  In the pursuit of speculative new particles, forces, and dimensions with
vanishingly small influence on normal matter, understanding the ultimate
physical limits of experimental sensitivity is essential. Here, I show that
quantum decoherence offers a window into otherwise inaccessible realms. There
is a standard quantum limit for diffusion that restricts some
entanglement-generating phenomena, like soft collisions with new particle
species, from having appreciable classical influence on normal matter. Such
phenomena are classically undetectable but can be revealed by the anomalous
decoherence they induce on non-classical superpositions with long-range
coherence in phase space. This gives strong, novel motivation for the
construction of matter interferometers and other experimental sources of large
superpositions, which recently have seen rapid progress. Decoherence is always
at least second order in the coupling strength, so such searches are best
suited for soft interactions (e.g., small momentum transfers), but not weak
ones (i.e., small coupling constants).
