Search for Light Pseudoscalar Bosons, Pair-Produced in Higgs Boson Decays in the Four-Electron Final State in Proton-Proton Collisions at (Formula presented)

A search for pairs of light neutral pseudoscalar bosons ((Formula presented)) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at (Formula presented), collected with the CMS detector in 2016-2018 and corresponding to an integrated luminosity of (Formula presented). The (Formula presented) 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 (Formula presented) are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below (Formula presented), reaching branching fraction sensitivities as low as (Formula presented). 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 axionlike particles with masses below 100 MeV .