Determination of the flavor-singlet form factor from B \to \eta^{(\prime)} l \nu decays

We study semileptonic decays B \to \eta^{(\prime)} l \nu in order to determine the flavor-singlet contribution (F^{\rm singlet}_+) to the B \to \eta^{(\prime)} form factors, which arises from the two-gluon emission in a decaying B meson. It has been recently pointed out that the unknown value of F^{\rm singlet}_+ prevents accurate theoretical estimates in the analysis of B \to \eta^\prime K decays in QCD factorization. We present certain methods to determine F^{\rm singlet}_+, using B \to \eta^{(\prime)} l \nu and B \to \pi l \nu decays. The uncertainty in determination of F^{\rm singlet}_+ can be significantly reduced, as the more accurate value of the B \to \pi form factor or |V_{ub}| become available through future studies