We present a systematic discussion of ${\Lambda }_b\to \Lambda$ transition form factors in the framework of soft-collinear effective theory (SCET). The universal soft form factor, which enters the symmetry relations in the limit of large recoil energy, is calculated from a sum-rule analysis of a suitable SCET correlation function. The same method is applied to derive the leading corrections from hard-collinear gluon exchange at first order in the strong coupling constant. We present numerical estimates for form factors and form-factor ratios and their impact on decay observables in ${\Lambda }_b\to \Lambda {\mu }^{+}{\mu }^{-}$ decays.

• Received 17 November 2011

DOI:https://doi.org/10.1103/PhysRevD.85.014035