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Register a new userScience and Technology of BOREXINO: A Real Time Detector for Low Energy Solar Neutrinos SOLAR NEUTRINOS
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BOREXINO, a real-time device for low energy neutrino spectroscopy is nearing completion of construction in the underground laboratories at Gran Sasso, Italy (LNGS). The experiments goal is the direct measurement of the flux of 7Be solar neutrinos of all flavors via neutrino-electron scattering in an ultra-pure scintillation liquid. Seeded by a series of innovations which were brought to fruition by large scale operation of a 4-ton test detector at LNGS, a new technology has been developed for BOREXINO. It enables sub-MeV solar neutrino spectroscopy for the first time. This paper describes the design of BOREXINO, the various facilities essential to its operation, its spectroscopic and background suppression capabilities and a prognosis of the impact of its results towards resolving the solar neutrino problem. BOREXINO will also address several other frontier questions in particle physics, astrophysics and geophysics.
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The study of solar neutrinos has given since ever a fundamental contribution both to astroparticle and to elementary particle physics, offering an ideal test of solar models and offering at the same time relevant indications on the fundamental interactions among particles. After reviewing the striking results of the last two decades, which were determinant to solve the long standing solar neutrino puzzle and refine the Standard Solar Model, we focus our attention on the more recent results in this field and on the experiments presently running or planned for the near future. The main focus at the moment is to improve the knowledge of the mass and mixing pattern and especially to study in detail the lowest energy part of the spectrum, which represents most of solar neutrino spectrum but is still a partially unexplored realm. We discuss this research project and the way in which present and future experiments could contribute to make the theoretical framemork more complete and stable, understanding the origin of some anomalies that seem to emerge from the data and contributing to answer some present questions, like the exact mechanism of the vacuum to matter transition and the solution of the so called solar metallicity problem.
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 ($bar{ u}_e$) are detected in an organic liquid scintillator through the inverse $beta$-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 $bar{ u}_e$ fluxes in the previously unexplored region below 8 MeV. A search for $bar{ u}_e$ in the solar neutrino flux is also presented: the presence of $bar{ u}_e$ 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 $bar{ u}_e$ flux of 384 cm$^{-2}$s$^{-1}$ (90% C.L.), assuming an undistorted solar $^{8}$B neutrinos energy spectrum, that corresponds to a transition probability $p_{ u_e rightarrow bar u_{e}}<$ 7.2$times$10$^{-5}$ (90% C.L.) for E$_{bar { u}_e}$ $>$ 1.8 MeV. At lower energies, by investigating the spectral shape of elastic scattering events, we obtain a new limit on solar $^{7}$Be-$ u_e$ conversion into $bar{ u}_e$ of $p_{ u_e rightarrow bar u_{e}}<$ 0.14 (90% C.L.) at 0.862 keV. 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 flares 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.
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 detectors 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 Bi-210 beta decays originating in the intrinsic contamination of the liquid scintillator with Pb-210. Assuming the CNO flux predicted by the high-metallicity Standard Solar Model and an exposure of 1000 daysx71.3 t, Borexino has a median sensitivity to CNO neutrino higher than 3 sigma. With the same hypothesis the expected experimental uncertainty on the CNO neutrino flux is 23%, provided the uncertainty on the independent estimate of the Bi-210 interaction rate is 1.5 cpd/100t. 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.
Liquid scintillator detectors play a central role in the detection of neutrinos from various sources. In particular, it is the only technique used so far for the precision spectroscopy of sub-MeV solar neutrinos, as demonstrated by the Borexino experiment at the Gran Sasso National Laboratory in Italy. The benefit of a high light yield, and thus a low energy threshold and a good energy resolution, comes at the cost of the directional information featured by water Cherenkov detectors, measuring $^8$B solar neutrinos above a few MeV. In this paper we provide the first directionality measurement of sub-MeV solar neutrinos which exploits the correlation between the first few detected photons in each event and the known position of the Sun for each event. This is also the first signature of directionality in neutrinos elastically scattering off electrons in a liquid scintillator target. This measurement exploits the sub-dominant, fast Cherenkov light emission that precedes the dominant yet slower scintillation light signal. Through this measurement, we have also been able to extract the rate of $^{7}$Be solar neutrinos in Borexino. The demonstration of directional sensitivity in a traditional liquid scintillator target paves the way for the possible exploitation of the Cherenkov light signal in future kton-scale experiments using liquid scintillator targets. Directionality is important for background suppression as well as the disentanglement of signals from various sources.
Borexino collaboration reported about first measurement of solar CNO-$ u$ interaction rate in Borexino detector. This result is consistent with Hydridic Earth model prediction about the contribution of $^{40}$K geo-antineutrino interactions in single Borexino events. The potassium abundance in the Earth in the range $1 div 1.5$% of the Earth mass could give the observed enhancement of counting rate above expected CNO-$ u$ counting rate. The Earth intrinsic heat flux must be in the range $200 div 300$ TW for this potassium abundance. This value of the heat flux can explain the ocean heating observed by the project ARGO. We consider that Hydridic Earth model actually corresponds better to CNO-$ u$ Borexino results than Silicate Earth model.
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