Searches for exclusive Higgs boson decays into D^∗γ and Z boson decays into D⁰γ and K⁰_sγ in pp collisions at √s = 13 TeV with the ATLAS detector

Searches for the exclusive decays of the Higgs boson into D^∗γ and of the Z boson into D⁰γ and K⁰_sγ can probe flavour-violating Higgs and Z boson couplings to light quarks. Searches for these decays are performed with a pp collision data sample corresponding to an integrated luminosity of 136.3 fb⁻¹ collected at √s = 13 TeV between 2016-2018 with the ATLAS detector at the CERN Large Hadron Collider. In the D^∗γ and D⁰γ channels, the observed (expected) 95% confidence-level upper limits on the respective branching fractions are B(H→D^∗γ) < 1.0(1.2) × 10⁻³, B(Z→D⁰γ) < 4.0(3.4) × 10⁻⁶, while the corresponding results in the K⁰_sγ channel are B(Z→K⁰_sγ) < 3.1(3.0) × 10⁻⁶.