plausible assumption that potential New Physics contributions to this amplitude can be neglected.[^2] Even if new electroweak penguin effects would be of comparable strength in $\bar b\to\bar s q\bar q$ and $\bar b\to\bar d q\bar q$ transitions, the latter would have to compete with a Cabibbo-enhanced tree amplitude in order to be significant in $B^+\to\pi^+\pi^0$ decays. We then find that [@NR; @me] $$\label{eps}
   \bar\varepsilon_{3/2} 
   \equiv \frac{\varepsilon_{3/2}}{\sqrt{1+\rho^2}}
   = \sqrt2\,R_{\rm SU(3)} \left|\frac{V_{us}}{V_{ud}}\right|
   \left[
   \frac{\mbox{B}(B^+\to\pi^+\pi^0)+\mbox{B}(B^-\to\pi^-\pi^0)}
        {\mbox{B}(B^+\to\pi^+ K^0)+\mbox{B}(B^-\to\pi^-\bar K^0)}
   \right]^{1/2} .$$ SU(3)-breaking corrections are described by the factor $R_{\rm SU(3)}=1.22\pm 0.05$, which can be calculated in a model-independent way using the QCD factorization theorem of [@fact]. The quoted error is an estimate of the theoretical uncertainty due to uncontrollable corrections of $O(\frac{1}{N_c}\frac{m_s}{m_b})$. Using preliminary data reported by the CLEO Collaboration [@CLEO] to evaluate the ratio of branching ratios in (\[eps\]), we obtain $$\label{epsval}
   \bar\varepsilon_{3/2} = 0.21\pm 0.06_{\rm exp}
   \pm 0.01_{\rm th} \,.$$ With a better measurement of the branching ratios the uncertainty in $\bar\varepsilon_{3/2}$ will be reduced significantly.

The parameter $\rho$ in (\[ampls\]) parameterizes the sum of all CP-violating contributions to the $B^+\to\pi^+ K^0$ decay amplitude. In the presence of New Physics, those contributions could arise from QCD as well as electroweak penguin operators. 