First Evidence ofpepSolar Neutrinos by Direct Detection in Borexino

Abstract

We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy range. We measured the rate of pep solar neutrino interactions in Borexino to be (3.1±0.6stat±0.3syst) counts/(day·100 ton) and provided a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day·100 ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. This unprecedented sensitivity was achieved by adopting novel data analysis techniques for the rejection of cosmogenic 11 C, the dominant background in the 1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6±0.3)×10 8 cm −2 s −1 and <7.7×10 8 cm −2 s −1 (95% C.L.), respectively, in agreement with the Standard Solar Model. These results represent the first measurement of the pep neutrino flux and the strongest constraint of the CNO solar neutrino flux to date.PACS numbers: 13.35. Hb, 14.60.St, 26.65.+t, 95.55.Vj, 29.40.Mc Over the past 40 years solar neutrino experiments [1][2][3][4][5] have proven to be sensitive tools to test both astrophysical and elementary particle physics models. Solar neutrino detectors have demonstrated that stars are powered by nuclear fusion reactions. Two distinct processes, the main pp fusion chain and the sub-dominant CNO cycle, are expected to produce solar neutrinos with different energy spectra and fluxes. Until now only fluxes from the pp chain have been measured:7 Be, 8 B, and, indirectly, pp. Experiments involving solar neutrinos and reactor anti-neutrinos [6] have shown that solar neutrinos undergo flavor oscillations.Results from solar neutrino experiments are consistent with the Mikheyev-Smirnov-Wolfenstein Large Mixing Angle (MSW-LMA) model [7], which predicts a transition from vacuum-dominated to matter-enhanced oscillations, resulting in an energy dependent ν e survival probability, P ee . Non-standard neutrino interaction models formulate P ee curves that deviate significantly from MSW-LMA, particularly in the 1-4 MeV transition region, see e.g. [8]. The mono-energetic 1.44 MeV pep neutrinos, which belong to the pp chain and whose StanarXiv:1110.3230v1 [hep-ex] 14 Oct 2011 2 dard Solar Model (SSM) predicted flux has one of the smallest uncertainties (1.2%) due to the solar luminosity constraint [9], are an ideal probe to test these competing hypotheses.The detection of neutrinos resulting from the CNO cycle has important implications in astrophysics, as it would be the first direct evidence of the nuclear process that is believed to fuel massive stars (>1.5M ). Furthermore, its measurement may help to resolve the solar metallicity problem [9,10]. The energy spectrum of neutrinos from the CNO cycle is the sum of three continuous spectra with end point energies of 1.19 ( 13 N), 1.73 ( 15 O) and 1.74 MeV ( 17 F), close to the pep neutrino energy. The total CNO flux is similar to that of the pep neutrinos but its predicted value is strongly dependent on th...

Cosmic-muon flux and annual modulation in Borexino at 3800 m water-equivalent depth

“…It is devoted to the spectroscopy of low-energy solar neutrinos via elastic scattering on electrons. Data taking started in May 2007 and led to measurements of solar -1 -neutrinos ( 7 Be [8,9], pep [10], 8 B [11], and a limit on CNO [10]), as well as antineutrinos from the Earth (geo-neutrinos) [12]. Borexino is also a powerful tool for both the study of cosmic muons that penetrate the Gran Sasso rock coverage and the neutrons and radioactive isotopes that they produce, which are relevant backgrounds for dark matter and neutrino experiments.…”

A Search for Low-energy Neutrinos Correlated with Gravitational Wave Events GW 150914, GW 151226, and GW 170104 with the Borexino Detector

“…Borexino first detected and then precisely measured the flux of the 7 Be solar neutrinos [20,21,30], has ruled out any significant day-night asymmetry of their interaction rate [26], has measured the 8 B-neutrino rate with 3 MeV threshold [23], has made the first direct observation of pep neutrinos [27], has made the first spectral measurement of pp-neutrinos [28] and has set the best upper limit on the flux of solar neutrinos produced in the CNO cycle [27]. The uniquely low background level of the Borexino detector made it possible to set new limits on the effective magnetic moment of the neutrino [21], on the stability of the electron for decay into a neutrino and a photon [31], on the heavy sterile neutrino mixing in 8 B decay [32], on the possible violation of the Pauli exclusion principle [33], on the flux of high energy solar axions [34], on antineutrinos from the Sun and other unknown sources [35], on Gamma-Ray bursts neutrino and antineutrino fluences [36] and on some other rare processes.…”

“…A detailed description of the detector could be found elsewhere [17][18][19][20][21][22][23][24][25][26][27][28][29].…”

Solar neutrinos: an update

“…Borexino reported in 2012 the first time direct observation of solar neutrinos in the 1.0 − 1.5 MeV energy range [28]. Borexino measured the rate of pep solar neutrino interactions in Borexino to be (3.1 ± 0.6(stat) ± 0.3(syst)) counts/(day x 100 ton) and provided a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day x 100 ton) (95% C.L.…”