2023 Articles

Search for a new heavy scalar particle decaying into a Higgs boson and a new scalar singlet in final states with one or two light leptons and a pair of 𝜏-leptons with the ATLAS detector

Aad, G.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Acharya, B. S.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S. V.; Addison, M. J.; Adelman, J.; Adiguzel, A.; Adye, T.; Affolder, A. A.; Afik, Y.; Agaras, M. N.; Agarwala, J.; Aggarwal, A.; Agheorghiesei, C.; Ahmad, A.; Ahmadov, F.; Ahmed, W. S.; Ahuja, S.; Ai, X.; Aielli, G.; Aikot, A.; Ait Tamlihat, M.; Aitbenchikh, B.; Aizenberg, I.; Akbiyik, M.; Åkesson, T. P. A.; Akimov, A. V.; Akiyama, D.; Akolkar, N. N.; Al Khoury, K.

A search for a new heavy scalar particle 𝑋 decaying into a Standard Model (SM) Higgs boson and a new singlet scalar particle 𝑆 is presented. The search uses a proton-proton (𝑝𝑝) collision data sample with an integrated luminosity of 140 fb⁻¹ recorded at a centre-of-mass energy of √𝒔 = 13 TeV with the ATLAS detector at the Large Hadron Collider. The most sensitive mass parameter space is explored in 𝑋 mass ranging from 500 to 1500 GeV, with the corresponding S mass in the range 200–500 GeV.

The search selects events with two hadronically decaying τ-lepton candidates from 𝐻 → 𝜏⁺𝜏⁻ decays and one or two light leptons (ℓ = 𝑒, 𝜇) from 𝑆 → 𝑉𝑉 (𝑉 = 𝑊, 𝑍) decays while the remaining V boson 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-section 𝜎(𝑝𝑝 → 𝑋 → 𝑆𝐻) assuming the same SM-Higgs boson-like decay branching ratios for the 𝑆 → 𝑉𝑉 decay. Upper limits on the visible cross-sections 𝜎(𝑝𝑝 → 𝑋 → 𝑆𝐻 → 𝑊𝑊𝜏𝜏) and σ(𝑝𝑝 → 𝑋 → 𝑆𝐻 → 𝑍𝑍𝜏𝜏) are also set in the ranges 3–26 fb and 6–33 fb, respectively.