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Gamma-ray bursts (GRBs) are the most energetic known events in the Universe. Though gamma-ray telescopes observe about one GRB event per day, the nature of this phenomenon is not yet totally understood. Many theoretical models predict emission of neutrinos of all types in a wide energy range. In this talk we review experimental searches of GRB neutrinos in MeV energy range. The searches of this kind had been performed by several experiments: SuperKamiokande, SNO and KamLAND. Also the similar study is now in progress in Borexino collaboration.

Based on the synthesis of the radar and LS8000 lightning detection network data, as well as the cloud resolving modeling, the effect of glaciogenic seeding on the electrical activity of a hail-hazardous cloud, which developed in the North Caucasus on May 14, 2012 was investigated. It has been proved that the introduction of a reagent leads to an increase in the frequency of intracloud discharges and in the total current of negative cloud-to-ground discharges. An increase in the peak current of the cloud-to-ground discharges of both polarities occurs in 10–15 minutes after the seeding termination. Seeding significantly increases the frequency of lightning discharges both in a cloud and a subcloud layer. As a result of seeding, the charge structure of a cloud turns out to be inverted: there is not a positive but a negative charge in its upper part.

Using numerical non-stationary three-dimensional Cu model the investigation of the evolution of thunderstorm with hail was carried out in North-Western region of Russia. Spatial and temporal variations of the main cloud and precipitation characteristics obtained, especially, cloud electrification intensity, volume charge density and electric field strength. It was shown that the main role in cloud electrification under given conditions has the polarization one. On the contrary, ice-ice collision was not so intensive because of small mass of cloud ice crystals.

Solar flares are sudden variations in brightness observed near the Sun's surface. Some theoretical models predict production of electron and muon neutrinos with energies up to few tens of MeV during solar flares. In 1980s the Homestake experiment reported excess of detected neutrino events possibly correlated with large solar flares. Since then the interest to similar studies by other neutrino detectors has increased. In this report we summarize the status of experimental searches and describe the methodology for the study of neutrinos from solar flares in Borexino liquid scintillator detector.

The three-dimensional nonstationary model of a convective cloud is used for investigating a thunderstorm with hail which developed over Pyatigorsk on May 29, 2012 and produced a severe hailstorm. The values of cloud characteristics (liquid water content, ice content, vertical velocity, etc.) are obtained. The importance ofconsidering wind shear is noted. The simulation results are used to analyze the transformation of precipitation field and the electric charge structure of the analyzed cloud during its development.

About 99 per cent of solar energy is produced through sequences of nuclear reactions that convert hydrogen into helium, starting from the fusion of two protons (the pp chain). The neutrinos emitted by 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. Here we report a complete study of the pp chain. We measure the neutrino–electron elastic-scattering rates for neutrinos produced by four reactions of the chain: the initial proton–proton fusion, the electron-capture decay of beryllium-7, the three-body proton–electron–proton ( pep ) fusion, here measured with the highest precision so far achieved, and the boron-8 beta decay, measured with the lowest energy threshold. We also set a limit on the neutrino flux produced by the 3 He–proton fusion (hep). These measurements provide a direct determination of the relative intensity of the two primary terminations of the pp chain ( pp -I and pp -II) and an indication that the temperature profile in the Sun is more compatible with solar models that assume high surface metallicity. We also determine the survival probability of solar electron neutrinos at different energies, thus probing simultaneously and with high precision the neutrino flavour-conversion paradigm, both in vacuum and in matter-dominated regimes. All components of the proton–proton nuclear fusion chain, in which hydrogen is converted into helium in the Sun, are described, with several implications for fundamental solar and particle physics.

About 99 per cent of solar energy is produced through sequences of nuclear reactions that convert hydrogen into helium, starting from the fusion of two protons (the pp chain). The neutrinos emitted by 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. Here we report a complete study of the pp chain. We measure the neutrino–electron elastic-scattering rates for neutrinos produced by four reactions of the chain: the initial proton–proton fusion, the electron-capture decay of beryllium-7, the three-body proton–electron–proton (pep) fusion, here measured with the highest precision so far achieved, and the boron-8 beta decay, measured with the lowest energy threshold. We also set a limit on the neutrino flux produced by the ³ He–proton fusion (hep). These measurements provide a direct determination of the relative intensity of the two primary terminations of the pp chain (pp-I and pp-II) and an indication that the temperature profile in the Sun is more compatible with solar models that assume high surface metallicity. We also determine the survival probability of solar electron neutrinos at different energies, thus probing simultaneously and with high precision the neutrino flavour-conversion paradigm, both in vacuum and in matter-dominated regimes.

The SOX (Short distance Oscillations with boreXino) experiment aims to investigate possible anomalous oscillatory behaviours in neutrinos, including the existence of sterile neutrinos, by exploiting the very low radioactive background of the Borexino detector. A calibration campaign is crucial to achieve a deeper understanding of the energy response and the spatial reconstruction accuracies of the detector. It will be performed with a suite of low-activity radioactive sources which will map the whole active volume, especially nearby the inner vessel. The calibration points at the border of the active zones will be extremely important to study the neutron detection efficiency. The calibration system, already used in Borexino Phase-I, allows the insertion of the sources without perturbing the radio-purity of the detector. The calibration campaign will take place a few months before the beginning of the SOX experiment. In this work, we describe in detail both the calibration hardware and the calibration strategy.

A spectral fitter based on the graphics processor unit (GPU) has been developed for Borexino solar neutrino analysis. It is able to shorten the fitting time to a superior level compared to the CPU fitting procedure. In Borexino solar neutrino spectral analysis, fitting usually requires around one hour to converge since it includes time-consuming convolutions in order to account for the detector response and pile-up effects. Moreover, the convergence time increases to more than two days when including extra computations for the discrimination of 11C and external γs. In sharp contrast, with the GPU-based fitter it takes less than 10 seconds and less than four minutes, respectively. This fitter is developed utilizing the GooFit project with customized likelihoods, pdfs and infrastructures supporting certain analysis methods. In this proceeding the design of the package, developed features and the comparison with the original CPU fitter are presented.

The Borexino experiment, located in the Laboratori Nazionali del Gran Sasso in Italy and widely known for its rich Solar Neutrino physics program, has recently celebrated the 10 years of data taking. Among the achievements of the Borexino experiment solar program are: a precision measurement of 7Be neutrino flux with uncertainty of 3%, limit on its day/night asymmetry, first spectral measurement of pp-neutrinos, first evidence of monoenergetic pep neutrinos at 5 sigma, 8B neutrinos detection with the lowest visible energy threshold of 3 MeV, observation of season modulation of the 7Be solar neutrino rate at 3.8 sigma and the best current limit on CNO neutrino flux. Borexino is now in its high-purity Phase II data taking, thanks to intense purification campaigns of scintillator in 2010-11 that were very successful in further reducing the already low backgrounds. The advanced tecniques of data analysis were improved, allowing to maximize the signal/noise ratio. The detector was thermally insulated in order to improve the fluid stability. As an outcome, quality of the data has significantly increased leading to new levels of sensitivity to all solar neutrino fluxes. This allows a more sensitive probe for CNO neutrinos relevant to the solar metallicity problem.