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We investigate three tree-dominated $B \to a_0 a_0$ decays for the first timein 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 asa meson based on the model of conventional two-quark$(q\bar q)$ structure. Allthe topologies of the Feynman diagrams such as the non-factorizable spectatorones and the annihilation ones are calculated in the pQCD approach. It is ofgreat 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-factorizablecontributions, which give rise to the large $B \to a_0 a_0$ decay rates in theorder of $10^{-6} \sim 10^{-5}$, although the $a_0$ meson has an extremelysmall vector decay constant $f_{a_0}$. Also observed are large directCP-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 atthe running Large Hadron Collider and the near future Super-B/Belle-IIexperiments. The future precision measurements combined with these pQCDpredictions might be helpful to explore the complicated QCD dynamics and theinner structure of the light scalar $a_0$, as well as to complementarilyconstrain the unitary angle $\alpha$.

Large non-factorizable contributions in $B \to a_0 a_0$ decays