Search for Higgs boson decays into a $Z$ boson and a light hadronically decaying resonance in 140 fb$^{-1}$ of 13 TeV $p$$p$ collisions with the ATLAS detector

The ATLAS collaboration
Aad, Georges , Aakvaag, Erlend , Abbott, Braden Keim , Abdelhameed, Sara , Abeling, Kira , Abicht, Nils Julius , Abidi, Haider , Aboelela, Mohammed , Aboulhorma, Asmaa , Abramowicz, Halina
CERN-EP-2024-261, 2024.

Abstract (data abstract)
A search for decays of the Higgs boson into a $Z$ boson and a light resonance, with a mass of 0.5-4 GeV, is performed using the full 140 fb$^{-1}$ dataset of 13 TeV proton-proton collisions recorded by the ATLAS detector during Run 2 of the LHC. Leptonic decays of the $Z$ boson and hadronic decays of the light resonance are considered. The resonance can be interpreted as a $J/\psi$ or $\eta_c$ meson, an axion-like particle, or a light pseudoscalar in two-Higgs-doublet models. Due to its low mass, it would be produced with high boost and reconstructed as a single small-radius jet of hadrons. A neural network is used to correct the Monte Carlo simulation of the background in a data-driven way. Two additional neural networks are used to distinguish signal from background. A binned profile-likelihood fit is performed on the final-state invariant mass distribution. Since no significant excess of events relative to the expected background is observed, 95% CL upper limits are set on the Higgs boson's branching fraction to a $Z$ boson and a light resonance.

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