Deep underground, in the silence of a cryostat (a cooled container), lie crystals of lead that once belonged to an ancient Roman ship. This material barely emits background radiation — over two thousand years, unstable isotopes have decayed, leaving it perfectly clean for hunting the rarest events. The RES-NOVA experiment was built to catch the neutrino response from a supernova explosion in our Galaxy. But the same crystals turned out to be an excellent trap for solar axions — light, invisible particles that no one has ever seen.

Axions are born in the Sun's hot plasma through several pathways. For instance, a photon can convert into an axion in the electric field of a nucleus: this is called the Primakoff effect. Or an electron, when decelerating, can emit such a particle. In a lead tungstate (PbWO4) crystal, the reverse process occurs: an axion is absorbed, transferring energy to an electron or producing a new photon. This creates a signal — a tiny temperature rise in the detector, which an ultrasensitive bolometer (a device for measuring minuscule heat flows) can detect.

Analysis of the first data from the 13-gram prototype already yielded limits on the axion-photon coupling strength. And this was done without complex background models, using a simple statistical method. In the future, with a ton-year exposure (a measure of accumulated data), RES-NOVA could simultaneously probe three different axion coupling channels: to photons, electrons, and nucleons. Thus, ancient lead becomes a bridge between astrophysics and particle physics.