Improved Measurement of Solar Neutrinos from the Carbon-Nitrogen-Oxygen Cycle by Borexino and Its Implications for the Standard Solar Model

We present an improved measurement of the carbon-nitrogen-oxygen (CNO) solar neutrino interaction rate at Earth obtained with the complete Borexino Phase-III dataset. The measured rate, R-CNO = 6.7(-0.8)(+2.0) counts/(day x 100 tonnes), allows us to exclude the absence of the CNO signal with about 7 sigma C.L. The correspondent CNO neutrino flux is 6.6(-0.9)(+2.0) x 10(8) cm-(2) s(-1), taking into account the neutrino flavor conversion. We use the new CNO measurement to evaluate the C and N abundances in the Sun with respect to the H abundance for the first time with solar neutrinos. Our result of N-CN = (5.78(-1.00)(+1.86)) x 10(-4) displays a similar to 2 sigma tension with the "low-metallicity" spectroscopic photospheric measurements. Furthermore, our result used together with the Be-7 and B-8 solar neutrino fluxes, also measured by Borexino, permits us to disfavor at 3.1 sigma C.L. the "low-metallicity" standard solar model B16-AGSS09met as an alternative to the "high-metallicity" standard solar model B16-GS98.