Epsilon_K at Next-to-Next-to-Leading Order: The Charm-Top-Quark Contribution

TL;DR

This paper presents a detailed NNLO QCD calculation of the charm-top-quark contribution to epsilon_K, refining the theoretical prediction and showing a significant increase over NLO estimates.

Contribution

The authors perform the first complete NNLO calculation of eta_ct, including three-loop anomalous dimensions and two-loop matching, improving precision in epsilon_K predictions.

Findings

eta_ct increased by ~7% at NNLO

epsilon_K prediction enhanced by ~3.3%

reduction in theoretical uncertainty

Abstract

We perform a next-to-next-to-leading order (NNLO) QCD analysis of the charm-top-quark contribution eta_ct to the effective Delta S = 2 Hamiltonian in the Standard Model. eta_ct represents an important part of the short distance contribution to the parameter epsilon_K. We calculate the three-loop anomalous dimension of the leading operator Q_S2, the three-loop mixing of the current-current and penguin operators into Q_S2, and the corresponding two-loop matching conditions at the electroweak, the bottom-quark, and the charm-quark scale. As our final numerical result we obtain eta_ct = 0.496 +/- 0.047, which is roughly 7% larger than the next-to-leading-order (NLO) value eta_ct(NLO) = 0.457 +/- 0.073. This results in a prediction for epsilon_K = (1.90 +/- 0.26) x 10^(-3), which corresponds to an enhancement of approximately 3.3% with respect to the value obtained using eta_ct(NLO).