In the scorching plasma of the solar interior, particles are born that barely interact with matter. One hypothesis suggests these are axion-like particles — light and elusive. Usually, their production is explained by the Primakoff effect: a photon in the electric field of a nucleus turns into an axion. But there is another path.

Transverse plasmons, collective oscillations of electrons in plasma, can resonantly convert into axions in the Sun's magnetic field. This mechanism predicts a particle flux that, for certain masses, is dozens of times larger than the Primakoff flux. The ANAIS-112 experiment, located in the Canfranc Underground Laboratory in Spain, has tested this hypothesis for the first time. The detector, containing 112.5 kg of sodium iodide, accumulated 625.75 kg×years of data, looking for faint annual modulations in the signal.

In the energy range 1–20 keV, no significant modulation was found. This allowed setting an upper limit on the axion-photon coupling constant: g_aγ < 1.32×10⁻⁹ GeV⁻¹ (90% confidence level) for a mass around 150 eV. For the first time, the constraints account for the contribution of the resonant channel, which dominates for masses 35–280 eV.

This result is not just a line in the record books. It narrows the search for dark matter particles and refines models of stellar physics. The silence in the detector speaks louder than any signal.