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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.

The Gerda collaboration is performing a search for neutrinoless double beta decay of 76 Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori Nazionali del Gran Sasso and has started operation in November 2011. The design, construction and first operational results are described, along with detailed information from the R&D phase.

The Pauli exclusion principle (PEP) has been tested for nucleons (n,p) in \\({}^{12}{\\rm C}\\) and \\({}^{16}{\\rm O}\\) nuclei, using the results of background measurements with the prototype of the Borexino detector, the Counting Test Facility (CTF). The approach consisted of a search for \\(\\gamma\\), n, p and/or \\(\\alpha\\)’s emitted in a non-Paulian transition of 1P- shell nucleons to the filled 1S1/2 shell in nuclei. Similarly, the Pauli-forbidden \\(\\beta^{\\pm}\\) decay processes were searched for. Due to the extremely low background and the large mass (4.2 tons) of the CTF detector, the following most stringent up-to-date experimental bounds on PEP violating transitions of nucleons have been established: \\(\\tau({}^{12}{\\rm C} \\rightarrow {}^{12}{\\rm\\widetilde C} + \\gamma) > 2.1\\cdot10^{27} \\mathrm y\\), \\(\\tau({}^{12}{\\rm C} \\rightarrow {}^{11}{\\rm\\widetilde B} + p) > 5.0\\cdot10^{26} {\\mathrm{y}}\\), \\(\\tau({}^{12}{\\rm C} ({}^{16}{\\rm O}) \\rightarrow {}^{11}{\\rm\\widetilde C} ({}^{15}{\\rm\\widetilde O} ) + n) > 3.7 \\cdot 10^{26} {\\mathrm{y}}\\), \\(\\tau({}^{12}{\\rm C} \\rightarrow {}^{8}{\\rm\\widetilde{Be}} + \\alpha) > 6.1 \\cdot 10^{23} \\mathrm y\\), \\(\\tau({}^{12}{\\rm C} \\rightarrow {}^{12}{\\rm\\widetilde N} + e^- + \\widetilde{\\nu_e}) > 7.6 \\cdot 10^{27} \\mathrm y\\) and \\(\\tau({}^{12}{\\rm C} \\rightarrow {}^{12}{\\rm\\widetilde B} + e^ + + \\nu_e) > 7.7 \\cdot 10^{27} \\mathrm y\\), all at \\(90 \\%\\) C.L.

Solar neutrinos have been pivotal to the discovery of neutrino flavour oscillations and are a unique tool to probe the reactions that keep the Sun shine. Although most of solar neutrino components have been directly measured, the neutrinos emitted by the keystone pp reaction, in which two protons fuse to make a deuteron, have so far eluded direct detection. The Borexino experiment, an ultra–pure liquid scintillator detector running at the Laboratori Nazionali del Gran Sasso in Italy, has now filled the gap, providing the first direct real time measurement of pp neutrinos and of the solar neutrino luminosity.

We observed, for the first time, solar neutrinos in the 1.0–1.5 MeV energy range. We determined the rate of pep solar neutrino interactions in Borexino to be 3.l±0.6stat±0.3syst counts/(day-100 ton). Assuming the pep neutrino flux predicted by the Standard Solar Model, we obtained a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day-100 ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. The necessary sensitivity was achieved by adopting data analysis techniques for the rejection of cosmogenic 11C, the dominant background in the 1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6±0.3)×l08cm−2s−1 and <7.7×l08 cm−2s−1 (95% C.L.), respectively, in agreement with both the High and Low Metallicity Standard Solar Models. These results represent the first direct evidence of the pep neutrino signal and the strongest constraint of the CNO solar neutrino flux to date.

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of 7 Li * . The Compton conversion of axion to a photon A+e→e+γ, axioelectric effect A+e+Z→e+Z, decay of axion in two photons A→2γ and Primakoff conversion on nuclei A+Z→γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons – g Ae g AN ≤(1.0–2.4)×10 -10 at m A ≤450 keV and g Aγ g AN ≤5×10 -9  GeV -1 at m A ≤10 keV are obtained (90%c.l.). For heavy axions with mass at 100

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