First Experimental Limit on the Thermal Solar Neutrino Flux

Abstract: The neutrino sky below 165\,keV is yet to be explored. This region provides a unique probe of stellar cooling mechanisms through the detection of thermal solar neutrinos and the low-energy tail of the $pp$ solar cycle. Here, we investigate prospects for probing this regime via neutrino capture on tritium. Analyzing KATRIN public data, we set the first experimental bound on the thermal solar neutrino flux $Φ/Φ{\mathrm{SSM}} < 1.86 \times 10^{18}$ at 95\%~CL ($1.58\times10^{18}$ at 90\%~CL), and show that a $100\;\text{kg}\cdot\text{yr}$ exposure would constrain the thermal solar neutrino component to $Φ/Φ{\mathrm{SSM}} \lesssim 10^4$ and detect the low-energy $pp$ flux at the Standard Solar Model (SSM) level. Neutrino--electron elastic scattering from $pp$ cycle neutrinos are identified as an irreducible background for neutrino capture searches.