C. Galbiati, M. Misiaszek and N. Rossi

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http://dx.doi.org/10.1140/epja/i2016-16086-1

Abstract

In this report we describe the unique capabilities of the α/β discrimination of the Borexino experiment. This capability is the direct result of years of development aimed at the design of an experiment that could withstand contamination from α-emitting nuclides. The combination of the excellent α/β discrimination and of the excellent radiopurity of the detector permitted to extract information on the solar neutrino interactions in Borexino without interference from α particles.

About half of the neutrinos detected from natural, underground sources come from the Earth’s mantle, rather than the crust, according to an analysis of new neutrino detection data.

“For those of us in the field, this is very impressive progress,” says Jason Detwiler of the University of Washington in Seattle, a member of the KamLAND group. “Their spectrum is very clean and beautifully and incontrovertibly demonstrates the presence of the geoneutrino signal.” Detwiler says that the number of mantle neutrinos seen by Borexino may have geophysical significance. ”The data are consistent with there being enough radiogenic heat to drive mantle convection,” he says, referring to the slow turnover of mantle material over geologic time.

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1. Research project objectives/ Research hypothesis

The scientific goal of this project is the search for signals on non-standard particle physics by looking for neutrino oscillations into sterile neutrinos. It is proposed to realize an experiment sensitive to a large fraction of the parameter space for short distance neutrino flavor oscillations into sterile components. The experiment, named SOX (Short distance Oscillations with boreXino), aims at the complete confirmation or at a clear disproof of the so called neutrino anomalies, a set of circumstantial evidences of electron neutrino disappearance observed at LSND, MiniBoone, with nuclear reactors and with solar neutrino Gallium detectors. The SOX project has been awarded by ERC Advanced Grant (Project Reference: 320873).

If new generations of particles do exist, it is plausible that new neutrino species exist as well, and mixing with known active neutrinos might occur. This scenario, though speculative, is supported by a set of non-conclusive but very significant experimental evidences coming both from particle physics and from cosmology. On one side, several experiments probing neutrino and anti-neutrino oscillations at relatively small values of L/E have observed a systematic lack of events in their detectors, a fact which might be explained in terms of neutrino oscillations into an unknown very weakly interacting component. On the other side, Cosmic Microwave Background Radiation (CMBR) experiments and Large Scale Structure (LSS) studies, when interpreted in the framework of standard cosmology, indicate that the total number of neutrinos in the Universe might be bigger than three.

2. Research project methodology

The BOREXINO detector, originally designed for precision study of low energy solar neutrinos and for the detection of anti-neutrinos originating from the Earth or from unknown astrophysical sources, is an ideal apparatus for the search of sterile neutrinos, both in neutrino and anti-neutrino mode. The experiment will be done by placing a well-designed artificial neutrino sources (⁵¹Cr, ¹⁴⁴Ce-¹⁴⁴Pr) close or inside the BOREXINO solar neutrino detector at the Laboratori Nazionali del Gran Sasso. The superb BOREXINO sensitivity, its large size, and very low radioactive background will be the key elements of the experiment. The well proved capability to detect both low energy electron neutrinos and anti-neutrinos make an ideal case for the study of short distance neutrino oscillations with artificial sources. ⁵¹Cr decays via electron capture into ⁵¹V, emitting two neutrino lines of 750 keV (90%) and 430 keV (10%), while ¹⁴⁴Pr decays β into ¹⁴⁴Nd with an end-point of 3 MeV. SOX will yield additional physics. The electroweak angle θ_W can be directly measured at MeV scale from the ν_e-e⁻ cross-section with an expected precision of 2.6%.

3. Expected impact of the research project on the development of science, civilization and society

SOX project has the potential to discover hypothetical sterile neutrinos. The discovery of the fourth family of neutrinos may be of fundamental importance for the theory of particles and for cosmological models. The precise measurements of the electroweak theory parameters at low energies will be performed.

Agostini M., et al. (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek, K. Jędrzejczak

See Synopsis: Still Waiting For Electron Decay

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http://dx.doi.org/10.1103/PhysRevLett.115.231802

Abstract

Borexino is a liquid scintillation detector located deep underground at the Laboratori Nazionali del Gran Sasso (LNGS, Italy). Thanks to the unmatched radio purity of the scintillator, and to the well understood detector response at low energy, a new limit on the stability of the electron for decay into a neutrino and a single monoenergetic photon was obtained. This new bound, τ≥6.6×1028  yr at 90% C.L., is 2 orders of magnitude better than the previous limit.

Agostini M., et al. (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek, K. Jędrzejczak

See Focus story: Neutrinos Detected from the Earth’s Mantle

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http://dx.doi.org/10.1103/PhysRevD.92.031101

Abstract

We report an improved geoneutrino measurement with Borexino from 2056 days of data taking. The present exposure is (5.5±0.3)×1031  proton×yr. Assuming a chondritic Th/U mass ratio of 3.9, we obtain 23.7+6.5−5.7(stat)+0.9−0.6(sys) geoneutrino events. The null observation of geoneutrinos with Borexino alone has a probability of 3.6×10−9 (5.9σ). A geoneutrino signal from the mantle is obtained at 98% C.L. The radiogenic heat production for U and Th from the present best-fit result is restricted to the range 23–36 TW, taking into account the uncertainty on the distribution of heat producing elements inside the Earth.

L. Ludhova, (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek

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http://dx.doi.org/10.1134/S1063779615020148

Abstract

Geo-neutrinos, electron anti-neutrinos produced in β-decays of naturally occurring radioactive isotopes in the Earth, are a unique direct probe of our planet’s interior. After a brief introduction about the Earth, the geo-neutrinos’ properties and the main aims of their study are discussed. An overview of the latest experimental results obtained by the Borexino collaboration is provided, followed by a short overview of future perspectives of this new inter-disciplinary field.

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http://dx.doi.org/10.1007/JHEP08(2013)038

Abstract

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http://dx.doi.org/10.1088/1475-7516/2013/08/049

Abstract

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_n = (3.10±0.11)10⁻⁴ n/(μ(g/cm²)). The distance profile between the parent muon track and the neutron capture point has the average value λ = (81.5±2.7) cm. Additionally the yields of a number of cosmogenic radioisotopes are measured for ¹²N, ¹²B, ⁸He, ⁹C, ⁹Li, ⁸B, ⁶He, ⁸Li, ¹¹Be, ¹⁰C and ¹¹C. 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 ¹¹C yield for which both codes return about 50% lower values. The predicted μ-n distance profile and the neutron multiplicity distribution are found to be overall consistent with data.

Download: http://iopscience.iop.org/1748-0221/7/10/P10018

Abstract

Borexino was the first experiment to detect solar neutrinos in real-time in the sub-MeV region. In order to achieve high precision in the determination of neutrino rates, the detector design includes an internal and an external calibration system. This paper describes both calibration systems and the calibration campaigns that were carried out in the period between 2008 and 2011. We discuss some of the results and show that the calibration procedures preserved the radiopurity of the scintillator. The calibrations provided a detailed understanding of the detector response and led to a significant reduction of the systematic uncertainties in the Borexino measurements.

Physics Letters B 716 (2012) 401–405

P. Alvarez Sanchez et al. (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek

Download: http://www.sciencedirect.com/science/article/pii/S0370269312009185

Abstract
We have measured the speed of muon neutrinos with the Borexino detector using short-bunch CNGS beams. The final result for the difference in time-of-flight between an 〈E〉=17 GeV muon neutrino and a particle moving at the speed of light in vacuum is δt=0.8±0.7_stat±2.9_sys ns, well consistent with zero.