Theory review on rare K decays: Standard Model and beyond
نویسنده
چکیده
with X = νν̄, ll, λq = V ∗ qsVqd. Without the dependence of the loop functions y q on the quark masses, CKM unitarity would imply vanishing FCNC (GIM mechanism). Now, looking at these dependences, combined with the scaling of the CKM elements, one can readily get a handle on the importance of each quark contribution. For X = νν̄, only the Z penguin and W box enter, y q ∼ m 2 q, and light quark contributions are suppressed. Since, in addition, ε ∼ 10 and Reλt ∼ Imλt, indirect CP-violation is small. For K, the Reλc,t and Imλt parts contribute. For X = ll, the photon penguin enters with its scaling y q ∼ log(mq) for mq → 0. Direct CPviolation is still short-distance dominated thanks to Imλu = 0, but not indirect CP-violation, completely dominated by the long-distance u-quark photon penguin, K1 → π γ → πll. For KL → l l, the structure is similar to K → πνν̄, up to the change Imλq ↔ Reλq (no single-photon penguin).
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