Discerning new physics in charm meson leptonic and semileptonic decays

TL;DR

This paper investigates potential new physics effects in charm meson leptonic and semileptonic decays, using recent lattice calculations and effective theory, to identify possible deviations from the Standard Model in future experiments.

Contribution

It revisits constraints on new physics in charm meson decays, focusing on differential distributions and asymmetries, and identifies observables sensitive to scalar Wilson coefficients and lepton flavor universality violation.

Findings

Current constraints are weak but future measurements could detect deviations.

The ratio R_{μ/e}(q^2) can vary between 0.9 and 1.2, allowing for potential lepton flavor universality violation.

Deviations from the Standard Model in the differential decay rate are limited to a few percent.

Abstract

Current experimental information on the charm meson decay observables in which the $c\to s\ell\nu_\ell$ transitions occur is well compatible with the Standard Model predictions. Recent precise lattice calculations of the $D_s$ meson decay constant and form factors in $D\to K\ell\nu$ decays offer a possibility to search for the small deviations from the Standard Model predictions in the next generation of the high intensity flavour experiments. We revisit constraints from these processes on the new physics contributions in the effective theory approach. We investigate new physics effects which might appear in the differential distributions for the longitudinally and transversely polarised $K^\ast$ in $D\to K^\ast\ell\nu_\ell$ decays. Present constraints from these observables are rather weak, but could be used to constrain new physics effects in the future. In the case of $D\to K\ell\nu$ we identify observables sensitive on new physics contribution coming from the scalar Wilson coefficient, namely the forward-backward and the transversal muon asymmetries. By allowing that new physics modifies only the second lepton generation but not the first one, we identify allowed region for the differential decay rate for the process $D\to K\mu\nu_\mu$ and find that it is allowed to deviate from the Standard Model prediction by only few percent. The lepton flavour universality violation can be tested in the ratio $R_{\mu/e}(q^2)\equiv \frac{d\Gamma^{(\mu)}}{dq^2}/\frac{d\Gamma^{(e)}}{dq^2}$. If the first lepton generation behaves as in the Standard Model, we find, using current constraint on the scalar Wilson coefficient, that the ratio $R_{\mu/e}(q^2)$ is currently allowed to be within the range $(0.9, 1.2)$, depending on the value of $q^2$.