Final results of Borexino Phase-I on low-energy solar neutrino spectroscopy

International audienceBorexino has been running since May 2007 at the LNGS laboratory in Italy with the primarygoal of detecting solar neutrinos. The detector, a large, unsegmented liquid scintillator calorimetercharacterized by unprecedented low levels of intrinsic radioactivity, is optimized for the study ofthe lower energy part of the spectrum. During the Phase-I (2007 - 2010), Borexino first detectedand then precisely measured the flux of the 7Be solar neutrinos, ruled out any significant day–nightasymmetry of their interaction rate, made the first direct observation of the pep neutrinos, and setthe tightest upper limit on the flux of CNO solar neutrinos. In this paper we discuss the signalsignature 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 detailedmodeling of the detector response, the ability to define an innermost fiducial volume with extremelylow background via software cuts, and the excellent pulse–shape discrimination capability of thescintillator that allows particle identification. We report a measurement of the annual modulationof the 7Be neutrino interaction rate. The period, the amplitude, and the phase of the observedmodulation are consistent with the solar origin of these events, and the absence of their annualmodulation is rejected with higher than 99% C.L. The physics implications of Phase-I results in thecontext of the neutrino oscillation physics and solar models are presented