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Improved anatomy of epsilon'/epsilon in the Standard Model (1507.06345v3)

Published 22 Jul 2015 in hep-ph, hep-ex, and hep-lat

Abstract: We present a new analysis of the ratio epsilon'/epsilon within the Standard Model (SM) using a formalism that is manifestly independent of the values of leading (V-A)x(V-A) QCD penguin, and EW penguin hadronic matrix elements of the operators Q_4, Q_9, and Q_10, and applies to the SM as well as extensions with the same operator structure. It is valid under the assumption that the SM exactly describes the data on CP-conserving K -> pi pi amplitudes. As a result of this and the high precision now available for CKM and quark mass parameters, to high accuracy epsilon'/epsilon depends only on two non-perturbative parameters, B_61/2 and B_83/2, and perturbatively calculable Wilson coefficients. Within the SM, we are separately able to determine the hadronic matrix element <Q_4>_0 from CP-conserving data, significantly more precisely than presently possible with lattice QCD. Employing B_61/2 = 0.57+-0.19 and B_83/2 = 0.76+-0.05, extracted from recent results by the RBC-UKQCD collaboration, we obtain epsilon'/epsilon = (1.9+-4.5) 10-4, substantially more precise than the recent RBC-UKQCD prediction and 2.9 sigma below the experimental value (16.6+-2.3) 10-4, with the error being fully dominated by that on B_61/2. Even discarding lattice input completely, but employing the recently obtained bound B_61/2 <= B_83/2 <= 1 from the large-N approach, the SM value is found more than 2 sigma below the experimental value. At B_61/2 = B_83/2 = 1, varying all other parameters within one sigma, we find epsilon'/epsilon = (8.6+-3.2) 10-4. We present a detailed anatomy of the various SM uncertainties, including all sub-leading hadronic matrix elements, briefly commenting on the possibility of underestimated SM contributions as well as on the impact of our results on new physics models.

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