Kaon Theory News

Abstract

During the last fifteen years B_{s,d} decays, B_{s,d}^0-\bar B^0_{s,d} mixings, CP-violating asymmetries S_{\psi K_S} and S_{\psi\phi} provided the dominant information about the pattern of flavour violation within the SM and its extensions. We emphasize that in the coming years K meson physics will certainly strike back through the measurements of the branching ratios of theoretically very clean decays K^+->pi^+\nu\bar\nu and K_L->\pi^0\nu\bar\nu and improved calculations of the ratio epsilon'/epsilon=\epe. We summarize the status of K^+->pi^+\nu\bar\nu and K_L->\pi^0\nu\bar\nu within the SM and simplified NP models and update the picture of flavour violation in the Littlest Higgs model with T-parity taking all available constraints into account. But the highlight of this talk are new results on \epe from lattice QCD and large N approach that give a strong indication for a new anomaly in flavour physics, this time coming from K mesons. Indeed, \epe within the SM is found to be significantly below the data of NA48 and KTeV collaborations. Combining lattice results on the non-perturbative parameters B6 and B8 with the assumption that the SM dynamics dominates the Delta I=1/2 rule a 3 sigma anomaly in \epe emerges. Moreover, the recently derived upper bounds on B6 and B8 from large N approach increase the confidence that the found anomaly could be an important signal of NP at work. Of particular importance are the correlations between K^+->pi^+\nu\bar\nu, K_L->\pi^0\nu\bar\nu and \epe. With future precise measurements of K^+->pi^+\nu\bar\nu and K_L->\pi^0\nu\bar\nu and improved calculations of \epe they will surely provide powerful tools for selecting the favourite NP models. We illustrate this with the help of the LHT model and simplified models in which NP in K\to\pi\nu\bar\nu and \epe is governed by Z with flavour violating couplings or by a heavy Z'.