2023 Articles

Search for flavour-changing neutral 𝑡𝑞𝐻 interactions with 𝐻 → 𝛾𝛾 in 𝑝𝑝 collisions at √𝒔 = 13 TeV using 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.; 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.; Alberghi, G. L.

A search for flavour-changing neutral interactions involving the top quark, the Higgs boson and an up-type quark 𝑞 (𝑞 = 𝑐, 𝑢) is presented. The proton-proton collision data set used, with an integrated luminosity of 139 fb⁻¹, was collected at √𝒔 = 13 TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process 𝑡 → 𝑞𝐻 in 𝑡𝑡̄ production and the production process 𝑝𝑝 → 𝑡𝐻, with the Higgs boson decaying into two photons, are investigated.

No significant excess is observed and upper limits are set on the 𝑡 → 𝑐𝐻 and the 𝑡 → 𝑢𝐻 branching ratios of 4.3 × 10⁻⁴ and 3.8 × 10⁻⁴, respectively, at the 95% confidence level, while the expected limits in the absence of signal are 4.7 × 10⁻⁴ and 3.9 × 10⁻⁴. Combining this search with ATLAS searches in the 𝐻 → 𝜏⁺𝜏⁻ and 𝐻 → 𝑏𝑏̄ final states yields observed (expected) upper limits on the 𝑡 → 𝑐𝐻 branching ratio of 5.8 × 10⁻⁴ (3.0 × 10⁻⁴) at the 95% confidence level. The corresponding observed (expected) upper limit on the 𝑡 → 𝑢𝐻 branching ratio is 4.0 × 10⁻⁴ (2.4 × 10⁻⁴).