Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four-τ 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 2m_τ<m_a<2m_b. Results are presented in the range of 4GeV<m_a<15GeV using the 140fb⁻¹ of proton–proton collisions at s=13TeV recorded with the ATLAS detector during Run2 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)/σ_SM(H))×B(H→aa→4τ) at 95% confidence level are provided, ranging from 0.03 to 0.10 depending on the a-boson mass.