2024

Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV 𝑝𝑝 collisions with the ATLAS detector

Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Aboelela, M.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Acharya, B. S.; Ackermann, A.; Bourdarios, C. A.; Adamczyk, 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.; Tamlihat, M. A.; Aitbenchikh, B.; Aizenberg, I.; Akbiyik, M.; Åkesson, T. P. A.; Akimov, A. V.; Akiyama, D.

A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark (𝑞 = 𝑢,𝑐) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two 𝑊 bosons, two 𝑍 bosons or a 𝜏⁺𝜏⁻ pair.

It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are 𝑡𝑡̄ and 𝐻𝑡 production. For the 𝑡𝑡̄ production, one top quark decays via 𝑡 → 𝐻𝑞. The proton–proton collision data set analysed amounts to (140 fb⁻¹) at (√𝒔 = 13 TeV). No significant excess beyond Standard Model expectations is observed and upper limits are set on the 𝑡 → 𝐻𝑞 branching ratios at 95 % confidence level, amounting to observed (expected) limits of 𝓑(𝑡 → 𝐻𝑢) < 2.8(3.0) × 10⁻⁴ and 𝓑(𝑡 → 𝐻𝑐) < 3.3 (3.8) × 10⁻⁴.

Combining this search with other searches for 𝑡𝐻𝑞 flavour-changing neutral-current interactions previously conducted by ATLAS, considering 𝐻 → 𝑏𝑏̄ and 𝐻 → 𝛾𝛾 decays, as well as 𝐻 → 𝜏+𝜏−decays with one or two hadronically decaying 𝜏-leptons, yields observed (expected) upper limits on the branching ratios of 𝓑(𝑡 → 𝐻𝑢) < 2.6 (1.8) × 10⁻⁴and 𝓑(𝑡 → 𝐻𝑐) < 3.4 (2.3) × 10⁻⁴.