Comment on form-factor shape and extraction of |Vub| from B → πlν

Thomas Becher, Richard J. Hill

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179 Scopus citations


We point out that current experimental data for partial B→πlν branching fractions reduce the theoretical input required for a precise extraction of |Vub| to the form-factor normalization at a single value of the pion energy. We show that the heavy-quark expansion provides a bound on the form-factor shape that is orders of magnitude more stringent than conventional unitarity bounds. We find |Vub|=(3.7±0. 2±0.1)×[0.8/F+(16 GeV2)]. The first error is from the experimental branching fractions, and the second is a conservative bound on the residual form-factor shape uncertainty, both of which will improve with additional data. Together with current and future lattice determinations of the form-factor normalization this result gives an accurate, model independent determination of |Vub|. We further extract semileptonic shape observables such as |VubF+(0)|=0.92±0.11±0. 03 and show how these observables can be used to test factorization and to determine low-energy parameters in hadronic B decays.

Original languageEnglish
Pages (from-to)61-69
Number of pages9
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
Issue number1
StatePublished - Feb 2 2006

Bibliographical note

Funding Information:
We thank A. Kronfeld, H. Quinn and I. Stewart for discussions. We are grateful to the Institute for Nuclear Theory (Seattle, WA) for hospitality where a part of this work was completed. Research supported by the Department of Energy under Grants DE-AC02-76SF00515 and DE-AC02-76CH03000. Fermilab is operated by Universities Research Association Inc., under contract with the US Department of Energy.

ASJC Scopus subject areas

  • Nuclear and High Energy Physics


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