Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun

For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton–proton (pp) chain and the carbon–nitrogen–oxygen (CNO) cycle. Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal—only a few counts per day above the background per 100 tonnes of target—that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate—as well as the flux of emitted CNO neutrinos—depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe

The HLMA project: determination of high delta-m^2 LMA mixing parameters and constraint on |U_e3| with a new reactor neutrino experiment

In the forthcoming months, the KamLAND experiment will probe the parameter space of the solar large mixing angle (LMA) MSW solution as the origin of the solar neutrino deficit with \nuebar's from distant nuclear reactors. If however the solution realized in nature is such that \Dm2_{sol} \gsim 2 \cdot 10^{-4} eV$^2$ (thereafter named the HLMA region), KamLAND will only observe a rate suppression but no spectral distortion and hence it will not have the optimal sensitivity to measure the mixing parameters. In this case, we propose a new medium baseline reactor experiment located at Heilbronn (Germany) to pin down the precise value of the solar mixing parameters. In this paper, we present the Heilbronn detector site, we calculate the \nuebar interaction rate and the positron spectrum expected from the surrounding nuclear power plants. We also discuss the sensitivity of such an experiment to |U_e3| in both normal and inverted neutrino mass hierarchy scenarios. We then outline the detector design, estimate background signals induced by natural radioactivity as well as by in-situ cosmic ray muon interaction, and discuss a strategy to detect the anti-neutrino signal 'free of background'.Comment: 22 pages, 5 figures; v2: added references, caption of Fig.4 and typos corrected; v3: accepted for publication in Astroparticle Physics, references added, typo in Sec. 6.3 correcte

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

Borexino has been running since May 2007 at the Laboratori Nazionali del Gran Sasso 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 Phase-I (2007-2010) Borexino first detected and then precisely measured the flux of the Be-7 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 solar neutrinos produced in the CNO cycle (carbon nitrogen oxigen) where carbon nitrogen and oxygen serve as catalysts in the fusion process. 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 Be-7 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

Remaining inconsistencies with solar neutrinos: can spin flavour precession provide a clue?

A few inconsistencies remain after it has been ascertained that LMA is the dominant solution to the solar neutrino problem: why is the SuperKamiokande spectrum flat and why is the Chlorine rate prediction over two standard deviations above the data. There also remains the ananswered and important question of whether the active neutrino flux is constant or time varying. We propose a scenario involving spin flavour precession to sterile neutrinos with three active flavours that predicts a flat SuperK spectrum and a Chlorine rate prediction more consistent with data. We also argue that running the Borexino experiment during the next few years may provide a very important clue as to the possible variability of the solar neutrino flux.Comment: 3 pages, 2 figures, contribution to TAUP 2009 (Rome

Precision Measurement of the (7)Be Solar Neutrino Interaction Rate in Borexino

A direct measurement of the 0.862 MeV 7Be solar neutrino interaction rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso yields 46.0±1.5stat+1.6−1.5syst counts/day/(100 tons). Our result is the first direct measurement of a sub-MeV solar neutrino rate with an accuracy better than 5%. The hypothesis of no oscillation for 7Be solar neutrinos is rejected at 4.9σ C.L. Using the latest Standard Solar Model (SSM) flux predictions, the result leads directly to a precise determination of the survival probability for solar νe's in vacuum, and permits us to probe with unprecedented sensitivity the transition between the matter-enhanced and vacuum-dominated neutrino oscillation regimes characteristic of the MSW-LMA solution to the solar neutrino problem