Z pass not showing anything not even in z buffer

Hello all,

Im trying to render a particle animation with bloom and transparency, I did bake the animation into the cache. Im planning on adding depth of field later on so i wanted to add a Z pass to my render. The problem is that the z pass doesnt seem to be picking up anything at all. I have tried adding a map range node, a normalizer or looking at the z buffer but nothing shows. Does anyone have any idea why this might be happening?

This is the render as it is

this are my nodes

Thanks for the help

Try math node.