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Improved Measurements of the Branching Fractions for B0 --> pi+pi- and B0 --> K+pi-, and a Search fro B0 --> K+K-

We present measurements of the branching fractions for the charmless two-body decays $\Bz\to\pip\pim$ and $\Bz\to\Kp\pim$, and a search for the decay $\Bz\to\Kp\Km$. We include the effects of final-state radiation from the daughter mesons for the first time, and quote branching fractions for the inclusive processes $\Bz\to h^+ h^{\prime -} nγ$, where $h$ and $h^\prime$ are pions or kaons. The maximum value of the sum of the energies of the $n$ undetected photons, $E_γ^{\rm max}$, is mode-dependent. Using a data sample of approximately 227 million \upsbb decays collected with the \babar\ detector at the \pep2 asymmetric-energy \epem collider at SLAC, we measure: \begin{eqnarray*} {\cal B} (\Bz \to \pip\pim nγ;\: E_γ^{\rm max}=150\mev) & = & (5.1\pm 0.4\pm 0.2)\times 10^{-6}, {\cal B} (\Bz \to \Kp\pim nγ;\: E_γ^{\rm max}=105\mev) & = & (18.1\pm 0.6\pm 0.6)\times 10^{-6}, {\cal B} (\Bz \to \Kp\Km nγ;\: E_γ^{\rm max}=59\mev) & < & 0.5 \times 10^{-6} (90% {\rm confidence level}), \end{eqnarray*} where the first uncertainty is statistical and the second is systematic. Theoretical calculations can be used to extrapolate from the above measurements the non-radiative branching fractions, ${\cal B}^0$. Using one such calculation, we find: \begin{eqnarray*} {\cal B}^0 (\Bz \to \pip\pim) & = & (5.5\pm 0.4\pm 0.3)\times 10^{-6}, {\cal B}^0 (\Bz \to \Kp\pim) & = & (19.1\pm 0.6\pm 0.6)\times 10^{-6}, {\cal B}^0 (\Bz \to \Kp\Km) & < & 0.5 \times 10^{-6} (90% {\rm confidence level}). \end{eqnarray*} Meaningful comparison between theory and experiment, as well as combination of measurements from different experiments, can be performed only in terms of these non-radiative quantities.