Improved measurement of solar neutrinos from the Carbon-Nitrogen-Oxygen cycle by Borexino and its implications for the Standard Solar Model

TL;DR

This paper reports an improved measurement of CNO solar neutrinos by Borexino, which constrains solar composition models and provides insights into the Sun's C and N abundances, challenging low metallicity models.

Contribution

The study presents the first measurement of C and N abundances in the Sun using solar neutrinos, and compares these results with different solar metallicity models.

Findings

Measured CNO neutrino interaction rate with 7σ significance.

Disfavours low metallicity solar models at 3.1σ confidence level.

Provides new constraints on solar C and N abundances.

Abstract

We present an improved measurement of the CNO solar neutrino interaction rate at Earth obtained with the complete Borexino Phase-III dataset. The measured rate R$_{\rm CNO}$ = $6.7^{+2.0}_{-0.8}$ counts/(day$ \cdot$ 100 tonnes), allows us to exclude the absence of the CNO signal with about 7$\sigma$ C.L. The correspondent CNO neutrino flux is $6.6^{+2.0}_{-0.9} \times 10^8$ 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 $N_{\rm CN}$ = $(5.78^{+1.86}_{-1.00})\times10^{-4}$ displays a $\sim$2$\sigma$ tension with the "low metallicity" spectroscopic photospheric measurements. On the other hand, our result used together with the $^7$Be and $^8$B solar neutrino fluxes, also measured by Borexino, permits to disfavour at 3.1$\sigma$ C.L. the "low metallicity" SSM B16-AGSS09met as an alternative to the "high metallicity" SSM B16-GS98.