We report on a relation between the decay constants of \rho-like J^{ PC}=1^{ --} vector mesons, which arises solely from the perturbative analysis of the VV, TT and VT correlators at order \alpha_s^0 in the large-N_c limit. We find f_{ V}^T/f_{ V}=1/\sqrt{ 2} for highly excited states together with a pattern of alternation in sign. Quite remarkably, recent lattice determinations reported f_{ \rho}^T/f_{ \rho}=0.72(2), in excellent agreement with our large-N_c result. This seems to suggest a pattern like f_{ Vn}^T/f_{ Vn}=(-1)^n/\sqrt{ 2} for the whole (1^{ --}) states. In order to test this conjecture in real QCD we construct a set of spectral sum rules, which turn out to comply nicely with this scenario.
Categories: hep-ph