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For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton–proton ( pp ) chain and the carbon–nitrogen–oxygen (CNO) cycle 1 , 2 . Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously 3 ; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal—only a few counts per day above the background per 100 tonnes of target—that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate—as well as the flux of emitted CNO neutrinos—depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe. Direct experimental evidence of the carbon–nitrogen–oxygen fusion cycle in the Sun is provided by the detection of neutrinos emitted during this process.

We evaluate the diffuse gamma-ray flux at TeV energies produced by hadronic interactions of cosmic rays with the gas contained in the galactic disk. We consider different assumptions for the cosmic ray distribution, including the recently emerged possibility of a harder cosmic ray spectrum in the inner Galaxy. We show that observational data provided by Argo-YBJ, HESS, HAWC and Milagro, can already discriminate among different hyphoteses. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.

We describe a new generation of standard solar models (SSMs), Barcelona 2016 or B16 for short, that includes recent updates on some important nuclear reaction rates, a more consistent treatment of the equation of state and a novel and flexible treatment of opacity uncertainties. Two large sets of SSMs, each based on a different canonical set of solar abundances with high and low metallicity, are calculated and compared with different ensembles of solar observables including solar neutrinos, surface helium abundance, depth of convective envelope and sound speed profile.

Neutrinos emitted in the carbon, nitrogen, oxygen (CNO) fusion cycle in the Sun are a sub-dominant, yet crucial component of solar neutrinos whose flux has not been measured yet. The Borexino experiment at the Laboratori Nazionali del Gran Sasso (Italy) has a unique opportunity to detect them directly thanks to the detector’s radiopurity and the precise understanding of the detector backgrounds. We discuss the sensitivity of Borexino to CNO neutrinos, which is based on the strategies we adopted to constrain the rates of the two most relevant background sources, pep neutrinos from the solar pp -chain and 210 Bi beta decays originating in the intrinsic contamination of the liquid scintillator with 210 Pb. Assuming the CNO flux predicted by the high-metallicity Standard Solar Model and an exposure of 1000 days × 71.3 t, Borexino has a median sensitivity to CNO neutrino higher than 3 σ . With the same hypothesis the expected experimental uncertainty on the CNO neutrino flux is 23%, provided the uncertainty on the independent estimate of the 210 Bi  interaction rate is 1.5 cpd / 100 ton  . Finally, we evaluated the expected uncertainty of the C and N abundances and the expected discrimination significance between the high and low metallicity Standard Solar Models (HZ and LZ) with future more precise measurement of the CNO solar neutrino flux.

We present the predictions of updated standard solar models and we briefly discuss the solar composition problem, i.e. the conflict between helioseismology and standard solar models implementing the state-of-the-art photospheric abundances, emphasizing the importance of measuring neutrinos produced in the CNO cycle for its comprehension.

We analyze the subset of high energy neutrino events observed by IceCube above 60 TeV, combined with the information provided by passing muons, aiming to probe the flavor of cosmic neutrinos. First, we compare the observed track-to-shower ratio with the predictions for a cosmic neutrino population, taking into account the different production mechanisms and the uncertainties due to neutrino oscillations. Our results corroborate the hypotheses that cosmic neutrinos have been seen. In addition, we show that the possibility of neutrinos decay is disfavored at about 2σ level of significance for both the normal and inverted neutrino mass hierarchy.

About 99 per cent of solar energy is produced through sequences of nuclear reactions that convert hydrogen into helium, starting from the fusion of two protons (the pp chain). The neutrinos emitted by five of these reactions represent a unique probe of the Sun’s internal working and, at the same time, offer an intense natural neutrino beam for fundamental physics. Here we report a complete study of the pp chain. We measure the neutrino–electron elastic-scattering rates for neutrinos produced by four reactions of the chain: the initial proton–proton fusion, the electron-capture decay of beryllium-7, the three-body proton–electron–proton (pep) fusion, here measured with the highest precision so far achieved, and the boron-8 beta decay, measured with the lowest energy threshold. We also set a limit on the neutrino flux produced by the ³ He–proton fusion (hep). These measurements provide a direct determination of the relative intensity of the two primary terminations of the pp chain (pp-I and pp-II) and an indication that the temperature profile in the Sun is more compatible with solar models that assume high surface metallicity. We also determine the survival probability of solar electron neutrinos at different energies, thus probing simultaneously and with high precision the neutrino flavour-conversion paradigm, both in vacuum and in matter-dominated regimes.

We briefly review the physics of Big Bang Nucleosynthesis (BBN), discussing theoretical and observational uncertainties in primordial abundances determinations. We present, moreover, some recent results on active-sterile neutrino oscillations in the early universe and on their effects on BBN.