Isospin breaking in heavy-meson decay constants

Evaluation of Borelized QCD sum rules in the so-called local-duality limit of infinitely large Borel mass parameter provides an alternate route for extraction of the dependence of the decay constants of heavy–light mesons on the mass m_q of the involved light quark q: For appropriate choices of the two-point correlation functions of currents interpolating the hadrons under study, the local-duality limit forces all nonperturbative contributions parametrized by vacuum condensates to such kind of correlator to vanish. As a consequence, the sought m_q dependence of the heavy–light meson decay constants proves to be controlled primarily by the correlator contributions from perturbative QCD. Our knowledge of the analytic behaviour of the latter as functions of m_q enables us to derive the m_q dependence of the decay constants of both pseudoscalar and vector heavy–light mesons, for which we estimate strong isospin breaking to be of the order of 1 MeV for both charm and beauty sectors.