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 The Borexino Collaboration Physics , 2009, DOI: 10.1103/PhysRevC.81.034317 Abstract: The Pauli exclusion principle (PEP) has been tested for nucleons ($n,p$) in $^{12}C$ with the Borexino detector.The approach consists of a search for $\gamma$, $n$, $p$ and $\beta^\pm$ emitted in a non-Paulian transition of 1$P_{3/2}$- shell nucleons to the filled 1$S_{1/2}$ shell in nuclei. Due to the extremely low background and the large mass (278 t) of the Borexino detector, the following most stringent up-to-date experimental bounds on PEP violating transitions of nucleons have been established: $\tau({^{12}\rm{C}}\to{^{12}\widetilde{\rm{C}}}+\gamma) \geq 5.0\cdot10^{31}$ y, $\tau({^{12}\rm{C}}\to{^{11}\widetilde{\rm{B}}}+ p) \geq 8.9\cdot10^{29}$ y, $\tau({^{12}\rm{C}}\to{^{11}\widetilde{\rm{C}}}+ n) \geq 3.4 \cdot 10^{30}$ y, $\tau({^{12}\rm{C}}\to{^{12}\widetilde{\rm{N}}}+ e^- + \widetilde{\nu_e}) \geq 3.1\cdot 10^{30}$ y and $\tau({^{12}\rm{C}}\to{^{12}\widetilde{\rm{B}}}+ e^+ + \nu_e) \geq 2.1 \cdot 10^{30}$ y, all at 90% C.L. The corresponding upper limits on the relative strengths for the searched non-Paulian electromagnetic, strong and weak transitions have been estimated: $\delta^2_{\gamma} \leq 2.2\cdot10^{-57}$, $\delta^2_{N} \leq 4.1\cdot10^{-60}$ and $\delta^2_{\beta} \leq 2.1\cdot10^{-35}$.
 Physics , 2012, DOI: 10.1007/JHEP07(2012)112 Abstract: We study production of sterile neutrinos in the atmosphere and their detection at Super-Kamiokande. A sterile neutrino in the mass range $1\,{\rm MeV} \lesssim M_N \lesssim 105\,{\rm MeV}$ is produced by muon or pion decay, and decays to an electron-positron pair and an active neutrino. Such a decay of the sterile neutrino leaves two electron-like Cherenkov rings in the detector. We estimate the sterile neutrino flux from the well-established active neutrino fluxes and study the number of the decay events in the detector. The upper bounds for the active-sterile mixings are obtained by comparing the $2e$-like events from the sterile neutrino decays and the observed data by Super-Kamiokande. The upper bound for the muon type mixing $\Theta_\mu$ is found to be $|\Theta_\mu|^2 \lesssim 5 \times 10^{-5}$ for $20 \,{\rm MeV} \lesssim M_N \lesssim 80\,{\rm MeV}$, which is significantly loosened compared to the previous estimation. We demonstrate that the opening angle and the total energy of the rings may serve as diagnostic tools to discover the sterile neutrinos in further data accumulation and future upgraded facilities. The directional asymmetry of the events is a sensitive measure of the diminishment of the sterile neutrino flux due to the decays on the way to the detector.
 Physics , 2014, DOI: 10.1134/S1063779615020185 Abstract: Borexino is a unique detector able to perform measurement of solar neutrinos fluxes in the energy region around 1 MeV or below due to its low level of radioactive background. It was constructed at the LNGS underground laboratory with a goal of solar $^{7}$Be neutrino flux measurement with 5\% precision. The goal has been successfully achieved marking the end of the first stage of the experiment. A number of other important measurements of solar neutrino fluxes have been performed during the first stage. Recently the collaboration conducted successful liquid scintillator repurification campaign aiming to reduce main contaminants in the sub-MeV energy range. With the new levels of radiopurity Borexino can improve existing and challenge a number of new measurements including: improvement of the results on the Solar and terrestrial neutrino fluxes measurements; measurement of pp and CNO solar neutrino fluxes; search for non-standard interactions of neutrino; study of the neutrino oscillations on the short baseline with an artificial neutrino source (search for sterile neutrino) in context of SOX project.
 Physics , 2001, DOI: 10.1016/S0550-3213(02)00476-5 Abstract: The present experimental status does not exclude weak-strength non-standard interactions of neutrinos with electrons. These interactions can be revealed in solar neutrino experiments. Our discussion covers several aspects related to this issue. First, we perform an analysis of the Super Kamiokande and SNO data to investigate their sensitivity to such interactions. In particular, we show that the \nu_e oscillation into sterile neutrinos can be still allowed if \nu_e has extra interactions of the proper strength. Second, we suggest that the Borexino detector can provide good signatures for these non-standard interactions. Indeed, in Borexino the shape of the recoil electron spectrum from the \nu e \to \nu e scattering essentially does not depend on the solar neutrino conversion details, since most of the signal comes from the mono-energetic ^7Be neutrinos. Hence, the partial conversion of solar \nu_e into a a nearly equal mixture of \nu_\mu and \nu_\tau, as is indicated by the atmospheric neutrino data, offers the chance to test extra interactions of \nu_\tau, or of \nu_e itself.
