  We show that the best variational ground state of the spin-$\frac{1}{2}$
Kagome antiferromagnetic Heisenberg model with nearest-neighboring exchange
coupling(NN-KAFH) is a $Z_{2}$ spin liquid state, rather than the widely
believed $U(1)$ Dirac spin liquid state. The spinon excitation in the $Z_{2}$
spin liquid state has a small gap of about $1/40$ of the spinon band width. We
find that while the $Z_{2}$ and the $U(1)$ spin liquid state have a large
overlap on finite clusters and are thus very close in energy, they host totally
different spinon excitation spectrum. The strength of the RVB theory becomes
particularly clear in such a situation, as it provides not only a variational
understanding of the ground state structure, but also a comprehensive picture
for the excitation spectrum of the system. Our result indicates that the
spin-$\frac{1}{2}$ NN-KAFH should be better understood as a nearly critical
system, rather than a prototypical gapped $Z_{2}$ spin liquid system.
