Probing New Physics in $B → πK$ Decays

Over the last two decades the $B \to \pi K$ decays 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 give a contribution 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 give an up-to-date 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 employ an isospin relation between the amplitudes of the $B \to \pi K$ decays to determine the relevant parameters, making only minimal use of the $SU(3)$ flavour symmetry. Another essential constraint is provided by the mixing-induced CP asymmetry of $B_d^0 \to \pi^0 K_{\rm S}$. 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.