Direct CP Violation in B Decays

We discuss several aspects of direct CP asymmetries in B decays, which are very useful in spite of hadronic uncertainties in asymmetry calculations. 1) Asymmetries in decays to $D^{(*)}K^{(*)}$, $\pi^+\pi^-, \rho^+\rho^-$, providing precision tests for the CKM phase $\gamma$. 2) Null tests in $B^+\to J/\psi K^+, \pi^+\pi^0$, where a nonzero asymmetry provides evidence for New Physics. 3) Isospin and broken flavor SU(3) relations among CP asymmetries in $B\to K\pi, \pi\pi$ predicting $A_{CP}(B^0\to K^0\pi^0)$ and $A_{CP}(B^0\to \pi^0\pi^0)$. 4) The significance of $A_{CP}(B^0\to K^+\pi^-)\ne A_{CP}(B^+\to K^+\pi^0)$. 5) A potentially stringent constraint on $\gamma$ from $A_{CP}(B^+\to K^+\pi^0)$ and $R_c\equiv 2\Gamma(B^+\to K^+\pi^0)/\Gamma(B^+\to K^0\pi^+)$. 6) The role of direct CP asymmetries in $b\to s\bar qq$ decays for studying the origin of potential New Physics.