Gluonic contribution to B→η(′) form factors

We calculate the flavor-singlet contribution to the B ->eta((')) transition form factors from the gluonic content of the eta((')) meson in the large-recoil region using the perturbative QCD approach. The formulation for the eta-eta(') mixing in the quark-flavor and singlet-octet schemes is compared, and employed to determine the chiral enhancement scales associated with the two-parton twist-3 eta((')) meson distribution amplitudes. It is found that the gluonic contribution is negligible (around 5%) in the B ->eta form factors, but reaches 10%-40% in the B ->eta(') ones. Its impact on the accommodation of the measured B ->eta K-(') branching ratios in the perturbative QCD and QCD-improved factorization approaches is elaborated.