Search for Higgs boson decays into a 𝑍� boson and a light hadronically decaying resonance in 𝑝�𝑝� collisions at SQUARE ROOT𝑠�=13 TeV with the ATLAS detector

Abstract

A search for decays of the Higgs boson into a 𝑍 boson and a light resonance, with a mass of 0.5-3.5 GeV, isperformed using the full 140 fb-1 dataset of 13 TeV proton-proton collisions recorded by the ATLAS detectorduring LHC Run 2. Leptonic decays of the 𝑍 boson and hadronic decays of the light resonance are considered.The resonance can be interpreted as a 𝐽DIVISION SLASH𝜓 or 𝜂𝑐 meson, an axion-like particle, or a light pseudoscalar predictedin two-Higgs-doublet models. Due to its low mass, this resonance is produced with a high Lorentz boost in thelaboratory frame and therefore reconstructed as a single small-radius jet of hadrons. A neural network is usedto correct the Monte Carlo simulation of the total expected background using data from sideband regions. Twoadditional neural networks are used to distinguish signal from background, enhancing the purity of the signalregion. A binned profile-likelihood fit is performed on the final-state invariant mass distribution. No significantexcess of events relative to the expected background is observed, and upper limits at 95% confidence level areset on the Higgs boson's branching fraction to a 𝑍 boson and a light resonance. The exclusion limit is TILDE OPERATOR+D9110% forthe lower masses, and increases for higher masses. Upper limits on the effective coupling 𝐶eff𝑍𝐻DIVISION SLASHΛ of an axion-likeparticle to a Higgs boson and 𝑍 boson are also set at 95% confidence level, and range from 0.9 to 2 TeV-1.