|Vcd|, |Vcs| and fD(s) from (semi-)leptonic D(s)-decays: Signals of New Physics?

We confront the recent improved measurements of the D(s) (semi-)leptonic decays with Lattice QCD (LQCD) with nf=3 flavours and QCD spectral sum rules (QSSR) predictions. D→μνμ leptonic width data compared with theoretical determinations of fD, leads to the value of the CKM mixing angle: |Vcd|=0.230(10)exp(9)th. Measured ratio of the D semi-leptonic widths combined with LQCD and QSSR predictions leads to the average |Vcd|/|Vcs|=0.2175(88), and then to |Vcs|=1.068(47). We consider the previous determinations as improvements of the existing estimates. Using the average data of the Ds→μνμ (respectively Ds→τντ) branching ratios, one obtains: |Vcs|fDsμ=(259±12)MeV (respectively |Vcs|fDsτ=(274±13)MeV), which can be compared with the average of the Standard Model (SM) values from LQCD and QSSR fDs=(240±7) MeV. If one uses the present determination of |Vcs|, there is an agreement with the SM prediction within 1σ. If instead, we impose the unitarity constraint |Vcs|⩽1, or assume (as frequently done) |Vcs|=|Vud|, we would obtain a deviation from the SM expectations ranging from 1.5 to 3σ, therefore, signaling some New Physics beyond the SM.