Borexino has been running since May 2007 at the LNGS laboratory in Italy with the primary goal of detecting solar neutrinos. The detector, a large, unsegmented liquid scintillator calorimeter characterized by unprecedented low levels of intrinsic radioactivity, is optimized for the study of the lower energy part of the spectrum. During the Phase-I (2007Phase-I ( -2010), Borexino first detected and then precisely measured the flux of the 7 Be solar neutrinos, ruled out any significant day-night asymmetry of their interaction rate, made the first direct observation of the pep neutrinos, and set the tightest upper limit on the flux of CNO solar neutrinos. In this paper we discuss the signal signature and provide a comprehensive description of the backgrounds, quantify their event rates, describe the methods for their identification, selection or subtraction, and describe data analysis. Key features are an extensive in situ calibration program using radioactive sources, the detailed modeling of the detector response, the ability to define an innermost fiducial volume with extremely low background via software cuts, and the excellent pulse-shape discrimination capability of the scintillator that allows particle identification. We report a measurement of the annual modulation of the 7 Be neutrino interaction rate. The period, the amplitude, and the phase of the observed modulation are consistent with the solar origin of these events, and the absence of their annual modulation is rejected with higher than 99% C.L. The physics implications of Phase-I results in the context of the neutrino oscillation physics and solar models are presented.

A search for the solar neutrino effective magnetic moment has been performed using data from 1291.5 days exposure during the second phase of the Borexino experiment. No significant deviations from the expected shape of the electron recoil spectrum from solar neutrinos have been found, and a new upper limit on the effective neutrino magnetic moment of μ eff ν < 2.8 × 10 −11 μ B at 90% C.L. has been set using constraints on the sum of the solar neutrino fluxes implied by the radiochemical gallium experiments. Using the limit for the effective neutrino moment, new limits for the magnetic moments of the neutrino flavor states, and for the elements of the neutrino magnetic moments matrix for Dirac and Majorana neutrinos, are derived.

We present the simultaneous measurement of the interaction rates R pp , R Be , R pep of pp, 7 Be, and pep solar neutrinos performed with a global fit to the Borexino data in an extended energy range (0.19-2.93) MeV with particular attention to details of the analysis methods. This result was obtained by analyzing 1291.51 days of Borexino Phase-II data, collected after an extensive scintillator purification campaign. Using counts per day ðcpdÞ=100 ton as unit, we find R pp ¼ 134 AE 10ðstatÞ þ6 −10 ðsysÞ, R Be ¼ 48.3 AE 1.1ðstatÞ þ0.4 −0.7 ðsysÞ; and R HZ pep ¼ 2.43 AE 0.36ðstatÞ þ0.15 −0.22 ðsysÞ assuming the interaction rate R CNO of CNO-cycle (Carbon, Nitrogen, Oxigen) solar neutrinos according to the prediction of the high metallicity standard solar model, and R LZ pep ¼ 2.65 AE 0.36ðstatÞ þ0.15 −0.24 ðsysÞ according to that of the low metallicity model. An upper limit R CNO < 8.1 cpd=100 ton (95% C.L.) is obtained by setting in the fit a constraint on the ratio R pp =R pep (47.7 AE 0.8 cpd=100 ton or 47.5 AE 0.8 cpd=100 ton according to the high or low metallicity hypothesis).

Abstract. The solar neutrino experiment Borexino, which is located in the Gran Sasso underground laboratories, is in a unique position to study muon-induced backgrounds in an organic liquid scintillator. In this study, a large sample of cosmic muons is identified and tracked by a muon veto detector external to the liquid scintillator, and by the specific light patterns observed when muons cross the scintillator volume. The yield of muon-induced neutrons is found to be Y n = (3.10 ± 0.11) · 10 −4 n/(µ · (g/cm 2 )). The distance profile between the parent muon track and the neutron capture point has the average value λ = (81.5 ± 2.7) cm. Additionally the yields of a number of cosmogenic radioisotopes are measured for 12 N, 12 B, 8 He, 9 C, 9 Li, 8 B, 6 He, 8 Li, 11 Be, 10 C and 11 C. All results are compared with Monte Carlo simulation predictions using the Fluka and Geant4 packages. General agreement between data and simulation is observed for the cosmogenic production yields with a few exceptions, the most prominent case being 11 C yield for which both codes return about 50% lower values. The predicted µ-n distance profile and the neutron multiplicity distribution are found to be overall consistent with data.

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