  We present ALMA observations of the 880um continuum and CO J= 3-2 line
emission from the transition disk around [PZ99] J160421.7-213028, a solar mass
star in the Upper Scorpius OB association. Analysis of the continuum data
indicates that 80% of the dust mass is concentrated in an annulus extending
between 79 and 114AU in radius. Dust is robustly detected inside the annulus,
at a mass surface density 100 times lower than that at 80 AU. The CO emission
in the inner disk also shows a significantly decreased mass surface density,
but we infer a cavity radius of only 31AU for the gas. The large separation of
the dust and gas cavity edges, as well as the high radial concentration of
millimeter-sized dust grains, is qualitatively consistent with the predictions
of pressure trap models that include hydrodynamical disk-planet interactions
and dust coagulation/fragmentation processes.
