CMS-EXO-24-031

CMS-EXO-24-031 ; CERN-EP-2025-235
Search for light pseudoscalar bosons, pair-produced in Higgs boson decays in the four-electron final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
24 November 2025
Submitted to Phys. Rev. Lett.
Abstract: A search for pairs of light neutral pseudoscalar bosons (A) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at $ \sqrt{s} = $ 13 TeV, collected with the CMS detector in 2016--2018 and corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The A boson decays into a highly collimated electron-positron pair. A novel multivariate algorithm using tracks and calorimeter information is developed to identify these distinctive signatures, and events are selected with two such merged electron-positron pairs. No significant excess above the standard model background predictions is observed. Upper limits on the branching fraction for $ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to4\mathrm{e} $ are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 m, reaching branching fraction sensitivities as low as 10$^{-5} $. This is the first search for Higgs boson decays to four electrons via light pseudoscalars at the LHC. It significantly improves the experimental sensitivity to axion-like particles with masses below 100 MeV.
Links: e-print arXiv:2511.19563 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of the four-electron system ($ m_{4\mathrm{e}} $) for selected events (points), compared to the background-only fit (red) with its 68% and 95% CL uncertainty bands (green and yellow). A non-stacked benchmark signal (blue) for a Higgs boson decaying to a pair of ALPs with $ m_{ {\mathrm{A}} }= $ 20 MeV and $ c\tau= $ 10 $\mu$m is overlaid and normalized to a branching ratio of 4.6 $ \times$ 10$^{-5} $, which corresponds to the 95% CL upper limit value set by this analysis. The lower panel shows the same data after subtracting the background fit.
png pdf	Figure 2: Observed (solid points) and expected (dashed lines) 95% CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to\mathrm{e}\mathrm{e} $), shown as a function of the ALP mass for benchmark proper decay lengths of 1 m (upper left), 10 m (upper right), and 100 m (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed 95% CL upper limit, shown as a color scale, as a function of the ALP mass $ m_{ {\mathrm{A}} } $ and proper decay length $ c\tau $.
png pdf	Figure 2-a: Observed (solid points) and expected (dashed lines) 95% CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to\mathrm{e}\mathrm{e} $), shown as a function of the ALP mass for benchmark proper decay lengths of 1 m (upper left), 10 m (upper right), and 100 m (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed 95% CL upper limit, shown as a color scale, as a function of the ALP mass $ m_{ {\mathrm{A}} } $ and proper decay length $ c\tau $.
png pdf	Figure 2-b: Observed (solid points) and expected (dashed lines) 95% CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to\mathrm{e}\mathrm{e} $), shown as a function of the ALP mass for benchmark proper decay lengths of 1 m (upper left), 10 m (upper right), and 100 m (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed 95% CL upper limit, shown as a color scale, as a function of the ALP mass $ m_{ {\mathrm{A}} } $ and proper decay length $ c\tau $.
png pdf	Figure 2-c: Observed (solid points) and expected (dashed lines) 95% CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to\mathrm{e}\mathrm{e} $), shown as a function of the ALP mass for benchmark proper decay lengths of 1 m (upper left), 10 m (upper right), and 100 m (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed 95% CL upper limit, shown as a color scale, as a function of the ALP mass $ m_{ {\mathrm{A}} } $ and proper decay length $ c\tau $.
png pdf	Figure 2-d: Observed (solid points) and expected (dashed lines) 95% CL upper limits on the Higgs boson branching fraction to a pair of ALPs decaying into electron-positron pairs ($ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to\mathrm{e}\mathrm{e} $), shown as a function of the ALP mass for benchmark proper decay lengths of 1 m (upper left), 10 m (upper right), and 100 m (lower left). The green and yellow bands represent the one and two standard deviation confidence intervals around the expected limits. The lower right panel shows a map of the observed 95% CL upper limit, shown as a color scale, as a function of the ALP mass $ m_{ {\mathrm{A}} } $ and proper decay length $ c\tau $.
png pdf	Figure 3: A map of the observed 95% CL upper limit on the Higgs boson branching fraction for $ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to4\mathrm{e} $, as a function of the ALP mass and the ratio of the ALP coupling to electrons to the energy scale of the ALP effective interaction.

Summary

This analysis establishes the first direct limits on the Higgs boson exotic decay $ \mathrm{H}\to {\mathrm{A}} {\mathrm{A}} \to4\mathrm{e} $ for an axion-like particle A with a mass of $ \mathcal{O}$(10) MeV, reaching branching fraction sensitivities as low as 10$^{-5} $. The limiting factor for the sensitivity of this analysis is the number of events in data, while the leading systematic uncertainty arises from the identification efficiency of merged electron-positron pairs. This search explores previously inaccessible parameter space and provides the most stringent constraints to date on this model, establishing a new benchmark for future axion-like particle searches at the LHC.

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