Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four-tau final state at sqrt(s) = 13 TeV with the ATLAS detector

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into tau-leptons, H->aa->tau+tau-tau+tau-, is presented. In models with Yukawa-like couplings, the decay to tau-leptons is favoured for light a-bosons, with mass in the range of 2m_tau < m_a < 2 m_b. Results are presented in the range of 4 GeV < m_a < 15 GeV using the 140 fb-1 of proton-proton collisions at sqrt(s) = 13TeV recorded with the ATLAS detector during Run2 of the Large Hadron Collider. This search focuses on the scenario where, for both di-tau pairs, one of the tau-leptons decays to hadrons and neutrinos, while the other decays to a muon and neutrinos. In this mass range, the a -> tau+tau- is Lorentz-boosted and a dedicated muon removal technique is used to reconstruct the di-tau pairs. No significant excess above the Standard Model background prediction is observed. Upper limits on (sigma(H)/sigma_SM(H)) x BR(H->aa->4tau) at 95% confidence level are provided, ranging from 0.03 to 0.10 depending on the a-boson mass.