CMS-PAS-EXO-24-031

CMS-PAS-EXO-24-031
Search for light pseudoscalar a bosons in $ \mathrm{H}\to\mathrm{a}\mathrm{a}\to4\mathrm{e} $ decays in pp collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
19 May 2025

Abstract: A search for pairs of light neutral bosons ($ \mathrm{a} $) resulting from the decay of a Higgs boson is performed. The $ \mathrm{a} $ bosons have a mass in the range of 10 to 100 MeV and decay into an electron-positron pair. The search is conducted using proton-proton collision data at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Due to the low mass of the $ \mathrm{a} $ bosons their decay products are highly collimated and are merged into single detector objects. A novel multivariate identification technique 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 prediction is observed. Upper limits on the branching fraction for $ \mathrm{H}\to\mathrm{a}\mathrm{a}\to4\mathrm{e} $ are set at the 95% confidence level, reaching sensitivities as low as $ \mathcal{O}(10^{-5}) $. The measured limits are further interpreted in an effective axion-like particle and Higgs-coupling model, confirming the LHC's potential as a unique axion-like particle search facility.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of gluon-gluon fusion, via an intermediate heavy quark loop, producing a Higgs boson which decays to two axion-like particles, each further decaying to electron-positron pairs.
png pdf	Figure 2: An $ \mathrm{H}\to\mathrm{a}\mathrm{a}\to4\mathrm{e} $ event with two merged electron pairs. In each pair, the two reconstructed GSF tracks, one for each electron, are shown as the yellow lines, while the cyan region on the circle represents the corresponding reconstructed supercluster. The blue and red shapes represent the energy deposit from HCAL and ECAL respectively.
png pdf	Figure 3: (upper) Invariant mass distribution of four electrons ($ m_{4\mathrm{e}} $) for selected events (points), compared to a fit of the background (red), with one (green) and two (yellow) standard deviation uncertainty bands, and the extracted signal (blue), shown as an example for a Higgs boson decaying to pairs of axion-like particles ($ \mathrm{a} $) with a mass of 20 MeV and a mean proper decay length to electron pairs $ c\tau = 10\,\mu\text{m} $. (lower) The lower panel shows the same data after subtracting the background fit.
png pdf	Figure 4: As a function of the mass of an axion-like particle ($ \mathrm{a} $), the observed (points) and expected (dashed) upper limits at 95% CL on the Higgs boson branching fraction for Higgs boson decays to a pairs in the four-electron final state, for scenarios with $ \mathrm{a} $ decay lengths of 1$ \,\mu\text{m} $ (upper left), 10$ \,\mu\text{m} $ (upper right), and 100$ \,\mu\text{m} $ (lower left). The one (green) and two (yellow) standard deviation bands are also shown. The observed limits on the branching fraction in the 2d decay length vs. A mass plane is also shown (lower right).
png pdf	Figure 4-a: As a function of the mass of an axion-like particle ($ \mathrm{a} $), the observed (points) and expected (dashed) upper limits at 95% CL on the Higgs boson branching fraction for Higgs boson decays to a pairs in the four-electron final state, for scenarios with $ \mathrm{a} $ decay lengths of 1$ \,\mu\text{m} $ (upper left), 10$ \,\mu\text{m} $ (upper right), and 100$ \,\mu\text{m} $ (lower left). The one (green) and two (yellow) standard deviation bands are also shown. The observed limits on the branching fraction in the 2d decay length vs. A mass plane is also shown (lower right).
png pdf	Figure 4-b: As a function of the mass of an axion-like particle ($ \mathrm{a} $), the observed (points) and expected (dashed) upper limits at 95% CL on the Higgs boson branching fraction for Higgs boson decays to a pairs in the four-electron final state, for scenarios with $ \mathrm{a} $ decay lengths of 1$ \,\mu\text{m} $ (upper left), 10$ \,\mu\text{m} $ (upper right), and 100$ \,\mu\text{m} $ (lower left). The one (green) and two (yellow) standard deviation bands are also shown. The observed limits on the branching fraction in the 2d decay length vs. A mass plane is also shown (lower right).
png pdf	Figure 4-c: As a function of the mass of an axion-like particle ($ \mathrm{a} $), the observed (points) and expected (dashed) upper limits at 95% CL on the Higgs boson branching fraction for Higgs boson decays to a pairs in the four-electron final state, for scenarios with $ \mathrm{a} $ decay lengths of 1$ \,\mu\text{m} $ (upper left), 10$ \,\mu\text{m} $ (upper right), and 100$ \,\mu\text{m} $ (lower left). The one (green) and two (yellow) standard deviation bands are also shown. The observed limits on the branching fraction in the 2d decay length vs. A mass plane is also shown (lower right).
png pdf	Figure 4-d: As a function of the mass of an axion-like particle ($ \mathrm{a} $), the observed (points) and expected (dashed) upper limits at 95% CL on the Higgs boson branching fraction for Higgs boson decays to a pairs in the four-electron final state, for scenarios with $ \mathrm{a} $ decay lengths of 1$ \,\mu\text{m} $ (upper left), 10$ \,\mu\text{m} $ (upper right), and 100$ \,\mu\text{m} $ (lower left). The one (green) and two (yellow) standard deviation bands are also shown. The observed limits on the branching fraction in the 2d decay length vs. A mass plane is also shown (lower right).
png pdf	Figure 5: The observed upper limits at 95% CL 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 and the energy scale of the ALP effective interaction.

Tables
png pdf	Table 1: Scale factors (SFs) with their statistical uncertainties for different CMS data-taking eras.

Summary

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