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.
The solar neutrino experiment Borexino, which is located in the Gran Sasso underground laboratories, is in a unique position to study muon-induced backgrounds in an organic liquid scintillator. In ...this study, a large sample of cosmic muons is identified and tracked by a muon veto detector external to the liquid scintillator, and by the specific light patterns observed when muons cross the scintillator volume. The yield of muon-induced neutrons is found to be Y sub(n) = (3.10 + or - 0.11) times 10 super(-4) sub(n)/( mu times (g/cm super(2))). The distance prole between the parent muon track and the neutron capture point has the average value lambda = (81.5 + or - 2.7) cm. Additionally the yields of a number of cosmogenic radioisotopes are measured for super(12)N, super(12)B, super(8)He, super(9)C, super(9)Li, super(8)B, super(6)He, super(8)Li, super(11)Be, super(10)C and super(11)C. All results are compared with Monte Carlo simulation predictions using the Fluka and GEANT4 packages. General agreement between data and simulation is observed for the cosmogenic production yields with a few exceptions, the most prominent case being super(11)C yield for which both codes return about 50% lower values. The predicted mu -n distance prole and the neutron multiplicity distribution are found to be overall consistent with data.
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.
Borexino is a liquid scintillator detector sited underground in the Laboratori Nazionali del Gran Sasso (Italy). Its physics program, until the end of this year, is focussed on the study of solar ...neutrinos, in particular from the Beryllium, pp, pep and CNO fusion reactions. Knowing the reaction chains in the sun provides insights towards physics disciplines such as astrophysics (star physics, star formation, etc.), astroparticle and particle physics. Phase II started in 2011 and its aim is to improve the phase I results, in particular the measurements of the neutrino fluxes from the pep and CNO processes. By the end of this year, data taking from the sun will be over and a new project is scheduled to launch: Short distance Oscillation with boreXino (SOX), which uses a Cerium source for neutrinos (100÷150 kCi of activity) and aims to confirm or rule out the presence of sterile neutrinos. This particle is hypothesised to justify the reactor, Gallium and LSND anomalies found and can reject extensions to the standard model. The work presented is a summary of the solar neutrino results achieved so far, which lead not only to a precise study of the processes in the sun, but also to more Standard Model oriented measurements (such as the stability of the charge, i.e. the life time of the electron). Furthermore, the perspectives of the SOX program are discussed showing the experiment sensitivity to a fourth neutrino state covering almost entirely 3σ of the preferred region of the anomalous neutrino experiments, and additional applications of the detector such as the study of geo-neutrinos.
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.
The Borexino experiment located in the Gran Sasso National Laboratory, is an organic liquid scintillator detector conceived for the real time spectroscopy of low energy solar neutrinos. The phase-I ...of the data taking campaign (2007 – 2010) has allowed the first independent measurements of 7Be and pep solar neutrino fluxes as well as the first measurement of anti-neutrinos from the Earth. After a purification of the scintillator, Borexino is now in phase-II since 2011. Thanks to the unprecedented background levels, we have performed the first flux measurement of neutrinos from the fundamental pp reaction which powers the Sun. We review this breakthrough result and other recent results, including the latest review of our terrestrial neutrino analysis. We also discuss the upcoming measurements on middle energy solar neutrino spectral components (pep, CNO) and the new project SOX devoted to the study of sterile neutrinos via the use of a neutrino source placed in close proximity of the detector’s active material.
We report on searches for neutrinos and antineutrinos from astrophysical sources performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso in Italy. Electron antineutrinos (ν¯e) ...are detected in an organic liquid scintillator through the inverse β-decay reaction. In the present work we set model-independent upper limits in the energy range 1.8–16.8 MeV on neutrino fluxes from unknown sources that improve our previous results, on average, by a factor 2.5. Using the same data set, we first obtain experimental constraints on the diffuse supernova ν¯e fluxes in the previously unexplored region below 8 MeV. A search for ν¯e in the solar neutrino flux is also presented: the presence of ν¯e would be a manifestation of a non-zero anomalous magnetic moment of the neutrino, making possible its conversion to antineutrinos in the strong magnetic field of the Sun. We obtain a limit for a solar ν¯e flux of 384 cm–2 s–1 (90% C.L.), assuming an undistorted solar 8B neutrinos energy spectrum, that corresponds to a transition probability pνe→ν¯e< 7.2 × 10–5 (90% C.L.) for Eν¯e > 1.8 MeV. At lower energies, by investigating the spectral shape of elastic scattering events, we obtain a new limit on solar 7Be-νe conversion into ν¯e of pνe→ν¯e< 0.14 (90% C.L.) at 0.862 MeV. Last, we investigate solar flares as possible neutrino sources and obtain the strongest up-to-date limits on the fluence of neutrinos of all flavor neutrino below 3–7 MeV. Assuming the neutrino flux to be proportional to the flare’s intensity, we exclude an intense solar flare as the cause of the observed excess of events in run 117 of the Cl-Ar Homestake experiment.
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