Can Up FCNC solve the $\Delta A_{CP}$ puzzle?

TL;DR

This paper examines whether flavor-violating gauge interactions in the up sector can explain the large CP asymmetry difference observed in D meson decays, concluding that such models are incompatible with mixing constraints.

Contribution

It demonstrates that up FCNC models cannot simultaneously explain the $ abla A_{CP}$ and satisfy $D^{0}-ar{D}^{0}$ mixing constraints, suggesting alternative models like a fourth family extension are more promising.

Findings

Up FCNC models are excluded by mixing constraints.

Direct collider searches can test the proposed models.

Fourth family models better explain the observed CP asymmetry.

Abstract

We investigate the attempt using flavor violation gauge interaction in the up sector to explain the LHCb recently observed large $\Delta A_{CP}$ ($A_{CP}(D^{0}\to K^{+}K^{-})-A_{CP}(D^{0}\to \pi^{+}\pi^{-})$). We study an Abelian model that only right-handed up quarks is charged under it and the 1-3 coupling is maximized. The simultaneous 1-3 2-3 mixing is realized by a quark mixing of 1-2 generation. Given the easy identification of top quark, the model can be directly tested by $\Delta F=1$ and $\Delta F=2$ processes at the hadron colliders as associated top production $g c \to t Z^{\prime}$ or same-sign top scattering $u u\to t t$. The direct search bounds are still consistent with the assumption that $ut$ and $ct$ couplings are equal but the same-sign top scattering bound is expected to be reached very soon. However, since there is no CKM-like suppression, the corresponding parameter space for generating $\Delta A_{CP}$ is completely excluded by the $D^{0}-\bar{D}^{0}$ mixing. We conclude that the up FCNC type models cannot explain the $\Delta A_{CP}$ while to be consistent with the $D^{0}-\bar{D}^{0}$ mixing constraint at the same time. On the other hand, a model as SM with fourth family extension has better chance to explain the large $\Delta A_{CP}$ consistently.