The $B \to \pi K$ Puzzle: A Status Report

We discuss the theoretical interpretation of the $B\to\pi K$ system in the light of new data. Using the branching ratio and direct CP asymmetry of $B^0_d\to\pi^-K^+$, the picture of the direct CP violation in $B_d^0\to\pi^+\pi^-$ could be clarified: we predict $A_{CP}^{dir}(B_d\to\pi^+\pi^-)=-0.24\pm0.04$, which favours the BaBar measurement, and extract $\gamma=(70.0^{+3.8}_{-4.3})^\circ$, in agreement with the Standard-Model fits of the unitarity triangle. All $B\to\pi K$ modes with colour-suppressed electroweak penguin contributions are found in excellent agreement with the Standard Model. The data for the ratios $R_{c,n}$ of the charged and neutral $B\to\pi K$ branching ratios, which are sizeably affected by electroweak penguin contributions, have moved quite a bit towards the Standard-Model predictions, which are almost unchanged, thereby reducing the "$B\to\pi K$ puzzle". On the other hand, the mixing-induced CP violation of $B^0_d\to\pi^0 K_S$ still looks puzzling and could be accommodated through a modified electroweak penguin sector with a large CP-violating new-physics phase, while the observed non-vanishing difference between the direct CP asymmetries of $B^\pm\to\pi^0K^\pm$ and $B_d\to\pi^\mp K^\pm$ seems to be caused by hadronic and not by new physics.