Search

The combined analysis of the BaBar, Belle, and LHCb data on $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $B_c\to J/\Psi\tau\nu$ 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 $\tau$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\to\tau\nu)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\to\tau\nu)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\to\tau\nu)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $\Lambda_b\to \Lambda_c\tau\nu$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(\Lambda_b\to \Lambda_c\tau\nu)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data., Comment: 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

The importance of charm decays in the context of flavor is motivated. A theory overview on recent developments in leptonic and semileptonic decays, hadronic two-body decays, and rare decays is presented. Standard Model (SM) predictions as well as tests of the SM are pointed out, {\it e.g.}~how to discover direct CP violation in charm decays and how to search for physics beyond the SM. Differences between rare charm and beauty decays are discussed., Comment: To be published in the proceedings of The International Conference on B-Physics at Frontier Machines - BEAUTY2018, La Biodola, Elba Island, Italy, 6-11 May, 2018

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 figures

We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0 \to D^+ \tau^{-} \bar{\nu}_{\tau}$ and $B^- \to D^0 \tau^{-} \bar{\nu}_{\tau}$ relative to $\bar{B}{}^0 \to D^+ \mu^{-} \bar{\nu}_{\mu}$ and $B^- \to D^0 \mu^{-} \bar{\nu}_{\mu}$, 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 $R(D^{+})^{\tau/\mu}$ and $R(D^{0})^{\tau/\mu}$ can be amplified by $\sim4\%$ and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV., Comment: 5 pages, 3 figures; v3: several minor modifications, references added, published version

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 figures

Motivated by the recent measurement of $D^0\to\rho^0\gamma$ improved standard model (SM) predictions for branching ratios and CP asymmetries of radiative charm decay are given. Weak annihilation induced decays are probes of non-perturbative QCD approaches. Rare decays probe the SM and physics beyond the SM, e.g. leptoquark and supersymmetric models. Opportunities with $\Lambda_c\to p\gamma$ for future polarization measurements are presented., Comment: To be published in the proceedings of The European Physical Society Conference on High Energy Physics, Venice, Italy, 5-12 Jul 2017

Non-factorizable two loop corrections to heavy to light flavor changing neutral current transitions due to matrix elements of current-current operators are calculated analytically for arbitrary momentum transfer. This extends previous works on $b\to(d,s)\ell^+\ell^-$ transitions. New results for $c\to u\ell^+\ell^-$ transitions are presented. Recent works on polylogarithms are used for the master integrals. For $b\to s\ell^+\ell^-$ transitions, the corrections are most significant for the imaginary parts of the effective Wilson coefficients in the large hadronic recoil range. Analytical results and ready-to-use fitted results for a specific set of parameters are provided., Comment: 14 pages plus references and appendix. The results are supplemented to the source files of this paper; v2: typos corrected and clarifications and references added, supplemented files (+ analytical results in terms of GPLs) and description thereof improved, results unchanged; v3: files and text (fitted results for negative momentum transfer and factorizable results) added, results unchanged

We present standard model (SM) estimates for exclusive $c \to u \gamma$ processes in heavy quark and hybrid frameworks. Measured branching ratios ${\cal{B}}(D^0 \to (\phi, \bar K^{*0}) \gamma)$ are at or somewhat exceeding the upper range of the SM and suggest slow convergence of the $1/m_D, \alpha_s$-expansion. Model-independent constraints on $|\Delta C|=|\Delta U|=1$ dipole operators from ${\cal{B}}(D^0 \to \rho^0 \gamma)$ data are obtained. Predictions and implications for leptoquark models are worked out. While branching ratios are SM-like CP asymmetries $\lesssim 10 \%$ can be induced. In SUSY deviations from the SM can be even larger with CP asymmetries of $O(0.1)$. If $\Lambda_c$-baryons are produced polarized, such as at the $Z$-pole, an angular asymmetry in $\Lambda_c \to p \gamma$ decays can be studied that is sensitive to chirality-flipped contributions., Comment: 23 pages plus references and appendices, 7 figures; v2: clarifications and references added, conclusions unchanged; v3: In section IV.B on leptoquark models matching corrected and subsequent text, tables and equations adapted. Phenomenological results and conclusions unchanged

The standard theoretical framework to deal with weak decays of heavy mesons is the so-called weak effective Hamiltonian. It involves the short-distance Wilson coefficients, which depend on the renormalisation scale $\mu$. For specific calculations one has to evolve the Wilson coefficients down from the electroweak scale $\mu=M_W$ to the typical mass scale of the decay under consideration. This is done by solving a renormalisation group equation for the effective operator basis. In this paper the results of a consistent two-step running of the $c \to u \,\ell^+\ell^-$ Wilson coefficients for dimension-6 operators are presented. This running involves the intermediate scale $\mu=m_b$ (with $M_W > m_b > m_c$) where the bottom quark is integrated out. The matching coefficients and anomalous dimensions are taken to the required order by generalizing and extending results from $b \to s$ or $s \to d$ transitions available in the literature., Comment: 13 pages, 1 figure

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

Updated standard model predictions for $D_{(s)} \to P l^+ l^-$, $l=e,\mu$, $P=\pi,K$ and inclusive decays are presented. Model-independent constraints on $|\Delta C|=|\Delta U|=1$ Wilson coefficients are worked out. New physics (NP) opportunities arise in semileptonic branching ratios for very large couplings only, however, are not excluded outside the resonance regions yet. The NP potential of resonance-assisted CP asymmetries and angular observables is worked out. Predictions are given for leptoquark models, and include lepton flavor violating and dineutrino decays. Whether NP can be seen depends on flavor patterns, and vice versa., Comment: 19 pages plus references and appendices, 5 figures; v2: $\eta^{(\prime)}$ contribution fixed, references added, conclusion unchanged; v3: typos corrected and clarifications and references added, conclusions unchanged