Search for Higgs boson exotic decays into Lorentz-boosted light bosons in the four-τ final state at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si21.svg"> <mml:mrow> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo linebreak="goodbreak">=</mml:mo> <mml:mn>13</mml:mn> <mml:mspace width="0.16em"/> <mml:mtext>TeV</mml:mtext> </mml:mrow> </mml:math> 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 4 GeV < 𝑚 𝑎 < 15 GeV using the 140 fb -1 of proton-proton collisions at √ 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-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)) × (𝐻 → 𝑎𝑎 → 4𝜏) at 95 % confidence level are provided, ranging from 0.03 to 0.10 depending on the 𝑎-boson mass.ferred decays is 𝑎 → 𝜏 + 𝜏 -.Due to the low mass of the 𝑎-boson, this decay is Lorentz-boosted and the two 𝜏-leptons overlap in the detector.This search focuses on the case in which, for each 𝑎-boson, one of the 𝜏-leptons decays to a muon 𝜏 → 𝜇𝜈 𝜇 𝜈 𝜏 and the other decays to hadrons 𝜏 → hadrons + 𝜈 𝜏 , referred to as 𝜏 had .In this decay mode, the muon can be identified close to the hadrons and, once removed from the hadron reconstruction cone, it does not impact the identification of the hadronically decaying 𝜏-lepton.This paper presents a search for events with two di-𝜏 pairs identified through a muon-removal technique, consistent with a decay 𝐻 → 𝑎𝑎 → 𝜏 + 𝜏 -𝜏 + 𝜏 -in the mass range 4 GeV < m a < 15 GeV.When 𝑚 𝑎 is higher, the two 𝜏-leptons do not overlap, requiring a different analysis strategy.The CMS Collaboration has previously searched for exotic Higgs boson decays 𝐻 → 𝑎𝑎 → 𝜏 + 𝜏 -𝜏 + 𝜏 -using only tracks to identify the hadronically decaying 𝜏-leptons [20].The ATLAS Collaboration has also searched for exotic Higgs boson decays in the 𝐻 → 𝑎𝑎 → 𝜇 + 𝜇 -𝜏 + 𝜏 -decay channel using only tracks to identify the 𝜏-lepton decays [21].The muon-removal technique allows an improved detection of the hadronically decaying 𝜏-leptons using both tracks and calorimeter information, as described in Ref. [22] and is used for the first time by the ATLAS Collaboration in this paper.A similar muon-removal reconstruction has also been used by the CMS Collaboration in the search for exotic Higgs boson decays in the 𝐻 → 𝑎𝑎 → 𝜇 + 𝜇 -𝜏 + 𝜏 -decay channel [23].This paper complements many previous searches for exotic Higgs boson decays 𝐻 → 𝑎𝑎 conducted by the ATLAS and CMS Collaborations.In addition to the results mentioned above, searches are performed in the 𝓁 + 𝓁 -𝓁 + 𝓁 -[24,25] (where 𝓁 is an electron or a muon), 𝑏 b𝜏 + 𝜏 -[26,

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