A hunter sets snares in the woods. They know the animal's habits but not whether it will ever show up. In much the same way, physicists set up detectors in hopes of catching particles of dark matter — the invisible substance thought to make up most of the Universe's mass.
A new study offers hunters an unusual tool: not a trap, but a measuring tape. The authors proved that for any detector sensitive to dark matter scattering off electrons, there is a hard upper limit on sensitivity. It depends only on three macroscopic properties of the material: its density, plasma frequency (how fast electrons oscillate), and dielectric function (its response to an electric field).
Surprisingly, all three parameters vary within a very narrow range across very different materials — from silicon to germanium. It's as if all traps had roughly the same mesh size. So the sensitivity limit turns out to be almost universal. Changing the detector material is like painting your snares a different color: the animal couldn't care less.
The authors tested their theory on analytical and numerical models. They confirmed that for large momentum transfers, the dielectric function follows a simple scaling law. The tape measure shows: no matter how hard you try, you can't push the signal beyond that line. This isn't bad news — it's a sober benchmark for future experiments.
The plasma frequency of most materials lies in the ultraviolet range, so they shine under UV light even if they look dull to the eye.