Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four-𝜏� final state at √𝑠� = 13 TeV with the ATLAS detector

Abstract

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into𝜏-leptons, 𝐻 → 𝑎𝑎 → 𝜏+𝜏−𝜏+𝜏−, is presented. In models with Yukawa-like couplings, the decay to 𝜏-leptons isfavoured for light 𝑎-bosons, with mass in the range of 2𝑚𝜏 < 𝑚𝑎 < 2𝑚𝑏. Results are presented in the range of4 GeV < 𝑚𝑎 < 15 GeV using the 140 fb−1 of proton–proton collisions at √𝑠 = 13 TeV recorded with the ATLASdetector during Run 2 of the Large Hadron Collider. This search focuses on the scenario where, for both di-𝜏pairs, one of the 𝜏-leptons decays to hadrons and neutrinos, while the other decays to a muon and neutrinos. Inthis mass range, the 𝑎 → 𝜏+𝜏− is Lorentz-boosted and a dedicated muon removal technique is used to reconstructthe di-𝜏 pairs. No significant excess above the Standard Model background prediction is observed. Upper limitson (𝜎(𝐻)∕𝜎SM(𝐻)) × (𝐻 → 𝑎𝑎 → 4𝜏) at 95% confidence level are provided, ranging from 0.03 to 0.10 dependingon the 𝑎-boson mass.