I'm writing a story in which a scientist finds her way to the realm of the dead. The realm is a place that exists "behind" our world, on another plane of existence. One accesses this place by entering a different state of being rather than going through any sort of portal.
What kind of scientist would make this kind of discovery? Which scientific discipline?
Spiritual issues are not the domain of science, which deals with this material reality. Since you are writing fiction just pick a discipline. You'll get criticized whichever you choose. Or opt for a non-scientist who happens to stumble upon this. Or maybe a medium or psychic.