  We investigate three tree-dominated $B \to a_0 a_0$ decays for the first time
in the perturbative QCD(pQCD) approach at leading order in the standard model,
with $a_0$ standing for the light scalar $a_0(980)$ state, which is assumed as
a meson based on the model of conventional two-quark$(q\bar q)$ structure. All
the topologies of the Feynman diagrams such as the non-factorizable spectator
ones and the annihilation ones are calculated in the pQCD approach. It is of
great interest to find that, contrary to the known $B \to \pi \pi$ decays, the
$B \to a_0 a_0$ decays are governed by the large non-factorizable
contributions, which give rise to the large $B \to a_0 a_0$ decay rates in the
order of $10^{-6} \sim 10^{-5}$, although the $a_0$ meson has an extremely
small vector decay constant $f_{a_0}$. Also observed are large direct
CP-violating asymmetries around $15\%$ and $30\%$ for the $B^0 \to a_0^0 a_0^0$
and $a_0^+ a_0^-$ modes. These sizable predictions could be easily examined at
the running Large Hadron Collider and the near future Super-B/Belle-II
experiments. The future precision measurements combined with these pQCD
predictions might be helpful to explore the complicated QCD dynamics and the
inner structure of the light scalar $a_0$, as well as to complementarily
constrain the unitary angle $\alpha$.
