  We report a search for diphoton events with large missing transverse energy
produced in ppbar collisions at sqrt(s)=1.96 TeV. The data were collected with
the D0 detector at the Fermilab Tevatron Collider, and correspond to 6.3 fb-1
of integrated luminosity. The observed missing transverse energy distribution
is well described by the standard model prediction, and 95% C.L. limits are
derived on two realizations of theories beyond the standard model. In a gauge
mediated supersymmetry breaking scenario, the breaking scale Lambda is excluded
for Lambda < 124 TeV. In a universal extra dimension model including
gravitational decays, the compactification radius R_c is excluded for R_c-1 <
477 GeV.
