Bound onCPNonconservation inKL→2γDecay

The surprisingly small upper bound recently established for $K_L\to {\mu }^{+}+{\mu }^{-}$ decay is exploited, via unitarity, to set an upper bound on $K_L$ decay to a two-photon state with spin-parity $0^{+}$. Special circumstances operate here to lend a fair degree of credibility to the analysis. The fraction of $K_L\to 2\gamma$ decays which proceed in this $\mathrm{CP}$-nonconserving mode is less than ∼0.37.