Size of direct CP violation in singly Cabibbo-suppressed D decays

TL;DR

This paper estimates the size of direct CP violation in charm decays within the Standard Model, suggesting that certain suppressed amplitudes could explain recent experimental observations by LHCb.

Contribution

It provides a theoretical estimate of CP violation effects in charm decays, highlighting the potential significance of suppressed penguin amplitudes in explaining experimental data.

Findings

Standard Model predictions are an order of magnitude smaller than observed.

Large tree-level annihilation amplitudes could enhance CP violation effects.

Suppressed penguin amplitudes may account for the LHCb measurements.

Abstract

The first experimental evidence for direct CP violation in charm-quark decays has recently been presented by the LHCb collaboration in the difference between the D -> K+ K- and D -> pi+ pi- time-integrated CP asymmetries. We estimate the size of the effects that can be expected within the Standard Model and find that at leading order in 1/mc they are an order of magnitude smaller. However, tree-level annihilation type amplitudes are known to be large experimentally. This implies that certain formally 1/mc-suppressed penguin amplitudes could plausibly account for the LHCb measurement. Simultaneously, the flavor-breaking parts of these amplitudes could explain the large difference between the D -> K+ K- and D -> pi+ pi- decay rates.