Charm mixing in the Standard Model

TL;DR

This paper analyzes charm meson mixing within the Standard Model, highlighting potential CP violation levels and the impact of higher-order effects, challenging assumptions that such CP violation indicates new physics.

Contribution

It demonstrates that CP violation in charm mixing can be significant within the Standard Model and clarifies the role of higher-dimension operators and flavor symmetry breaking effects.

Findings

CP violating phase of order per mille cannot be excluded in the Standard Model.

Higher-dimension operators contribute significantly to mixing parameters.

Simplified assumptions about CKM couplings can lead to incorrect conclusions.

Abstract

In this talk we report on a study on the mixing of neutral charmed mesons and argue that, at the present stage, a CP violating weak phase of the order of some per mille can not be excluded in the Standard Model. It is shown how some, seemingly reasonable, simplifying assumptions about CKM couplings lead to the wrong conclusion that CP violation of this amount is an unambiguous indication of new physics. The presented results rely on a recent short-distance analysis of the Delta C=2 transition, which confirms the expectation that the dominant contribution is due to effects of flavour symmetry breaking appearing in higher orders of the Heavy Quark Expansion. We investigate meson-antimeson transitions with an intermediate state coupling to the meson's sea quark background, present in dimension 10 and 12, using a factorisation approach to simplify the operator basis. On account of a lifting of GIM suppression by one power of m_s/m_c, the contribution to y=Delta Gamma/2 Gamma is found to exceed that of the formally leading dimension six by a factor close to ten.