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

Abstract

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.

Results from the first use of low radioactivity argon in a dark matter search

“…We refer to the LSV and WCV detectors together as the veto system of DarkSide-50. The DarkSide-50 detectors are located in Hall C of LNGS, in close proximity to and sharing many facilities with the Borexino experiment [6,7].…”

“…Figure 16 shows the в -spectrum of 14C convolved with the fitted LSV response, compared to the Phase-II data. The fitting function includes the energy resolution and ionization quenching (assuming a Birks' constant of 0.012 cm/MeV [6] and the model described in [11]), and an additional constant background term. The fit is performed over the photoelectron distribution of clusters in random coincidence with the LAr TPC between 25 and 120 PE.…”

“…The light yields derived from fits to the 14C в -spectrum, the two 60Co y-rays, and the 208Tl y-ray in Phase-II are summarized in table 5. The calculation of the light yield from the 14C в -spectrum and the 60Co y-rays assume a quenching model described by a Birks' constant k B = 0.012 cm/MeV [6]. …”

Neutrino versus antineutrino oscillation parameters at DUNE and Hyper-Kamiokande experiments

“…Neutrino oscillation parameters are mostly constrained by solar neutrino experiments [40][41][42][43][44][45], the SuperKamiokande atmospheric neutrino data [37], and accelerator data from MINOS [46], T2K [47], and the NuMI Off-Axis ν e Appearance Experiment (NOνA) [48]. These imply the existence of two hierarchical mass-squared differences ∆m 2 21 and ∆m 2 32 and three nonzero mixing angles θ 13 , θ 12 , θ 23 .…”

Borexino: geo-neutrino measurement at Gran Sasso, Italy

“…A thorough account of the effort to characterize the performances of the detector has been published in Bellini et al [2014], where also the full methodology adopted for the solar neutrino analysis is described.…”