This paper reports the first experimental bound on the flux of thermal solar neutrinos — very low‑energy neutrinos from the Sun — using public data from the KATRIN experiment. The authors looked for neutrinos with energies between roughly 0.1 eV and 10 keV and set an upper limit on the thermal solar neutrino flux of Φ/Φ_SSM < 1.86 × 10^18 at 95% confidence (1.58 × 10^18 at 90% confidence). Here Φ_SSM is the prediction from the Standard Solar Model, the usual theoretical reference for solar neutrino rates.

The detection idea they study is neutrino capture on tritium, a simple reaction ν_e + 3H → 3He + e^−. A captured neutrino shows up as an electron whose kinetic energy is the usual beta‑decay endpoint Q_β (18.591 keV) plus the neutrino energy. The team used KATRIN public spectra above a retarding energy of 18,576 eV and fit for an extra contribution from solar thermal neutrinos. They performed a Bayesian fit using the PyMC statistical tool and included detector efficiency factors (transport 0.5, geometric acceptance 0.18, and a 0.43 fraction for the molecular final state). Those inputs led to the stated experimental limits over the 0.1 eV–10 keV band.

The authors also estimate what future tritium experiments might do. For a very large exposure of about 100 kilogram·years, a tritium capture experiment could constrain the thermal solar neutrino component to about Φ/Φ_SSM ≲ 10^4 and could detect the low‑energy tail of the ordinary pp (proton–proton) solar neutrino flux at the Standard Solar Model level. For comparison, their plots use a reference 100 gram tritium target to show how the expected capture rates change with energy.

This work matters because thermal solar neutrinos are produced by a variety of thermal processes in stellar cores and carry information about stellar cooling, the Sun’s core temperature, and its chemical composition. Until now the low‑energy neutrino sky below 165 keV was essentially unexplored; the lowest energy solar neutrinos previously measured were pp neutrinos above 165 keV by Borexino. Probing the sub‑165 keV range would open a new window on solar and stellar physics.