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

The CMS collaboration
Hayrapetyan, Aram , Makarenko, Vladimir , Tumasyan, Armen , Adam, Wolfgang , Andrejkovic, Janik Walter , Benato, Lisa , Bergauer, Thomas , Dragicevic, Marko , Giordano, Cristina , Hussain, Priya Sajid
CMS-EXO-24-031, 2025.

Abstract (data 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 $\sqrt{s} = 13~\mathrm{TeV}$, collected with the CMS detector in 2016--2018 and corresponding to an integrated luminosity of $138~\mathrm{fb}^{-1}$. 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 $ H\to A A\to4 e$ are set at $95\%$ confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 $\mu\mathrm{m}$ ; 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.

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