  We consider the zero-temperature properties of the spin-half two-dimensional
Shastry-Sutherland antiferromagnet by using a high-order coupled cluster method
(CCM) treatment. We find that this model demonstrates various groundstate
phases (N\'{e}el, magnetically disordered, orthogonal dimer), and we make
predictions for the positions of the phase transition points. In particular, we
find that orthogonal-dimer state becomes the groundstate at ${J}^{d}_2/J_1 \sim
1.477$. For the critical point $J_2^{c}/J_1$ where the semi-classical N\'eel
order disappears we obtain a significantly lower value than $J_2^{d}/J_1$,
namely, ${J}^{c}_2/J_1$ in the range $[1.14, 1.39]$. We therefore conclude that
an intermediate phase exists between the \Neel and the dimer phases. An
analysis of the energy of a competing spiral phase yields clear evidence that
the spiral phase does not become the groundstate for any value of $J_2$. The
intermediate phase is therefore magnetically disordered but may exhibit
plaquette or columnar dimer ordering.
