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Title: How well is the sub-100 MeV atmospheric neutrino flux known?
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status 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.
Posed since: (empty)