The photon polarization in radiative DD decays, phenomenologically

We work out the phenomenology of untagged time-dependent analysis with radiative $D^0$-decays into CP eigenstates, which allows to probe the photon polarization by means of the charm mesons' finite width difference. We show that $D^0 \to \phi \gamma$ or $D^0 \to \bar K^{0 *} \gamma$ decays, which are SM-dominated, or SM-like, respectively, together with U-spin allow to obtain chirality-predictions for radiative decay amplitudes. The order of magnitude of wrong-chirality contributions in the SM can be cross-checked with an up-down asymmetry in $D^0 \to \bar K_1^0 (\to \bar K \pi \pi) \gamma$. We explore the sensitivity to new physics in $|\Delta c|=|\Delta u|=1$ dipole couplings in the decays $D^0 \to \rho^0 \gamma$. We point out the possibility to test the SM with $D_s \to K_1^+( \to K \pi \pi) \gamma$ decays.Comment: 15 pages plus references and appendices, 7 figure

Null tests from angular distributions in D→P1P2l+l−D \to P_1 P_2 l^+l^-, l=e,μl=e,\mu decays on and off peak

We systematically analyze the full angular distribution in $D \to P_1 P_2 l^+ l^-$ decays, where $P_{1,2}=\pi,K$, $l=e,\mu$. We identify several null tests of the standard model (SM). Notably, the angular coefficients $I_{5,6,7}$, driven by the leptons' axial-vector coupling $C_{10}^{(\prime)}$, vanish by means of a superior GIM-cancellation and are protected by parity invariance below the weak scale. CP-odd observables related to the angular coefficients $I_{5,6,8,9}$ allow to measure CP-asymmetries without $D$-tagging. The corresponding observables $A_{5,6,8,9}$ constitute null tests of the SM. Lepton universality in $|\Delta c| =|\Delta u|=1$ transitions can be tested by comparing $D \to P_1 P_2 \mu^+ \mu^-$ to $D \to P_1 P_2 e^+ e^-$ decays. Data for $P_1 P_2=\pi^+ \pi^-$ and $K^+ K^-$ on muon modes are available from LHCb and on electron modes from BESIII. Corresponding ratios of dimuon to dielectron branching fractions are at least about an order of magnitude away from probing the SM. In the future electron and muon measurements should be made available for the same cuts as corresponding ratios $R_{P_1 P_2}^D$ provide null tests of $e$-$\mu$-universality. We work out beyond-SM signals model-independently and in SM extensions with leptoquarks.Comment: 18 pages plus references and appendix, 7 figure