In 1987, astronomers saw a supernova flare up in a neighboring galaxy — the Large Magellanic Cloud. It was a rare gift: the explosion was close enough to catch not just light but also neutrinos, those nearly undetectable particles that pass straight through Earth. Since then, data from that explosion have been used as a filter: if some hypothetical particle could carry away too much energy from a dying star, its properties get constrained.
One such particle is the axion — a dark matter candidate that is light and almost intangible. The mass range above one electronvolt, known as the 'Turner window,' took a particular hit. Three lines of defense seemed to seal it shut: the very fact that the star cooled, the absence of extra photons in Japan's Kamioka II detector, and the absence of neutrons from solar axions at the Sudbury Observatory. But the authors recalculated how strongly axions are absorbed in the supernova's interior, and it turns out the second line, the missing photons, has weakened considerably. The Turner window has cracked open.
Now scientists have a new hunting plan. In the depths of carbon stars, sodium-23 is synthesized, acting like a heat pump: it transfers stellar energy into axions. These stream toward Earth with an energy of 440 kiloelectronvolts, just right to be caught by a sodium iodide (NaI) crystal. The same sodium serves as both target and detector: when it absorbs an axion, it emits a gamma-ray photon that is immediately registered. Facilities built to search for dark matter, like DAMA/LIBRA, are already standing and waiting: they just need to be pointed at this new signal. Two years of data collection could probe a wide range of interactions, including QCD axions with masses above ten electronvolts.
An NaI crystal is essentially table salt grown in a lab. The same stuff you have in your salt shaker, but ultra-pure and brick-sized.
In Greg Egan's novel Schild's Ladder, new particles born in the hearts of stars change the physics of the entire universe.