The semi-leptonic decay B -> pi l nu is one of the most important reactions for the determination of the CKM matrix element V_ub. However, in order to extract V_ub from data one needs an accurate theoretical calculation of the hadronic matrix element describing the B to pi transition. In collaboration with A. Khodjamirian, R. Rueckl and O. Yakovlev I calculated the perturbative O(alpha_s) correction to the light-cone QCD sum rule for the B -> pi transition form factor f^+ (hep-ph/9706303). The correction to the product f_B f^+ in leading twist approximation was found to be about 30%, that is similar in magnitude to the corresponding O(alpha_s) correction in the two-point sum rule for f_B. The resulting cancellation of large QCD corrections in f^+ eliminates one important uncertainty in the theoretical prediction for B -> pi e nu (and therefore in the extraction of |V_ub|). We further calculated the perturbative O(alpha_s) corrections to the light-cone QCD sum rule for the B^* B pi and D^* D pi coupling constants (hep-ph/9903421). Assuming vector meson dominance, these couplings determine the form factor f^+ at zero recoil. Combining all available information we obtained predictions on B -> pi l nu, D -> pi l nu and D -> K l nu (hep-ph/0001297). Together with O. Yakovlev I derived a new sum rule, which predicts the form factor in the whole kinematical range. It has the advantage that it no longer relies on vector meson dominance or similar model assumptions (hep-ph/0007317).
Talks on this subject:
Talk given by O.Yakovlev at PHENO 2000: Symposium on Phenomenology for the Nu Century, Madison, Wisconsin, April 17-19, 2000
(hep-ph/0105161).
Talk given by O.Yakovlev at the 4th Workshop on Continuous Advances in QCD, Minneapolis, May 12-14, 2000
(hep-ph/0007344).
Talk given at the UK Phenomenology Workshop on
Collider Physics, Durham, 19-24 September, 1999
(hep-ph/9911530).
Talk given by O. Yakovlev at IV Int. Workshop on Progress in Heavy Quark Physics, Rostock, September 20-22,
1997
(hep-ph/9712399).