the $\pi K$ isospin eigenstate with $I=\frac 32$. In the SM the parameters of this amplitude are determined, up to an overall strong-interaction phase $\phi$, in the limit of SU(3) flavor symmetry [@NR]. SU(3)-breaking corrections can be calculated in the factorization approximation [@Stech], so that theoretical uncertainties enter only at the level of nonfactorizable SU(3)-breaking corrections to a subleading decay amplitude. Moreover, it has recently been shown that even these nonfactorizable corrections can be calculated in a model-independent way up to terms that are power suppressed in $\Lambda/m_b$ and vanish in the heavy-quark limit [@fact].

General parametrization of the decay amplitudes
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In the presence of New Physics, the analysis of $B^\pm\to\pi K$ decays becomes more complicated. A convenient and completely general parametrization of the two decay amplitudes is $$\begin{aligned}
\label{ampls}
   {\cal A}(B^+\to\pi^+ K^0) &=& P\,(1-i\rho\,e^{i\phi_\rho})
    \,,\nonumber\\
   -\sqrt2\,{\cal A}(B^+\to\pi^0 K^+) &=& P \Big[ 
    1-i\rho\,e^{i\phi_\rho} - \varepsilon_{3/2}\,e^{i\phi}
    (e^{i\gamma} - a\,e^{i\phi_a} - ib\,e^{i\phi_b}) \Big] \,,\end{aligned}$$ where $P$ is the dominant penguin amplitude defined as the sum of all CP-conserving terms in the $B^\pm\to\pi^\pm K^0$ decay amplitudes, $\varepsilon_{3/2}$, $\rho$, $a$, $b$ are real hadronic parameters, and $\phi$, $\phi_\rho$, $\phi_a$, $\phi_b$ are strong-interaction phases. The weak phase $\gamma=\mbox{arg}(V_{ub}^*)$ and the terms $i\rho$ and $ib$ 