An improved observable for the forward-backward asymmetry in B -> K* l+ l- and Bs -> phi l+ l-

TL;DR

This paper introduces a new observable for B -> K* l+ l- decays that minimizes theoretical uncertainties, enhances experimental measurability, and improves sensitivity to new physics effects in various Standard Model extensions.

Contribution

A novel ratio of integrated forward-backward asymmetries is proposed, reducing form factor dependence and increasing sensitivity to new physics in B meson decays.

Findings

The new observable has reduced theoretical uncertainties.

It is more experimentally accessible than the zero of the FBA spectrum.

It shows strong sensitivity to the phase of Wilson coefficient C_10.

Abstract

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.