change sign under a CP transformation, whereas all other parameters stay invariant. The terms proportional to $\varepsilon_{3/2}$ in (\[ampls\]) parameterize the isospin amplitude $A_{3/2}$. The contribution proportional to $e^{i\gamma}$ comes from the matrix elements of the current–current operators $Q_1^u$ and $Q_2^u$ in the effective Hamiltonian, which mediate the tree process $\bar b\to\bar u u\bar s$. The quantities $a$ and $b$ parameterize the effects of electroweak penguins. It is crucial that only isospin-violating terms can contribute to the amplitude $A_{3/2}$. All isospin-conserving contributions reside in $P$ and $\rho$.

Let us discuss the various terms entering the decay amplitudes in detail. The parameter $\varepsilon_{3/2}$ characterizes the relative strength of tree and QCD penguin contributions. Information about it can be derived by using SU(3) flavor symmetry to relate the tree contribution to the isospin amplitude $A_{3/2}$ to the corresponding contribution in the decay $B^+\to\pi^+\pi^0$. Since the final state $\pi^+\pi^0$ has isospin $I=2$ (because of Bose symmetry), the amplitude for the latter process does not receive any contribution from QCD penguins. Moreover, in the SM electroweak penguins in $\bar b\to\bar d q\bar q$ transitions are negligible, and thus only the tree topology contributes to the $B^+\to\pi^+\pi^0$ decay amplitude. In our analysis we make the 