On the dispersive two-photon K_L -> μ^+ μ^- amplitude

We present a full account of the two-loop electroweak, two-photon mediated short-distance dispersive $K_L \toμ^+μ^-$ decay amplitude. QCD corrections change the sign of this amplitude and reduce it by an order of magnitude. Thus, the QCD-corrected two-loop amplitude represents only a small fraction (with the central value of 5 %) of the one-loop weak short-distance contribution, and has the same sign. In combination with a recent measurement, the standard-model prediction of the short-distance amplitude, completed in this paper, provides a constraint on the otherwise uncertain long-distance dispersive amplitude.

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