Narrow Results

The neutrino event rate in the Borexino scintillator is very low (~ 0.5 events per day per ton) and concentrated in an energy region well below the 2.6 MeV threshold of natural radioactivity. The intrinsic radioactive contaminants in the photomultipliers (PMTs), in the Stainless Steel Sphere, and in other detector components, play special requirements on the system required to contain the scintillator. The liquid scintillator must be shielded from the Stainless Steel Sphere and from the PMTs by a thick barrier of buffer fluid. The fluid barrier, in addition, needs to be segmented in order to contain migration of radon and daughters emanated by the Stainless Steel Sphere and by the PMTs. These requirements were met by designing and building two spherical vessel made of thin nylon film. The inner vessel contains the scintillator, separating it from the surrounding buffer. The buffer region itself is divided into two concentric shells by the second, outer nylon vessel. In addition, the two nylon vessels must satisfy stringent requirements for radioactivity and for mechanical, optical and chemical properties. This paper describes the requirements of the the nylon vessels for the Borexino experiment and offers a brief overview of the construction methods adopted to meet those requirements.

The performance of the Borexino Outer Detector is described. It is a large water Cherenkov detector for identifying cosmic muons, penetrating the whole detector system. The Outer Detector is important for tagging cosmic muon generated background events in Borexino. Here, we present the muon identification efficiency and show the capability of muon tracking reconstruction.

Borexino was designed to measure solar neutrinos in the MeV or sub-MeV energy range. The unprecedented radiopurity of the detector has allowed the detection of geo-neutrinos and the determination of competitive limits on the rate of rare or forbidden processes. In this paper, we review the basic principle of neutrinos and antineutrinos detection in Borexino and we describe the results of the geo-neutrinos measurements and their implications. Then we summarize the search for Borexino events correlated with gamma ray bursts and for axion induced signals, and the limits achieved on Pauli forbidden transitions and on the electron charge conservation.

0ν2β decay is a very powerful tool for probing the physics beyond the particle Standard Model. After the recent discovery of neutrino flavor oscillation, we know that neutrinos must have a mass (at least two of them). The 0ν2β decay discovery could fix the neutrino mass scale and its nature (Majorana particle). The unique characteristics of the Borexino detector and its Counting Test Facility (CTF) can be employed for high sensitivity studies of ¹¹⁶Cd0ν2β decay: the CAMEO project. A first step foresees 24 enriched ¹¹⁶CdWO₄ crystals for a total mass of 65 kg in the Counting Test Facility; then, 370 enriched ¹¹⁶CdWO₄ crystals, for a total mass of 1 ton in the Borexino detector. Measurements of ¹¹⁶CdWO₄ crystals and Monte Carlo simulations have shown that the CAMEO experiment sensitivity will be , for the 65 kg phase, and for the 1 ton phase; consequently the limit on the effective neutrino mass will be ≤ 60 meV, and ≤ 20 meV, respectively. This work is based upon the experiments performed by the INR (Kiev) (and from 1998 also by the University of Florence) at the Solotvina Underground Laboratory (Ukraine). The current status of 0ν2β, and future projects of 0ν2β decay research are also briefly reviewed.

The Borexino experiment, located at Gran Sasso Laboratory in Italy, is operational since 2007. The unprecedented ultra-low background of the inner scintillating core has allowed to measure the fluxes of different components of the solar neutrino spectrum for the first time, as well as neutrino physical properties. In this work we present the recent results obtained with Borexino Phase-II data. Furthermore, we discuss the prospects for the SOX project, which will start at the beginning of 2018 and it will be able to test the long-standing issue of the existence of a sterile neutrino.

In more than ten years of operation, Borexino has performed a precision measurement of the solar neutrino spectrum, resolving almost all spectral components originating from the proton-proton fusion chain. The presentation will review the results recently released for the second data taking phase 2012–2016 during which the detector excelled by its unprecedentedly low background levels. New results on the rate of pp, ⁷Be, pep and ⁸B neutrinos as well as their implications for solar neutrino oscillations and metallicity are discussed.

Borexino measured with unprecedented accuracy the fluxes of solar neutrinos emitted at all the steps of the pp fusion chain. Still missing is the measurement of the flux of neutrinos produced in the CNO cycle. A positive measurement of the CNO neutrino flux is of fundamental importance for understanding the evolution of stars and addressing the unresolved controversy on the solar abundances. The measurement of the CNO neutrino flux in Borexino is challenging because of the low intensity of this component (CNO cycle accounts for about 1% of the energy emitted by Sun), the lack of prominent spectral features and the presence of background sources. The main background component is ²¹⁰Bi decaying in the liquid scintillator of Borexino that creates events with an energy distribution very close to the one of CNO neutrino interactions. Since 2015 the collaboration undertook significant efforts to achieve an independent measurement of the background affecting a CNO measurement, whose impact on the sensitivity to a CNO signal will be discussed.

