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 $\tau$-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 $4\mathrm{GeV}<m_a<15\mathrm{GeV}$ using the $140{\mathrm{fb}}^{-1}$ of proton–proton collisions at $\sqrt{s}=13\mathrm{TeV}$ recorded with the ATLAS detector during $\mathrm{Run}2$ 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 $(\sigma \left(H\right)/{\sigma }_{\text{SM}}\left(H\right))\times B(H\to aa\to 4\tau )$ at $95\%$ confidence level are provided, ranging from 0.03 to 0.10 depending on the $a$-boson mass.