Measurement of solar $pp$ neutrino flux with the new PandaX-4T data

We report a new measurement of the solar proton–proton ($pp$) neutrino flux via neutrino–electron elastic scattering using the PandaX-4T Run 2 data set collected between 2024 and 2026, corresponding to an exposure of 1.9 tonne$\cdot$yr. Before Run 2 data taking, the detector underwent a series of upgrades to improve its response and background conditions. Time variations of radioactive noble-gas impurities are constrained using the physics data themselves, complemented by measurements from the gas-assay system. The analysis introduced improvements in the data processing chain, detector response characterization, and background models. A blind spectral analysis was then performed on the electronic-recoil data across a wide energy range from 20 to 1000 keV. In combination with the Run 0 data published earlier, the fitted $pp$ flux is $(8.5\pm 3.5)\times {10}^{10}$ $\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, consistent with the prediction of the Standard Solar Model. With a statistical significance of $2.2\sigma$ above background, this marks the first positive indication of solar $pp$ neutrino–electron scattering below an electronic-recoil energy of 165 keV.