Borexino has been running since May 2007 at the Laboratori Nazionali del Gran Sasso laboratory in Italy with the primary goal of detecting solar neutrinos. The detector, a large, unsegmented liquid ...scintillator calorimeter characterized by unprecedented low levels of intrinsic radioactivity, is optimized for the study of the lower energy part of the spectrum. During Phase-I (2007-2010), Borexino first detected and then precisely measured the flux of the super(7)Be solar neutrinos, ruled out any significant day-night asymmetry of their interaction rate, made the first direct observation of the pep neutrinos, and set the tightest upper limit on the flux of solar neutrinos produced in the CNO cycle (carbon, nitrogen, oxigen) where carbon, nitrogen, and oxygen serve as catalysts in the fusion process. In this paper we discuss the signal signature and provide a comprehensive description of the backgrounds, quantify their event rates, describe the methods for their identification, selection, or subtraction, and describe data analysis. Key features are an extensive in situ calibration program using radioactive sources, the detailed modeling of the detector response, the ability to define an innermost fiducial volume with extremely low background via software cuts, and the excellent pulse-shape discrimination capability of the scintillator that allows particle identification. We report a measurement of the annual modulation of the super(7)Be neutrino interaction rate. The period, the amplitude, and the phase of the observed modulation are consistent with the solar origin of these events, and the absence of their annual modulation is rejected with higher than 99% C.L. The physics implications of Phase-I results in the context of the neutrino oscillation physics and solar models are presented.
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 report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso and the search for a day–night asymmetry of this ...interaction rate. The interaction rate of the 0.862MeV 7Be neutrinos is 46±1.6(stat)−1.6+1.5(syst) counts/(day · 100ton). The hypothesis of no oscillation for this solar neutrinos is inconsistent with our measurement at the 5.8σ C.L. Our result is the first direct measurement of solar neutrinos with an accuracy better than 5%. We report the survival probability for solar νe in the transition region between matter-enhanced and vacuum-driven oscillations. The measured day night asymmetry is Adn=0.001±0.012 (stat)±0.007 (syst), in agreement with the predicition of MSW–LMA neutrino 13 oscillations. This result discourages MSW oscillations with mixing parameters in the LOW region at more than 8.5σ, meaning that this region is, for the first time, strongly rejected without the assumption of CPT symmetry. The result can also be used to constrain some neutrino oscillation scenarios involving new physics.
The rate of neutrino-electron elastic scattering interactions from 862 keV (7)Be solar neutrinos in Borexino is determined to be 46.0±1.5(stat)(-1.6)(+1.5)(syst) counts/(day·100 ton). This ...corresponds to a ν(e)-equivalent (7)Be solar neutrino flux of (3.10±0.15)×10(9) cm(-2) s(-1) and, under the assumption of ν(e) 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 5.0 σ. A global solar neutrino analysis with free fluxes determines Φ(pp)=6.06(-0.06)(+0.02)×10(10) cm(-2) s(-1) and Φ(CNO)<1.3×10(9) cm(-2) s(-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.
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