Probing New Physics in $B\to\pi K$ Decays

Over the last two decades, $B \to \pi K$ modes have received a lot of attention. They are particularly interesting since the main contribution to these decays comes from QCD penguin topologies. Furthermore, electroweak penguin amplitudes enter at a level comparable to the tree topologies. In the past, a discrepancy was found in the correlation between the CP asymmetries of $B_d^0 \to \pi^0 K_{\rm S}$. We provide the state-of-the-art picture of this situation and consider new constraints, finding that the effect has become larger. An attractive explanation is offered by a modified electroweak penguin sector. We propose a new method to determine the relevant parameters. It employs an isospin relation between the amplitudes of the $B \to \pi K$ decays, and makes only minimal use of the $SU(3)$ flavour symmetry. Using current data as well as future scenarios, we demonstrate how the mixing-induced CP asymmetry of $B_d^0 \to \pi^0 K_{\rm S}$ plays a key role in this analysis. The application of our strategy at the next generation of $B$-physics experiments may establish New Physics and reveal new sources of CP violation in the electroweak penguin sector.