We report on the search for anti-neutrinos of yet unknown origin with the Borexino detector at the Laboratori Nazionali del Gran Sasso. In particular, a hypothetical anti-neutrino flux from the Sun ...is investigated. Anti-neutrinos are detected through the neutron inverse β decay reaction in a large liquid organic scintillator target. We set a new upper limit for a hypothetical solar ν¯e flux of 760 cm−2s−1, obtained assuming an undistorted solar 8B energy spectrum. This corresponds to a limit on the transition probability of solar neutrinos to anti-neutrinos of 1.3×10−4 (90% C.L.) for Eν¯>1.8 MeV, covering the entire 8B spectrum. Best differential limits on anti-neutrino fluxes from unknown sources are also obtained between the detection energy threshold of 1.8 MeV and 17.8 MeV with more than 2 years of data.
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
. 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
; 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.
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
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.
A search for the solar neutrino effective magnetic moment has been performed using data from 1291.5 days exposure during the second phase of the Borexino experiment. No significant deviations from ...the expected shape of the electron recoil spectrum from solar neutrinos have been found, and a new upper limit on the effective neutrino magnetic moment of μνeff<2.8×10−11 μB at 90% C.L. has been set using constraints on the sum of the solar neutrino fluxes implied by the radiochemical gallium experiments. Using the limit for the effective neutrino moment, new limits for the magnetic moments of the neutrino flavor states, and for the elements of the neutrino magnetic moments matrix for Dirac and Majorana neutrinos, are derived.
In the core of the Sun, energy is released through sequences of nuclear reactions that convert hydrogen into helium. The primary reaction is thought to be the fusion of two protons with the emission ...of a low-energy neutrino. These so-called pp neutrinos constitute nearly the entirety of the solar neutrino flux, vastly outnumbering those emitted in the reactions that follow. Although solar neutrinos from secondary processes have been observed, proving the nuclear origin of the Sun's energy and contributing to the discovery of neutrino oscillations, those from proton-proton fusion have hitherto eluded direct detection. Here we report spectral observations of pp neutrinos, demonstrating that about 99 per cent of the power of the Sun, 3.84 × 10(33) ergs per second, is generated by the proton-proton fusion process.
This paper presents a comprehensive geoneutrino measurement using the Borexino detector, located at Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The analysis is the result of 3262.74 days of ...data between December 2007 and April 2019. The paper describes improved analysis techniques and optimized data selection, which includes enlarged fiducial volume and sophisticated cosmogenic veto. The reported exposure of (1.29±0.05)×1032 protons ×year represents an increase by a factor of two over a previous Borexino analysis reported in 2015. By observing 52.6−8.6+9.4(stat)−2.1+2.7(sys) geoneutrinos (68% interval) from U238 and Th232, a geoneutrino signal of 47.0−7.7+8.4(stat)−1.9+2.4(sys) TNU with −17.2+18.3% total precision was obtained. This result assumes the same Th/U mass ratio as found in chondritic CI meteorites but compatible results were found when contributions from U238 and Th232 were both fit as free parameters. Antineutrino background from reactors is fit unconstrained and found compatible with the expectations. The null-hypothesis of observing a geoneutrino signal from the mantle is excluded at a 99.0% C.L. when exploiting detailed knowledge of the local crust near the experimental site. Measured mantle signal of 21.2−9.0+9.5(stat)−0.9+1.1(sys) TNU corresponds to the production of a radiogenic heat of 24.6−10.4+11.1 TW (68% interval) from U238 and Th232 in the mantle. Assuming 18% contribution of K40 in the mantle and 8.1−1.4+1.9 TW of total radiogenic heat of the lithosphere, the Borexino estimate of the total radiogenic heat of the Earth is 38.2−12.7+13.6 TW, which corresponds to the convective Urey ratio of 0.78−0.28+0.41. These values are compatible with different geological predictions, however there is a ∼2.4σ tension with those Earth models which predict the lowest concentration of heat-producing elements in the mantle. In addition, by constraining the number of expected reactor antineutrino events, the existence of a hypothetical georeactor at the center of the Earth having power greater than 2.4 TW is excluded at 95% C.L. Particular attention is given to the description of all analysis details which should be of interest for the next generation of geoneutrino measurements using liquid scintillator detectors.
The rate of neutrino-electron elastic scattering interactions from 862 keV Be7 solar neutrinos in Borexino is determined to be 46.0±1.5(stat)-1.6+1.5(syst)counts/(day*100ton). This corresponds to a ...νe-equivalent Be7 solar neutrino flux of (3.10±0.15)×109cm-2s-1 and, under the assumption of νe transition to other active neutrino flavours, yields an electron neutrino survival probability of 0.51±0.07 at 862 keV. The no flavor change hypothesis is ruled out at 5.0σ. A global solar neutrino analysis with free fluxes determines Φpp=6.06-0.06+0.02×1010cm-2s-1 and ΦCNO<1.3×109cm-2s-1 (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.
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×100 tonnes), allows us to exclude the absence of the CNO signal with about 7σ C.L. The correspondent CNO neutrino flux is 6.6_{-0.9}^{+2.0}×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})×10^{-4} displays a ∼2σ tension with the "low-metallicity" spectroscopic photospheric measurements. Furthermore, our result used together with the ^{7}Be and ^{8}B solar neutrino fluxes, also measured by Borexino, permits us to disfavor at 3.1σ C.L. the "low-metallicity" standard solar model B16-AGSS09met as an alternative to the "high-metallicity" standard solar model B16-GS98.
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