  We study the decay B -> K* l+ l- in the QCD factorization approach and
propose a new integrated observable whose dependence on the form factors is
almost negligible, consequently the non--perturbative error is significantly
reduced and indeed its overall theoretical error is dominated by perturbative
scale uncertainties. The new observable we propose is the ratio between the
integrated forward--backward asymmetry in the [4,6] GeV^2 and [1,4] GeV^2
dilepton invariant mass bins. This new observable is particularly interesting
because, when compared to the location of the zero of the FBA spectrum, it is
experimentally easier to measure and its theoretical uncertainties are almost
as small; moreover it displays a very strong dependence on the phase of the
Wilson coefficient C_10 that is otherwise only accessible through complicated
CP violating asymmetries. We illustrate the new physics sensitivity of this
observable within the context of few extensions of the Standard Model, namely
the SM with four generations, an MSSM with non--vanishing source of flavor
changing neutral currents in the down squark sector and a Z' model with tree
level flavor changing couplings.
