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, $H\to aa\to {\tau }^{+}{\tau }^{-}{\tau }^{+}{\tau }^{-}$, 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 $2m_{\tau }<m_a<2m_b$. Results are presented in the range of $4\mathrm{G}\mathrm{e}\mathrm{V}<m_a<15\mathrm{G}\mathrm{e}\mathrm{V}$ using the $140{\mathrm{f}\mathrm{b}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 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 $a\to {\tau }^{+}{\tau }^{-}$ 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 $(\sigma (H)\mathrm{/}{\sigma }_{\mathrm{S}\mathrm{M}}(H))\times \mathcal{B}(H\to aa\to 4\tau )$ at $95\mathrm{\%}$ confidence level are provided, ranging from $0.03$ to $0.10$ depending on the $a$-boson mass.