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SABRE aims to directly measure the annual modulation of the dark matter interaction rate with NaI(Tl) crystals. A modulation compatible with the standard hypothesis, in which our Galaxy is immersed in a dark matter halo, has been measured by the DAMA experiment in the same target material. Other direct detection experiments, using different target materials, seem to exclude the interpretation of such modulation in the simplest scenario of WIMP-nucleon elastic scattering. The SABRE experiment aims to carry out an independent search with sufficient sensitivity to confirm or refute the DAMA claim. The goal of the SABRE experiment is to achieve the lowest background rate for a NaI(Tl) experiment (order of 0.1 cpd/kg/keVee in the energy region of interest for dark matter). This challenging goal could be achievable by operating high-purity crystals inside a liquid scintillator veto for active background rejection. In addition, twin detectors will be located in the northern and southern hemispheres to identify possible contributions to the modulation from seasonal or site-related effects. The SABRE project includes an initial Proof-of-Principle phase at LNGS (Italy), to assess the radio-purity of the crystals and the efficiency of the liquid scintillator veto. This paper describes the general concept of SABRE and the expected sensitivity to WIMP annual modulation.

We present here a characterization of the low background NaI(Tl) crystal NaI-33 based on a period of almost one year of data taking (891 kg×days exposure) in a detector configuration with no use of organic scintillator veto. This remarkably radio-pure crystal already showed a low background in the SABRE Proof-of-Principle (PoP) detector, in the low energy region of interest (1–6 keV) for the search of dark matter interaction via the annual modulation signature. As the vetoable background components, such as 40K, are here sub-dominant, we reassembled the PoP setup with a fully passive shielding. We upgraded the selection of events based on a Boosted Decision Tree algorithm that rejects most of the PMT-induced noise while retaining scintillation signals with > 90% efficiency in 1–6 keV. We find an average background of 1.39 ± 0.02 counts/day/kg/keV in the region of interest and a spectrum consistent with data previously acquired in the PoP setup, where the external veto background suppression was in place. Our background model indicates that the dominant background component is due to decays of 210Pb, only partly residing in the crystal itself. The other location of 210Pb is the reflector foil that wraps the crystal. We now proceed to design the experimental setup for the physics phase of the SABRE North detector, based on an array of similar crystals, using a low radioactivity PTFE reflector and further improving the passive shielding strategy, in compliance with the new safety and environmental requirements of Laboratori Nazionali del Gran Sasso.

Ultra-pure NaI(Tl) crystals are the key element for a model-independent verification of the long standing DAMA result and a powerful means to search for the annual modulation signature of dark matter interactions. The SABRE collaboration has been developing cutting-edge techniques for the reduction of intrinsic backgrounds over several years. In this paper we report the first characterization of a 3.4 kg crystal, named NaI-33, performed in an underground passive shielding setup at LNGS. NaI-33 has a record low 39K contamination of 4.3 ± 0.2 ppb as determined by mass spectrometry. We measured a light yield of 11.1 ± 0.2 photoelectrons/keV and an energy resolution of 13.2% (FWHM/E) at 59.5 keV. We evaluated the activities of 226Ra and 228Th inside the crystal to be 5.9±0.6μBq/kg and 1.6±0.3μBq/kg, respectively, which would indicate a contamination from 238U and 232Th at part-per-trillion level. We measured an activity of 0.51 ± 0.02 mBq/kg due to 210Pb out of equilibrium and a α quenching factor of 0.63 ± 0.01 at 5304 keV. We illustrate the analyses techniques developed to reject electronic noise in the lower part of the energy spectrum. A cut-based strategy and a multivariate approach indicated a rate, attributed to the intrinsic radioactivity of the crystal, of ∼1 count/day/kg/keV in the [5–20] keV region.

For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton–proton ( pp ) chain and the carbon–nitrogen–oxygen (CNO) cycle 1 , 2 . Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously 3 ; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal—only a few counts per day above the background per 100 tonnes of target—that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate—as well as the flux of emitted CNO neutrinos—depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe. Direct experimental evidence of the carbon–nitrogen–oxygen fusion cycle in the Sun is provided by the detection of neutrinos emitted during this process.

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. Spectral observations of the low-energy neutrinos produced by proton–proton fusion in the Sun demonstrate that about 99 per cent of the Sun’s power is generated by this process. Sun's elusive pp neutrinos tracked down The Sun's energy output derives from a sequence of nuclear reactions that converts hydrogen into helium, most of it from the fusion of two protons (the proton–proton or pp reaction) accompanied by the release of a low-energy neutrino. These neutrinos have proved elusive: only solar neutrinos from secondary reactions had been directly observed. But here the Borexino collaboration reports observations of the pp neutrinos themselves, so providing a direct view of the principal fusion process that powers the Sun.

