Subleading Shape Functions and the Determination of |V_{ub}|

Abstract

It is argued that the dominant subleading shape-function contributions to the endpoint region of the charged-lepton energy spectrum in B -> X_u l nu decays can be related in a model-independent way to an integral over the B -> X_s gamma photon spectrum. The square root of the fraction of B -> X_u l nu events with charged-lepton energy above E_0=2.2 GeV can be calculated with a residual theoretical uncertainty from subleading shape-function effects that it safely below the 10% level. These effects have therefore a minor impact on the determination of |V_{ub}|.

8 pages, 1 figure. Final version to appear in Physics Letters B. By a smarter choice of weight function, m_b and lambda_1 have been eliminated from the final results. A paragraph has been added mentioning the potentially sizeable uncertainty due to duality violations and spectator-dependent effects. The discussion of the results obtained in refs. [7,8] has been made more accurate