Heavy pseudoscalar leading-twist distribution amplitudes within QCD theory in background fields

In this paper, we study the leading-twist distribution amplitude (DA) of the heavy pseudoscalars (HPs), such as eta(c), eta(b), and B-c, within the QCD theory in the background fields. New sum rules up to dimension-six condensates for both the HP decay constants and their leading-twist DA moments are presented. From the sum rules for the HP decay constants, we obtain f(eta c) = 453 +/- 4 MeV, f(Bc) = 498 +/- 14 MeV, and f(eta b) = 811 +/- 34 MeV. Based on the sum rules for the HPs' leading-twist DA moments, we construct a new model for the eta(c), eta(b), and B-c leading-twist DAs. Our present HP DA model is also adaptable for the light pseudoscalar DAs, such as the pion and kaon DAs. Thus, it shall be applicable for a wide range of QCD exclusive processes. As an application, we apply the eta(c) leading-twist DA to calculate the B-c -> eta(c) transition form factor f(+)(Bc ->eta c) (q(2)). At the maximum recoil region, we obtain f(+)(Bc ->eta c) (0) = 0.612(-0.052)(+0.053). Furthermore we predict the branching ratio for the semi-leptonic decay B-c -> eta(c)l nu and obtain B(B-c -> eta(c)l nu) = (7.70(-1.48)(+1.65)) x 10(-3) for massless leptons, which is consistent with the light-cone sum rules estimation obtained in the literature.