How well is the sub-100 MeV atmospheric neutrino flux known?
In plain words
For heavy dark matter, the main neutrino background comes from cosmic rays hitting the atmosphere and producing low-energy neutrinos, whose number is poorly measured. Better knowledge of this flux decides how deep heavy dark matter searches can go.
Precise statement
The CEvNS background for WIMP masses above about $100\,\mathrm{GeV}$ comes from atmospheric neutrinos with $E_\nu$ below about $100\,\mathrm{MeV}$, where the flux has never been measured directly and model uncertainties are of order 20 percent. The quantity is this flux, with its site and solar-cycle dependence, to about 10 percent precision. An answer is a measurement or a validated calculation of the sub-$100\,\mathrm{MeV}$ atmospheric $\nu_e$ and $\nu_\mu$ fluxes at underground laboratory sites.
What would settle it
A measurement of low-energy atmospheric neutrinos in a large detector (e.g., JUNO, Super-Kamiokande with gadolinium, DUNE) or by the first atmospheric CEvNS detection in a dark matter detector.
Status in the literature
Unverified note
Fan, Liu and Zhou (Chin. Phys. C 49, 103001, 2025) estimated the atmospheric flux at CJPL to be about 30 percent lower than at LNGS, so the fog boundary for heavy WIMPs is site dependent.