The effect of S-wave interference on the B 0 → K ∗0 ℓ + ℓ− angular observables

The rare decay B 0 → K ∗0 ℓ + ℓ − is a flavour changing neutral current decay with a high sensitivity to physics beyond the Standard Model. Nearly all theoretical predictions and all experimental measurements so far have assumed a K ∗0 P-wave that decays into the K + π − final state. In this paper the addition of an S-wave within the K +π− system of B 0 → K ∗0 ℓ + ℓ − and the subsequent impact of this on the angular distribution of the final state particles is explored. The inclusion of the S-wave causes a distinction between the values of the angular observables obtained from counting experiments and those obtained from fits to the angular distribution. The effect of a non-zero S-wave on an angular analysis of B 0 → K ∗0 ℓ + ℓ − is assessed as a function of dataset size and the relative size of the S-wave amplitude. An S-wave contribution, equivalent to what is measured in B 0 → J/ψK ∗0 at BaBar, leads to a significant bias on the angular observables for datasets of above 200 signal decays. Any future experimental analysis of the K + π − ℓ + ℓ − final state will have to take the S-wave contribution into account.