 Statistics , 2011, DOI: 10.1103/PhysRevLett.107.011802 Abstract: Results are reported from a search for active to sterile neutrino oscillations in the MINOS long-baseline experiment, based on the observation of neutral-current neutrino interactions, from an exposure to the NuMI neutrino beam of $7.07\times10^{20}$ protons on target. A total of 802 neutral-current event candidates is observed in the Far Detector, compared to an expected number of $754\pm28\rm{(stat.)}\pm{37}\rm{(syst.)}$ for oscillations among three active flavors. The fraction $f_s$ of disappearing \numu that may transition to $\nu_s$ is found to be less than 22% at the 90% C.L.
 Physics , 2014, Abstract: The Borexino experiment, located in the Gran Sasso National Laboratory, is an organic liquid scintillator detector conceived for the real time spectroscopy of low energy solar neutrinos. The data taking campaign phase I (2007 - 2010) has allowed the first independent measurements of 7Be, 8B and pep fluxes as well as the first measurement of anti-neutrinos from the earth. After a purification of the scintillator, Borexino is now in phase II since 2011. We review here the recent results achieved during 2013, concerning the seasonal modulation in the 7Be signal, the study of cosmogenic backgrounds and the updated measurement of geo-neutrinos. We also review the upcoming measurements from phase II data (pp, pep, CNO) and the project SOX devoted to the study of sterile neutrinos via the use of a 51Cr neutrino source and a 144Ce-144Pr antineutrino source placed in close proximity of the active material.
 The Borexino Collaboration Physics , 2011, DOI: 10.1103/PhysRevLett.107.141302 Abstract: A direct measurement of the 0.862 MeV 7Be solar neutrino interaction rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso yields 46.0$\pm1.5_{\rm stat}$$^{+1.6}_{-1.5\,\rm syst}$ counts/day/(100 tons). Our result is the first direct measurement of a sub-MeV solar neutrino rate with an accuracy better than 5%. The hypothesis of no oscillation for 7Be solar neutrinos is rejected at 4.9$\sigma$ C.L. Using the latest Standard Solar Model (SSM) flux predictions, the result leads directly to a precise determination of the survival probability for solar $\nu_e$'s in vacuum, and permits us to probe with unprecedented sensitivity the transition between the matter-enhanced and vacuum-dominated neutrino oscillation regimes characteristic of the MSW-LMA solution to the solar neutrino problem.
 Lino Miramonti Physics , 2006, Abstract: Borexino is a massive calorimetric liquid scintillation detector whose installation has been completed in the underground Gran Sasso Laboratory. The focus of the experiment is on the direct and real time measurement of the flux of neutrinos produced in the $^{7}Be$ electron capture reaction in the Sun. Furthermore, recent studies about the reduction of the $^{11}C$ background through suitable rejection techniques demonstrated the possibility to open an interesting additional observation window in the energy region of the pep and CNO solar neutrinos. Beyond the solar neutrino program, the detector will be also a powerful observatory for antineutrinos from Supernovae, as well as for geoneutrinos, profiting from a very low background from nuclear reactors.
 Physics , 2015, Abstract: The OPERA experiment, exposed to the CERN to Gran Sasso $\nu_\mu$ beam, collected data from 2008 to 2012. Four oscillated $\nu_\tau$ Charged Current interaction candidates have been detected in appearance mode, which are consistent with $\nu_\mu \to \nu_\tau$ oscillations at the atmospheric $\Delta m^2$ within the "standard" three-neutrino framework. In this paper, the OPERA $\nu_\tau$ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino.
 Physics , 2010, DOI: 10.1016/j.physletb.2010.12.030 Abstract: We report on the search for anti-neutrinos of yet unknown origin with the Borexino detector at the Laboratori Nazionali del Gran Sasso. In particular, a hypothetical anti-neutrino flux from the Sun is investigated. Anti-neutrinos are detected through the neutron inverse $\beta$ decay reaction in a large liquid organic scintillator target. We set a new upper limit for a hypothetical solar $\bar{\nu}_e$ flux of 760 ${\rm cm}^{-2}{\rm s}^{-1}$, obtained assuming an undistorted solar $^8$B energy spectrum. This corresponds to a limit on the transition probability of solar neutrinos to anti-neutrinos of $1.3\times10^{-4}$ (90\% C.L.) for $E_{\bar{\nu}}>1.8$ MeV, covering the entire $^{8}$B spectrum. Best differential limits on anti-neutrino fluxes from unknown sources are also obtained between the detection energy threshold of 1.8 MeV and 17.8 MeV with more than 2 years of data.
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