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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 2𝑚𝜏 < 𝑚𝑎 < 2𝑚𝑏. Results are presented in the range of 4 GeV < 𝑚𝑎 < 15 GeV using the 140 f b−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)∕𝜎SM (𝐻)) × Br(𝐻 → 𝑎𝑎 → 4𝜏) at 95 % confidence level are provided, ranging from 0.03 to 0.10 depending on the 𝑎-boson mass. 

A<sc>bstract</sc> A search for a new heavy scalar particleXdecaying into a Standard Model (SM) Higgs boson and a new singlet scalar particleSis presented. The search uses a proton-proton (pp) collision data sample with an integrated luminosity of 140 fb⁻¹recorded at a centre-of-mass energy of$$ \sqrt{s} $$ $\sqrt{s}$= 13 TeV with the ATLAS detector at the Large Hadron Collider. The most sensitive mass parameter space is explored inXmass ranging from 500 to 1500 GeV, with the correspondingSmass in the range 200–500 GeV. The search selects events with two hadronically decayingτ-lepton candidates fromH→τ⁺τ⁻decays and one or two light leptons (ℓ=e,μ) fromS→VV(V=W,Z) decays while the remainingVboson decays hadronically or to neutrinos. A multivariate discriminant based on event kinematics is used to separate the signal from the background. No excess is observed beyond the expected SM background and 95% confidence level upper limits between 72 fb and 542 fb are derived on the cross-sectionσ(pp→X→SH) assuming the same SM-Higgs boson-like decay branching ratios for theS→VVdecay. Upper limits on the visible cross-sectionsσ(pp→X→SH→WWττ) andσ(pp→X→SH→ZZττ) are also set in the ranges 3–26 fb and 6–33 fb, respectively. 

This Letter presents results from a combination of searches for Higgs boson pair production using $126–140{\mathrm{fb}}^{-1}$of proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$recorded with the ATLAS detector. At 95% confidence level (CL), the upper limit on the production rate is 2.9 times the standard model (SM) prediction, with an expected limit of 2.4 assuming no Higgs boson pair production. Constraints on the Higgs boson self-coupling modifier ${\kappa }_{\lambda }={\lambda }_{HHH}/{\lambda }_{HHH}^{\mathrm{SM}}$, and the quartic $HHVV$coupling modifier ${\kappa }_{2V}=g_{HHVV}/g_{HHVV}^{\mathrm{SM}}$, are derived individually, fixing the other parameter to its SM value. The observed 95% CL intervals are $-1.2<{\kappa }_{\lambda }<7.2$and $0.6<{\kappa }_{2V}<1.5$, respectively, while the expected intervals are $-1.6<{\kappa }_{\lambda }<7.2$and $0.4<{\kappa }_{2V}<1.6$in the SM case. Constraints obtained for several interaction parameters within Higgs effective field theory are the strongest to date, offering insights into potential deviations from SM predictions. © 2024 CERN, for the ATLAS Collaboration2024CERN 

A search for the nonresonant production of Higgs boson pairs in the $HH\to b\overline{b}{\tau }^{+}{\tau }^{-}$channel is performed using $140{\mathrm{fb}}^{-1}$of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimized to probe anomalous values of the Higgs boson self-coupling modifier ${\kappa }_{\lambda }$and of the quartic $HHVV$( $V=W,Z$) coupling modifier ${\kappa }_{2V}$. No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limit ${\mu }_{HH}<5.9\left(3.3\right)$is set at 95% confidence-level on the Higgs boson pair production cross section normalized to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of $-3.1<{\kappa }_{\lambda }<9.0$( $-2.5<{\kappa }_{\lambda }<9.3$) and $-0.5<{\kappa }_{2V}<2.7$( $-0.2<{\kappa }_{2V}<2.4$), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross sections assuming different kinematic benchmark scenarios. © 2024 CERN, for the ATLAS Collaboration2024CERN 

A bstract A search for Higgs boson pair production in events with two b -jets and two τ -leptons is presented, using a proton–proton collision dataset with an integrated luminosity of 139 fb − 1 collected at $$ \sqrt{s} $$ s = 13 TeV by the ATLAS experiment at the LHC. Higgs boson pairs produced non-resonantly or in the decay of a narrow scalar resonance in the mass range from 251 to 1600 GeV are targeted. Events in which at least one τ -lepton decays hadronically are considered, and multivariate discriminants are used to reject the backgrounds. No significant excess of events above the expected background is observed in the non-resonant search. The largest excess in the resonant search is observed at a resonance mass of 1 TeV, with a local (global) significance of 3 . 1 σ (2 . 0 σ ). Observed (expected) 95% confidence-level upper limits are set on the non-resonant Higgs boson pair-production cross-section at 4.7 (3.9) times the Standard Model prediction, assuming Standard Model kinematics, and on the resonant Higgs boson pair-production cross-section at between 21 and 900 fb (12 and 840 fb), depending on the mass of the narrow scalar resonance.