is the smallest and least known of all CKM matrix elements; it's currently determined primarily through the exclusive process B → π l 𝜈. I will present progress toward a lattice QCD determination of the matrix element from a novel transition -- the B → π π l 𝜈 process, where the π π system is in a P-wave and features the ρ(770) resonance as an enhancement. The calculation is performed an ensemble with isotropic clover fermions wiht fm and a pion mass of 320 MeV; for the b-quark we use the anisotropic clover action. After a brief overview of the theoretical framework, I will discuss some preliminary results.
