Download: http://prl.aps.org/abstract/PRL/v108/i5/e051302

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+-0.6(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.). 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 11C, 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]x10^8 cm^-2s-1 and 7.7×10^8 cm^-2s-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.

First Evidence of pep Solar Neutrinos by Direct Detection in Borexino

Neutrinos coming from the sun offer a window into the inner workings of our star. Scientists have detected the neutrino signature of several different solar nuclear reactions, but other steps in the fusion process have remained elusive. Now, the Borexino Collaboration, which runs a neutrino detector that lies a kilometer below the Gran Sasso mountain in Italy, reports in Physical Review Letters that they have obtained the first evidence of a relatively rare fusion reaction in the sun, while also placing strong limits on another.

Deep in their cores, most stars get their energy by fusing hydrogen into helium. This has been verified by Earth-bound measurements of solar neutrinos, which are one of the by-products of nuclear fusion. For our sun, the dominant pathway is the proton-proton, or pp, reaction chain.

Solar models predict that other reaction pathways occur in the sun. The proton-electron-proton, or pep, reaction produces deuterium that can feed into the pp chain, but only 1out of 400deuterium atoms are made through pep. The signature for the pep reaction is a neutrino with a distinct energy of 1.44mega-electron-volts, and the Borexino experiment was designed to detect neutrinos in this energy range. By carefully removing background signals from cosmic rays and other sources, such as gamma rays from the rocks surrounding the detector and from detector materials, the Borexino Collaboration (Bellini et al.) claims to have seen 3.1 pep neutrinos per day per 100tons of detector. The team also looked for neutrinos from a separate reaction network, the carbon-nitrogen-oxygen, or CNO, cycle, but was only able to set a stringent upper limit on the flux of these neutrinos. As more data are collected, the researchers may be able to discriminate between competing models of the sun as well as disentangle the different ways neutrino flavors can mix. – Michael Schirber

G. Bellini et al. (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek

Download: http://www.sciencedirect.com/science/article/pii/S037026931101389X

Abstract

We report the result of a search for a day–night asymmetry in the 7Be solar neutrino interaction rate in the Borexino detector at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The measured asymmetry is Adn=0.001±0.012 (stat)±0.007 (syst), in agreement with the prediction of MSW-LMA solution for neutrino oscillations. This result disfavors MSW oscillations with mixing parameters in the LOW region at more than 8.5 σ. This region is, for the first time, strongly disfavored without the use of reactor anti-neutrino data and therefore the assumption of CPT symmetry. The result can also be used to constrain some neutrino oscillation scenarios involving new physics.

G. Bellini et al. (Borexino Collaboration) M. Wójcik, G. Zuzel, M. Misiaszek

Download: http://prl.aps.org/abstract/PRL/v107/i14/e141302

Abstract

The rate of neutrino-electron elastic scattering interactions from 862 keV ⁷Be solar neutrinos in Borexino is determined to be 46.0±1.5(stat)_-1.6^+1.5(syst) counts/(day·100  ton). This corresponds to a ν_e-equivalent ⁷Be solar neutrino flux of (3.10±0.15)×10⁹ cm^-2 s^-1 and, under the assumption of ν_e transition to other active neutrino flavours, yields an electron neutrino survival probability of 0.51±0.07 at 862 keV. The no flavor change hypothesis is ruled out at 5.0 σ. A global solar neutrino analysis with free fluxes determines Φ_pp=6.06_-0.06^+0.02×10¹⁰ cm^-2 s^-1 and Φ_CNO<1.3×10⁹ cm^-2 s^-1 (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.