Search

We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0 \to D^+ ��^{-} \bar��_��$ and $B^- \to D^0 ��^{-} \bar��_��$ relative to $\bar{B}{}^0 \to D^+ ��^{-} \bar��_��$ and $B^- \to D^0 ��^{-} \bar��_��$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{��/��}$ and $R(D^{0})^{��/��}$ can be amplified by $\sim4\%$ and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV., 5 pages, 3 figures; v3: several minor modifications, references added, published version

We systematically analyze the full angular distribution in $D \to P_1 P_2 l^+ l^-$ decays, where $P_{1,2}=��,K$, $l=e,��$. 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 $|��c| =|��u|=1$ transitions can be tested by comparing $D \to P_1 P_2 ��^+ ��^-$ to $D \to P_1 P_2 e^+ e^-$ decays. Data for $P_1 P_2=��^+ ��^-$ 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 $e$-$��$-universality. We work out beyond-SM signals model-independently and in SM extensions with leptoquarks., 18 pages plus references and appendix, 7 figures

The combined analysis of the BaBar, Belle, and LHCb data on $B\to D����$, $B\to D^*����$ and $B_c\to J/������$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $��$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\to����)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\to����), 16 pages, 6 figures; v2: clarifying comments and few references added, matches version published in PRD; v3: typo corrected in eq. (28). Updated results based on the HFLAV average for spring 2019 are available in arXiv:1905.08253

We study (semi)leptonic rare charm decays and its opportunities in searches for physics beyond the Standard Model (BSM). In particular, we analyse the impact of potential BSM physics in $c\to ull'$ transitions, notably branching ratios, angular observables, asymmetries and Lepton Flavour Violating (LFV) decays. Testable effects are worked out model-independently and within Leptoquark models supplemented with flavour patterns to link $K$/$B$ decays., Comment: To be published in the proceedings of 51st Recontres de Moriond EW 2016, La Thuile, Italy, 12-19 Mar 2016

An analysis of charm mesons decaying semileptonically via Flavor Changing Neutral Currents is presented. We calculate the Wilson coefficients within the Standard Model. A window in the decay distribution, where physics beyond the Standard Model could be measured is identified. Exemplary, we study effects of leptoquark models., Comment: To be published in the proceedings of CHARM-2015, Detroit, MI, 18-22 May 2015