  We present a measurement of $B^{0} \rightarrow \omega \omega$, a charmless
decay into two vector mesons, using 772 $\times 10^6$ $B\overline{B}$ pairs
collected with the Belle detector at the KEKB $e^+e^-$ collider. The decay is
observed with a significance of 7.9 standard deviations. We measure a branching
fraction $\mathcal{B} = (1.53 \pm 0.29 \pm 0.17) \times 10^{-6}$, a fraction of
longitudinal polarization $f_L = 0.87 \pm 0.13 \pm 0.13$, and a time-integrated
$CP$ asymmetry $A_{CP}$ = $-0.44 \pm 0.43 \pm 0.11$, where the first
uncertainties listed are statistical and the second are systematic. This is the
first observation of $B^{0} \rightarrow \omega \omega$, and the first
measurements of $f_L$ and $A_{CP}$ for this decay.

High Energy Physics - Experiment