Comparing holographic dark energy models with statefinder
  We apply the statefinder diagnostic to the holographic dark energy models,
including the original holographic dark energy (HDE) model, the new holographic
dark energy model, the new agegraphic dark energy (NADE) model, and the Ricci
dark energy model. In the low-redshift region the holographic dark energy
models are degenerate with each other and with the $\Lambda$CDM model in the
$H(z)$ and $q(z)$ evolutions. In particular, the HDE model is highly degenerate
with the $\Lambda$CDM model, and in the HDE model the cases with different
parameter values are also in strong degeneracy. Since the observational data
are mainly within the low-redshift region, it is very important to break this
low-redshift degeneracy in the $H(z)$ and $q(z)$ diagnostics by using some
quantities with higher order derivatives of the scale factor. It is shown that
the statefinder diagnostic $r(z)$ is very useful in breaking the low-redshift
degeneracies. By employing the statefinder diagnostic the holographic dark
energy models can be differentiated efficiently in the low-redshift region. The
degeneracy between the holographic dark energy models and the $\Lambda$CDM
model can also be broken by this method. Especially for the HDE model, all the
previous strong degeneracies appearing in the $H(z)$ and $q(z)$ diagnostics are
broken effectively. But for the NADE model, the degeneracy between the cases
with different parameter values cannot be broken, even though the statefinder
diagnostic is used. A direct comparison of the holographic dark energy models
in the $r$--$s$ plane is also made, in which the separations between the models
(including the $\Lambda$CDM model) can be directly measured in the light of the
current values $\{r_0,s_0\}$ of the models.
