Radiative three-body D-meson decays in and beyond the standard model

We study radiative charm decays $$D \rightarrow P_1 P_2 \gamma $$ , $$P_{1,2}=\pi ,K$$ in QCD factorization at leading order and within heavy hadron chiral perturbation theory. Branching ratios including resonance contributions are around $$\sim 10^{-3}$$ for the Cabibbo-favored modes into $$K \pi \gamma $$ and $$\sim 10^{-5}$$ for the singly Cabibbo-suppressed modes into $$\pi ^+ \pi ^- \gamma , K^+ K^- \gamma $$ , and thus in reach of the flavor factories BES III and Belle II. Dalitz plots and forward–backward asymmetries reveal significant differences between the two QCD frameworks; such observables are therefore ideally suited for a data-driven identification of relevant decay mechanisms in the standard-model dominated $$D \rightarrow K \pi \gamma $$ decays. This increases the potential to probe new physics with the $$D \rightarrow \pi ^+ \pi ^- \gamma $$ and $$D \rightarrow K^+ K^- \gamma $$ decays, which are sensitive to enhanced dipole operators. CP asymmetries are useful to test the SM and look for new physics in neutral $$|\Delta C|=1$$ transitions. Cuts in the Dalitz plot enhance the sensitivity to new physics due to the presence of both s- and t, u-channel intermediate resonances.