Study of charmless hadronic B decays into the final states $K\pi, \pi\pi$, and KK, with the first observation of $B \to \pi^+ \pi^-$ and $B\to K^0\pi^0$

We have studied charmless hadronic decays of B mesons into two-body final states with kaons and pions. We present preliminary results based on 9.66 million $B\bar{B}$ pairs collected with the CLEO detector. We have made the first observation of the decay $B \to \pi^+\pi^-$, with the branching fraction of $Br(B \to \pi^+ \pi^-) = (4.7^{+1.8}_{-1.5} \pm 0.6) \times 10^{-6}$. We have also observed for the first time the decay $B \to K^0\pi^0$ with the branching fraction of $Br(B \to K^0 \pi^0) = (14.8^{+5.9+2.4}_{-5.1-3.3}) \times 10^{-6}$, thus completing the set of four $K\pi$ branching fraction measurements. We present improved measurements for the decays $B\to K^{\pm}\pi^{\mp}$, $B^{\pm}\to K^0\pi^{\pm}$, and $B^{\pm}\to K^{\pm}\pi^0$. We use these and other charmless hadronic B decays to make a first determination of the value of the weak phase ${\rm Arg}(V^*_{ub})=\gamma = {113^\circ}^{+25^\circ}_{-23^\circ}$.