Low-energy matrix elements of heavy-quark currents

Abstract

In QCD at energies well below a heavy-quark threshold, the heavy-quark vector current can be represented via local operators made of the lighter quarks and of the gluon fields. We extract the leading perturbative matching coefficients for the two most important sets of operators from known results. As an application, we analytically determine the ${\rm O}(\alpha_s^3)m_c^2/m_b^2$ effect of the bottom quark current on the $R(s)$ ratio below the bottom but above the charm threshold. For the low-energy representation of the charm quark current, the two most important operators are given by the total divergence of dimension-six gluonic operators. We argue that the charm magnetic moment of the nucleon is effectively measuring the forward matrix elements of these gluonic operators and predict the corresponding bottom magnetic moment. Similarly, the contribution of the charm current to $R(s\approx 1\,{\rm GeV}^2)$, which is associated with quark-disconnected diagrams, is dominantly determined by the decay constants of the $\omega$ and $\phi$ mesons with respect to the two gluonic operators.