for a total integrated luminosity of 207 fb$ ^{-1} $ The presence of these new physics effects implies the existence of new massive boson resonances with couplings to difermions and dibosons. The analysis employs an exact next-to-leading QCD order reweighting procedure. Assuming new boson states with masses well above the TeV scale, stringent bounds are set on the oblique parameters $ {\cal W} $ and $ {\cal Y} $,, which are zero in the Standard Model. These results were also combined with previous CMS constraints on $ {\cal W} $ and $ {\cal Y} $, searches to derive the CMS most stringent limits to date. One-dimensional fits, in which only one of the oblique parameters is permitted to deviate from zero, yield the following limits: $-3.9\times 10^{-4} < {\cal Y} <3.5\times 10^{-4}$, and $-2.1\times 10^{-4} < {\cal W} <1.6\times 10^{-4}$ at 95% CL. Within universal coupling scenarios with ${\cal W}, {\cal Y} > 0$, such as SILH, likelihood ratio criteria strongly constrain models with ${\cal W} > 7.8 \times 10^{-5}$ or ${\cal Y} > 7.6 \times 10^{-4}$. Taking into account the current CMS bounds on the Higgs signal strength modifier [49], compositeness Higgs scale limits at the TeV scale are effectively set in Y-universal and Little Higgs scenarios, independently of the value of the compositeness coupling strength $g*$. Limits are also set for Z$ ^\prime $ resonances in scenarios with large decay widths due to potentially significant decay branching fractions into diboson final states. |
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