Borexino, a large volume detector for low energy neutrino spectroscopy, is currently taking data underground since 2007 at the Laboratori Nazionali del Gran Sasso, Italy. The main goal of the experiment is the real-time measurement of solar neutrinos, especially the low energy part of the spectrum. Neutrinos are detected via neutrino-electron scattering in an ultra-pure organic liquid scintillator. The light generated by the interaction is detected by 2212 phototubes. During many years of data taking the experiment provided several remarkable results as the first evidence of pep neutrinos, the real-time detection of the pp neutrinos, the evidence of CNO neutrinos, and the detection of antineutrinos from the Earth. All these results are based on an accurate modelling of the detector’s response and performances. The contribution shows the design, the modelling of the detector’s response, and the performances. Moreover it will be discussed how the performances and the response were studied by means of extensive calibration campaigns.

About 99% of solar energy is produced through sequences of nuclear processes that convert Hydrogen into Helium in the so-called pp-chain. The neutrinos emitted in 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 research.

The Borexino experiment consists of a large-volume liquid-scintillator detector designed and constructed for real-time detection of low energy solar neutrinos. It is installed at the underground INFN Laboratori Nazionali del Gran Sasso (L’Aquila, Italy) and started taking data in May 2007. Borexino has been the only experiment so far capable of performing a complete study of the pp-chain by directly measuring the neutrino-electron elastic scattering rates for the neutrinos produced in four of its reactions: the initial proton–proton (pp) fusion, the electron capture of Beryllium-7, the proton–electron–proton (pep) fusion, and the Boron-8 β⁺ decay. A limit on the neutrino flux produced in the helium-proton fusion (hep) was also set. This set of measurements further probes the solar fusion mechanism via the direct determination of the relative intensity of the two primary terminations of the pp-chain, and the computation of the solar neutrino luminosity. Moreover, the Beryllium-7 and Boron-8 fluxes are indicative of the Sun’s core temperature, and their measurement shows a mild preference for the higher temperature expected from the high-metallicity Standard Solar Model scenario. Finally, the experimental survival probability of these solar electron neutrinos allows to simultaneously probe the MSW neutrino flavor conversion paradigm, both in vacuum and in matter-dominated regimes, at different energies.

The details of the strategy adopted by the Borexino collaboration for successfully isolating the spectral components of the pp-chain neutrinos signal from residual backgrounds in the total energy spectrum will be presented.

Till very recent the real-time solar neutrino experiments were detecting the tiny fraction of about 0.01% of the total neutrino flux above some MeV energy, the sub-MeV region remained explored only by radiochemical experiments without spectroscopical capabilities. The Borexino experiment, an unsegmented large volume liquid scintillator detector located in the Gran Sasso National Laboratory in central Italy, is at present the only experiment in the world acquiring the real-time solar neutrino data in the low-energy region, via the elastic scattering on electrons in the target mass. The data taking campaign started in 2007 and rapidly lead to the first independent measurement of the mono-cromatic line of ⁷Be of the solar neutrino spectrum at 862keV, which is of special interest because of the very loose limits coming from existing experiments. The latest measurement, after 41.3t · yr of exposure, is (49 ± 3_stat ± 4_syst)c/(day · 100t) and leaves the hypothesis of no oscillation inconsistent with data at 4σ level. It also represents the first direct measurement of the survival probability for solar in the vacuum-dominates oscillation regime. Recently Borexino was also able to measure of the ⁸B solar neutrinos interaction rate down to the threshold energy of 3 MeV, the lowest achieved so far. The inferred electron neutrino flux is . The corresponding mean electron neutrino survival probability, is at the effective energy of 8.9 MeV. Both measurements are in good agreement with other existing measurements and with predictions from the SSM in the hypothesis of MSW-LMA oscillation scenario. For the first time, thanks to the unprecedented radio-purity of the Borexino target and construction materials, we confirm with a single detector, the presence of a transition between the low energy vacuum-dominated and the high-energy matter-enhanced solar neutrino oscillations. A further confirmations of the LMA scenario is provided by the absence of a day-night asymmetry in the ⁷Be signal. These experimental results allow to improve the knowledge of the pp neutrino flux, to place an upper limit on the CNO flux and also to explore non standard neutrino properties, improving the upper limit on the neutrino effective magnetic moment. Calibration campaigns aiming to reduce the systematical errors on fiducial volume definition and detector energy response have been performed and data analysis is presently in progress. Borexino has also recently observed antineutrinos from the Earth, for the first time at more the 3σ C.L. and has measured a rate of events/(100ton-yr) at 68.3%(99.73%) C.L. Borexino is also a powerful supernova neutrino detector. Future prospects of the experiment include reducing the systematic error on the ⁷Be flux to below 5% and direct measurement of additional solar neutrino emissions such as pep, CNO and possibly pp.

Borexino is an excellent and well-understood detector for low energy, sub-MeV neutrinos (ν) and anti-neutrinos (), as proven by its recently published results. The European Community has recently approved a project for the construction of a ν or an source which will allow one to confirm or unambiguously reject the long standing neutrino anomalies suggested by the Liquid Scintillator Neutrino Detector (LSND), solar-ν gallium-based experiments and by reactor experiments. This note outlines the Short-distance Oscillations with Borexino (SOX) source project and discusses the sensitivities of its three proposed phases, as well as recent results from the solar-ν and geo- programs.