Isospin asymmetries in $B\to (K^*,\rho) \gamma/ l^+ l^-$ and $B \to K l^+ l^-$ in and beyond the Standard Model

Abstract

We compute the isospin asymmetries in $B \to (K^*,\rho) \gamma$ and $B\to (K,K^*,\rho) l^+l^-$ for low lepton pair invariant mass $q^2$, within the Standard Model (SM) and beyond the SM (BSM) in a generic dimension six operator basis. Within the SM the CP-averaged isospin asymmetries for $B \to (K,K^*,\rho) ll$, between $1{\rm GeV}^2 \leq q^2 \leq 4m_c^2$, are predicted to be small (below 1.5%) though with significant cancellation. In the SM the non-CP averaged asymmetries for $B \to \rho ll$ deviate by $\approx \pm 5%$ from the CP-averaged ones. We provide physical arguments, based on resonances, of why isospin asymmetries have to decrease for large $q^2$ (towards the endpoint). Two types of isospin violating effects are computed: ultraviolet (UV) isospin violation due to differences between operators coupling to up and down quarks, and infrared (IR) isospin violation where a photon is emitted from the spectator quark and is hence proportional to the difference between the up- and down-quark charges. These isospin violating processes may be subdivided into weak annihilation (WA), quark loop spectator scattering (QLSS) and a chromomagnetic contribution. Furthermore we discuss generic selection rules based on parity and angular momentum for the $B \to Kll$ transition as well as specific selection rules valid for WA at leading order in the strong coupling constant. We clarify that the relation between the $K$ and the longitudinal part of the $K^*$ only holds for leading twist and for left-handed currents. In general the $B \to \rho ll$ and $B \to K^*ll$ isospin asymmetries are structurally different yet the closeness of $\alpha_{\rm CKM}$ to ninety degrees allows us to construct a (quasi) null test for the SM out of the respective isospin symmetries. We provide an update on ${\cal B}(B^0 \to K^{*0}\gamma)/{\cal B}(B_s \to \phi \gamma)$ which is sensitive to WA.