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Ulrich Nierste
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I discuss hadronic decays of $D$ mesons with emphasis on the recent discovery of charm CP violation in $D^0to K^+K^-,pi^+pi^-$ decays. The measured difference $Delta a_{CP} , equiv ; a_{CP}^{mathrm{dir}}(D^0rightarrow K^+K^-) - a_{CP}^{mathrm{dir}}(D^0rightarrowpi^+pi^-) = ; (-15.4pm 2.9)cdot 10^{-4}$ of two direct CP asymmetries exceeds the SM prediction by a factor of 7. A possible explanation is an enhancement of the penguin amplitude entering $ a_{CP}^{mathrm{dir}}$ by QCD effects which are not understood yet. Alternatively, $Delta a_{CP}$ could be dominated by contributions from new physics. In order to distinguish these two hypotheses further CP asymmetries should be measured. To this end CP asymmetries resulting from the interference of two tree-level amplitudes auch as $a_{CP}^{mathrm{dir}}(D^0rightarrow K_SK_S)$ or $a_{CP}^{mathrm{dir}}(D^0 to K^{*0} K_S)$ are especially interesting.
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Proceedings of the CKM 2005 Workshop (WG5), UC San Diego, 15-18 March 2005.
We give arguments in favor of the compatibility with standard physics of some large nonleptonic branching fractions in Cabibbo--forbidden $D^+$ decays, contrary to a recent claim in the literature.
A global previous analysis of two-body nonleptonic decays of $D$ mesons has been extended to the decays involving light scalar mesons. The allowance for final state interaction also in nonresonant channels provides a fit of much improved quality and with less symmetry breaking in the axial charges. We give predictions for about 50 decay branching ratios yet to be measured. We also discuss long distance contributions to the difference $Delta Gamma$ between the $D_S$ and $D_L$ widths.
Indirect searches, and in particular rare decays, have proven to be a fruitful field to search for New Physics beyond the Standard Model. While the down-quark sector (B and K) have been studied in detail, less attention was devoted to charm decays due to the smaller expected values and higher theoretical uncertainties of their observables. Recently a renewed interest is growing in rare charm searches. In this article we review the current experimental status of searches for rare decays in charmed hadrons. While the Standard Model rates are yet to be reached, current experimental limits are already putting constraints on New Physics models.
We investigate the possibility that scalar leptoquarks generate consequential effects on the flavor-changing neutral-current decays of charmed hadrons into final states with missing energy ($ ot!!E$) carried away by either standard model or sterile neutrinos. We focus on scenarios involving the $R_2$, $tilde R_2$, and $bar S_1$ leptoquarks and take into account various pertinent constraints, learning that meson-mixing ones and those inferred from collider searches can be of significance. We find in particular that the branching fractions of charmed meson decays $Dto M! ot!!E$, $M=pi,rho$, and $D_sto K^{(*)}! ot!!E$ and singly charmed baryon decays $Lambda_c^+to p! ot!!E$ and $Xi_ctoSigma! ot!!E$ are presently allowed to attain the $10^{-7}$-$10^{-6}$ levels if induced by $R_2$ and that the impact of $tilde R_2$ is comparatively much less. In contrast, the contributions of $bar S_1$, which couples to right-handed up-type quarks and the sterile neutrinos, could lead to branching fractions as high as order $10^{-3}$. This suggests that these charmed hadron decays might be within reach of the BESIII and Belle II experiments or future super charm-tau factories and could serve as potentially promising probes of leptoquark interactions with sterile neutrinos.
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