# D-\bar D mixing and rare D decays in the Littlest Higgs model with non-unitarity matrix

## TL;DR

This paper investigates how the Littlest Higgs model with non-unitary quark mixing affects D meson mixing and rare decays, showing potential for observable CP asymmetries and enhanced decay rates.

## Contribution

It demonstrates that the Littlest Higgs model can produce sizable D-ar D mixing and rare decay rates, with possible observable CP asymmetries, due to non-unitary quark mixing and tree-level FCNCs.

## Key findings

- D-ar D$ mixing parameter can reach experimental values
- Branching ratio of $D o X_u o ext{gamma}$ can be significant
- Direct CP asymmetry in $D o X_u o ext{gamma}$ can be around 10%

## Abstract

We study the $D-\bar D$ mixing and rare D decays in the Littlest Higgs model. As the new weak singlet quark with the electric charge of 2/3 is introduced to cancel the quadratic divergence induced by the top-quark, the standard unitary $3\times 3$ Cabibbo-Kobayashi-Maskawa matrix is extended to a non-unitary $4\times 3$ matrix in the quark charged currents and Z-mediated flavor changing neutral currents are generated at tree level. In this model, we show that the $D-\bar D$ mixing parameter can be as large as the current experimental value and the decay branching ratio (BR) of $D\to X_u \ga$ is small but its direct CP asymmetry could be $O(10%)$. In addition, we find that the BRs of $D\to X_u \ell^{+} \ell^{-}$, $D\to X_u\nu \bar \nu$ and $D\to \mu^{+} \mu^{-}$ could be enhanced to be $O(10^{-9})$, $O(10^{-8})$ and $O(10^{-9})$, respectively.