Dark Weather over Gran Sasso: Daily Rhythm of Sub-GeV Dark Matter ✛

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At nine in the morning, the DarkSide-50 detector, 1400 meters beneath Gran Sasso, records a weak ionization pulse. By eleven at night, the frequency of such events drops noticeably. This is not an electronics glitch or a seasonal background variation.

This is how 'dark weather' manifests — a daily modulation caused by the scattering of dark matter particles inside the planet. A galactic wind of particles with masses tens of times smaller than a proton streams through the Solar System. Nothing like this exists in the Standard Model. As particles fly through Earth, they collide with nuclei. They lose energy. They change trajectory. For an underground detector, morning particles arrive almost directly, having passed through only a few hundred kilometers of rock. In the evening, the path lengthens to the full diameter of the planet—12 700 km. The flux weakens. Such a filter encodes information about the probability of nuclear scattering (cross section $σ_{SI}$_χn), while the detection itself in an argon target relies on ionization — interaction with electrons (cross section $σ_e$).

Numerical simulation (the DaMaSCUS package for Monte Carlo calculations) reproduced the distorted velocity distributions for different isoangles — angles between the detector and the galactic wind. The shape of the daily distribution is sensitive to the nuclear cross section. Analysis of DarkSide-50 data without time stamps gave an upper limit $σ_{SI}$_χn < 10⁻³³ cm² for masses 20–100 MeV. This is comparable to the best world constraints. Future detectors XENONnT and LZ with multi-year exposure will improve sensitivity by an order of magnitude. A network of setups in different hemispheres, like weather stations, will track phase and amplitude, confirming the reality of the signal.

Dark weather is not just a metaphor. It's a way to read the properties of invisible matter. Earth itself runs the experiment every day. Changing its perspective, it modulates the flux. And one day, the morning signal will tell us not only about the 'weather' underground, but about the fundamental laws of the Universe.