Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four- <italic>&#964;</italic> final state at s=13TeV with the ATLAS detector

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into τ-leptons, H→aa→τ+τ−τ+τ−, is presented. In models with Yukawa-like couplings, the decay to τ-leptons is favoured for light a-bosons, with mass in the range of 2 mτ <  ma < 2 mb. Results are presented in the range of 4 GeV <  ma < 15 GeV using the 140fb−1 of proton–proton collisions at s=13TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. This search focuses on the scenario where, for both di- τ pairs, one of the τ-leptons decays to hadrons and neutrinos, while the other decays to a muon and neutrinos. In this mass range, the a→τ+τ− is Lorentz-boosted and a dedicated muon removal technique is used to reconstruct the di- τ pairs. No significant excess above the Standard Model background prediction is observed. Upper limits on (σ(H)/σSM(H))×B(H→aa→4τ) at 95 % confidence level are provided, ranging from 0.03 to 0.10 depending on the a-boson mass.