We report an improved measurement of the partial-rate CP asymmetry in $B^{\pm }\to K^0{\pi }^{\pm }$ decays. The analysis is based on a data sample of $85\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}\mathrm{}\left(4S\right)\mathrm{}$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ storage ring. We measure ${\mathcal{A}}_{\mathrm{CP}}(K^0{\pi }^{\pm }{)=0.07\mathrm{}}_{-0.08}^{+0.09\mathrm{}}{}_{-0.03}^{+0.01\mathrm{}},$ where the first and second errors are statistical and systematic, respectively; the corresponding 90% confidence-level interval is $-0.10<\mathrm{}{\mathcal{A}}_{\mathrm{CP}}\left(K^0{\pi }^{\pm }\right)<\mathrm{}\mathrm{0.22.}\mathrm{}$

• Received 24 April 2003

DOI:https://doi.org/10.1103/PhysRevD.68.011103