Presently there is a big interest in models with additional dimensions which might provide solution to the gauge hierarchy problem. In Randall-Sundrum model typeII particles can be trapped on the brane, where they are expected to be metastable, they decay into continuum Kaluza-Klein modes. From the point of view of an observer in 3d-”brane” the particle disappear into the bulk of the additional dimension.

Tunneling of a particle through extra-dimension (V. A. Rubakov)

In a recent paper it was pointed out that this mechanism could result in a disappearing of ortho-positronium into the bulk of additional dimensions. The probability was calculated to be:

10^-9<Br(o-Ps extradim)<10^-8

which is an experimentally interesting region!