Z→π+π?, K+K?: A touchstone of the perturbative QCD approach

摘要

We study two rare decays, Z → π + π ? and K + K ? , in the perturbative QCD approach up to the next-to-leading order of the strong coupling and the leading power of 1 / m Z , m Z being the Z boson mass. The branching ratios B ( Z → π + π ? ) = ( 0.83 ± 0.02 ± 0.02 ± 0.04 ) × 10 ? 12 and B ( Z → K + K ? ) = ( 1.7 4 ? 0.05 + 0.03 ± 0.04 ± 0.02 ) × 10 ? 12 are obtained and can be measured at a Tera- Z factory. Because the subleading-power contributions to the branching ratios are negligible, and the leading one does not depend on any free parameter, the two channels can serve as a touchstone for the applicability of the perturbative QCD approach.