ИСТИНА

We present a study of the strong isospin-breaking (IB) effect, due in QCD to the difference between u- and d-quark masses, in the leptonic decay constants of charmed and beauty pseudoscalar and vector mesons using the method of QCD sum rules. We apply the sum-rule analysis to the decay constants of mesons containing one heavy quark and one light quark with the light mass in the range from the average u/d quark mass to the strange-quark mass. We then analyse the dependence of the decay constants on the light-quark mass and extract with good accuracy the IB ratios of decay constants at leading order in the mass difference (md−mu), obtaining: (fD+−fD0)/fD=0.0046(6), (fD∗+−fD∗0)/fD∗=0.0067(9), (fB0−fB+)/fB=0.0048(6), and (fB∗0−fB∗+)/fB∗=0.0045(5), which yield: fD+−fD0=0.95±0.13 MeV, fD∗+−fD∗0=1.69±0.27 MeV, fB0−fB+=0.92±0.13 MeV, fB∗0−fB∗+=0.82±0.11 MeV. In the case of the D-meson our finding is consistent with recent lattice QCD results, whereas it is much lower in the case of the B-meson showing a tension of ≈3 standard deviations.