CP Violation In KL Research Articles

Reanalysis of CP violation in KL→π+π-γ

We present a reanalysis of direct CP violation in the decay K_L -> pi^+ pi^- gamma. We point out an existing discrepancy between the theoretical and experimental definitions of epsilon'_{+-gamma}. Adopting the experimental definition of epsilon'_{+-gamma}, we estimate that epsilon'_{+-gamma}/epsilon could be as large as a few times 10^{-4} both within the standard model and beyond. We discuss these estimates in detail and we also show how a judicious choice of E_gamma cuts can increase the sensitivity of the observable epsilon'_{+-\gamma} to the underlying CP violation.

Final-state interactions and CP violation in KL--> pi + pi - e+e-

Using chiral perturbation theory we calculate the imaginary parts of the ${\mathit{K}}_{\mathit{L}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ form factors that arise from \ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$ and \ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\gamma}* rescattering. We discuss their influence on CP-violating variables in ${\mathit{K}}_{\mathit{L}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$. \textcopyright{} 1996 The American Physical Society.

Direct CP violation in KL → π+π−γ

CP violation due to interference between K L and K S decays into π + π − γ is analyzed in the Standard Model in conjunction with the large N c approach. The CP-violating parameter ϵ′ +− γ , which is the difference between η +− γ and η +−, receives dominant contributions from K 0− K 0 mixing and the gluon penguin diagram; its magnitude is calculated to be of order 10 −2ϵ. Genuine direct CP violation in K L → π + π − γ decay originating from the electromagnetic penguin diagram is of order (10 −3−10 −4)ϵ, depending on the region of the Dalitz plot.

CP violation in KL, S→2 γ transitions

In order to clarify the status of direct CP violation (in transition amplitudes), we study short-distance contributions to K 0( K 0 )→γγ within the standard model for large m t , including QCD corrections. Owing to a relaxed GIM suppression, CP violation in the decay amplitude turns out to more important than previously thought, namely of order 100 times direct CP violation (ε') in K L → ππ decays. This leads to the rare K L, S → γγ decays as potentially interesting candidates for testing direct CP violation at LEAR and φ factories. Still, it does not seem to be achievable by luminosity foreseeable for forthcoming φ factories.

How good is nonet symmetry for pseudoscalar mesons?

It is pointed out that the assumption of nonet symmetry for ΔS=1 weak interactions is equivalent to excluding non-penguin contributions (i.e. the so-called “figure-eight” graphs). The radiative decays KL→γγ and KL→π+π−γ provide a most sensitive test on nonet symmetry. We find that this symmetry for ΔS=1 weak transitions of the pseudoscalar nonet holds at the level of (15–25)%. Implications on the low-energy penguin dominance of the ΔI=12 K→ππ amplitude, the long-distance contribution to the KL−KS mass difference from single-particle intermediate states, and direct CP violation in KL→γγ are discussed.