Searches for exclusive Higgs boson decays into D^⁎γ and Z boson decays into D⁰γ and K_s ⁰γ in pp collisions at s=13TeV with the ATLAS detector

Searches for exclusive decays of the Higgs boson into D^⁎γ and of the Z boson into D⁰γ and K_s ⁰γ can probe flavour-violating Higgs boson 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=13TeV 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 K_s ⁰γ channel are B(Z→K_s ⁰γ)<3.1(3.0)×10⁻⁶.