Deep inside a former gold mine in South Dakota stands a tank holding ten tonnes of liquid xenon. It looks like a giant thermos, except that it catches not heat but rare nudges from invisible particles. Over two and a half years of observations, the LUX-ZEPLIN detector felt a single solitary hit that does not fit the usual noise. A xenon nucleus jerked as if it had been pushed with an energy of 248 thousand electronvolts, and that is far too strong for ordinary background.
This lone signal can be explained by two different keys to the same lock. The first key is heavy, with a cunningly cut bit: a dark matter particle changes its own mass in the collision, as though a piece of it snapped off on impact. The second key is light but thrown with enormous force: a tiny particle flies at nearly the speed of light because it was born in the decay of a heavier sibling. Both versions leave few traces at weak nudges, and those are exactly the traces the detector does not see.
The authors went through three variants of such an interaction, like three shapes of a key's bit. It turned out that for the light fast particle, only one shape of the bit, the most exotic one, hides weak nudges and leaves mostly strong ones. That is precisely the strong nudge the detector saw. For the heavy key with changing mass, various bit shapes work, but the more intricate the shape, the smaller the mass difference before and after the hit must be.
For now this single nudge remains a mystery, but a useful one. It has shown that even one hit can narrow the circle of suspects, if you sort it onto the right shelves. The detector keeps listening to the darkness, and every new nudge will either confirm one of the versions or hint that the lock is built quite differently. The thermos in the mine has not run empty yet.
For boosted dark matter with a pseudoscalar-pseudoscalar interaction, 22% of events fall into the 225-271 keV window, where the recorded candidate lies, whereas for the scalar operator only 0.14% do.
The idea of boosted dark matter resembles plots in which invisible particles gain energy from the decay or annihilation of heavier counterparts, as in some stories about dark sectors, but there is no direct reference to a known work of science fiction.