Cryogenic noble liquid detectors are presently considered one of the best options for WIMP Dark Matter searches, especially when extensions to multi ton scale sensitive masses are foreseen. The WArP experiment is the first one that exploits the unique characteristics of liquid Argon to make a highly sensitive search for WIMP Dark Matter candidates. In 2008, a double phase detector has been assembled in the Gran Sasso National Laboratory with 140 kg sensitive mass and a discovery potential in the range of 5 × 10-45 cm2 in the spin-independent WIMP-nucleon cross-section. In addition to standard neutrons and gamma-rays passive shields, WArP implements an 8 ton liquid Argon active shield with 4p coverage. The detector was commissioned and put into operation during the first half of 2009 for a first technical run. Detector design, construction and assembly are described, together with the very first results of this technical run.

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 detector’s 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 210 Bi beta decays originating in the intrinsic contamination of the liquid scintillator with 210 Pb. Assuming the CNO flux predicted by the high-metallicity Standard Solar Model and an exposure of 1000 days × 71.3 t, Borexino has a median sensitivity to CNO neutrino higher than 3 σ . With the same hypothesis the expected experimental uncertainty on the CNO neutrino flux is 23%, provided the uncertainty on the independent estimate of the 210 Bi  interaction rate is 1.5 cpd / 100 ton  . 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.

Cryogenic noble liquid detectors are presently considered one of the best options for WIMP Dark Matter searches, especially when extensions to multi ton scale sensitive masses are foreseen. The WArP experiment is the first one that exploits the unique characteristics of liquid Argon to make a highly sensitive search for WIMP Dark Matter candidates. In 2008, a double phase detector has been assembled in the Gran Sasso National Laboratory with 140 kg sensitive mass and a discovery potential in the range of 5 × 10−45 cm2 in the spin-independent WIMP-nucleon cross-section. In addition to standard neutrons and gamma-rays passive shields, WArP implements an 8 ton liquid Argon active shield with 4π coverage. The detector was commissioned and put into operation during the first half of 2009 for a first technical run. This first run lasted about three months and then it was stopped for some detector repairs and modifications in the summer of 2009. A second run was started at the beginning of 2010. Detector design, construction and assembly are described, together with the results of the technical run and the very first results of the 2010 run.

A bstract The Borexino detector measures solar neutrino fluxes via neutrino-electron elastic scattering. Observed spectra are determined by the solar- ν e survival probability P ee ( E ), and the chiral couplings of the neutrino and electron. Some theories of physics beyond the Standard Model postulate the existence of Non-Standard Interactions (NSI’s) which modify the chiral couplings and P ee ( E ). In this paper, we search for such NSI’s, in particular, flavor-diagonal neutral current interactions that modify the ν e e and ν τ e couplings using Borexino Phase II data. Standard Solar Model predictions of the solar neutrino fluxes for both high- and low-metallicity assumptions are considered. No indication of new physics is found at the level of sensitivity of the detector and constraints on the parameters of the NSI’s are placed. In addition, with the same dataset the value of sin 2 θ W is obtained with a precision comparable to that achieved in reactor antineutrino experiments .

A search for solar axions and axion-like particles produced in the p + d → 3 He + A ( 5.5 MeV ) reaction was performed using the complete dataset of the Borexino detector (3995 days of measurement live-time). The following interaction processes have been considered: axion decay into two photons ( A → 2 γ ) , inverse Primakoff conversion on nuclei ( A + Z → γ + Z ), the Compton conversion of axions to photons ( A + e → e + γ ) and the axio-electric effect ( A + e + Z → e + Z ). Model-independent limits on product of axion–photon ( g A γ ), axion–electron ( g Ae ), and isovector axion–nucleon ( g 3 A N ) couplings are obtained: | g A γ × g 3 A N | ≤ 2.3 × 10 - 11 GeV - 1 and | g Ae × g 3 A N | ≤ 1.9 × 10 - 13 at m A < 1 MeV (90% c.l.). The Borexino results exclude new large regions of g A γ , and g Ae coupling constants and axion masses m A , and leads to constraints on the products | g A γ × m A | and | g Ae × m A | for the KSVZ- and the DFSZ-axion models.