The Borexino collaboration announced the direct measurement of the 7Be solar neutrino flux with total uncertainty smaller than 5%. The day/night asymmetry of the 7Be solar neutrino flux was also measured with a total experimental uncertainty of 1%.
The new Borexino results are the most precise measurement of low energy solar neutrinos to date, and they represent an important step in understanding the phenomenon of neutrino oscillations, one of the main discoveries in elementary particles physics of the past decade. These observations also provide a sharper image of the workings of nuclear fusion reactions that sustain the Sun.
Borexino is a liquid scintillator unsegmented detector, running at the Gran Sasso underground Laboratories (LNGS) in the Italian Appennines. Thanks to its unprecedented low level of radioactive contamination, Borexino currently is the only experiment able to perform a real time measurement of solar neutrino interactions below few MeV.
The impact of the latest Borexino measurements is extremely relevant both in solar physics, in connection with the understanding of Sun-like stars, and in neutrino physics. In particular, the precision measurement of the 7Be solar neutrino flux allows a real time investigation of neutrino oscillations below few MeV and provides a unique opportunity to probe and validate the currently favored neutrino oscillation paradigm in the so far untested vacuum regime.
Furthermore, these new results, together with a previous Borexino measurement of the 8B solar neutrino flux, single out the Large Mixing Angle (LMA) region of the neutrino oscillation parameter space at the confidence level of >8.5 sigma, without including the data from the KamLAND antineutrino reactor experiment in a combined fit, i.e. with no need to rely on CPT conservation in fundamental particle interactions. This outcome is especially important in view of the recent experimental hints of possible differences between the oscillation parameters of neutrino and antineutrino (still to be confirmed). The independent determination of the LMA solution obtained by Borexino with neutrinos only is thus key in reinforcing the consistency of our understanding of neutrino oscillations.
Borexino is scheduled to continue data taking for the coming years and to further its investigation of solar neutrinos over the entire solar neutrino energy spectrum.
The international Borexino Collaboration involves institutions from Italy, US, Germany, Russia, Poland and France:
I.N.F.N. Laboratori Nazionale del Gran Sasso e sezioni di Genova, Milano, Dipartimento di Chimica dell'Universita' di Perugia, Italy
AstroParticle & Cosmology Laboratory (APC), France
J.I.N.R. Dubna, Russia, Kurchatov Institute - Moscow, Lomonosov Moscow State University - Skobeltsyn Institute of Nuclear Physics, Russian Academy Of Sciences B.P.Konstantinov Petersburg Nuclear Physics Institute, Russian federation
Max-Planck-Institut fuer Kernphysik Heidelberg, Technische Universitaet Muenchen, University of Hamburg, Germany
M. Smoluchowski Institute of Physics Jagellonian University Krakow, Poland
Princeton University, Department of Physics, University of Massachusetts, Amherst Department of Physics
Virginia Polytechnical Institute U. S. A.
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LNGS Head of Communication Unit
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The Borexino Experiment:
The main goal of Borexino is the measurement of the Be-7 solar neutrinos, a central feature in the on-going story of solar neutrinos and neutrino oscillations. Borexino will accomplish this goal by detecting neutrino-electron scattering events taking place in real time in its well shielded 100 tonnes mass fiducial volume.
The detector is an unsegmented liquid scintillator featuring 300 tonnes of sensitive mass, viewed by 2,200 photomultipliers, for the detection of the energy released by the neutrino-induced recoil electrons. The detector core is a transparent nylon spherical vessel (125 µm thick), 8.5 m of diameter, filled with liquid scintillator and surrounded by 1,000 tonnes of high-purity shielding buffer liquid. The scintillator mixture is PC (1-2-4,trimethylbenzene) and PPO (2,5 diphenyl oxazole, 1.5g/l) as a fluor, while the buffer liquid is PC with the addition of DMP as light quencher. The photomultipliers are supported by a stainless steel sphere, which also separates the inner part of the detector from an additional external shielding, provided by 2,400 tons of purified water. This outermost water shield is instrumented with 200 outward-pointing photomultipliers serving as a veto for penetrating muons, the only significant residual cosmic background at the 3800 mwe (meters of water equivalent) depth of the underground laboratory.