High Precision 7be Solar Neutrinos Measurement and Day Night Effect Obtained with Borexino

We report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso and the search for a day–night asymmetry of this interaction rate. The interaction rate of the 0.862MeV 7Be neutrinos is 46±1.6(stat)−1.6+1.5(syst) counts/(day · 100ton). The hypothesis of no oscillation for this solar neutrinos is inconsistent with our measurement at the 5.8σ C.L. Our result is the first direct measurement of solar neutrinos with an accuracy better than 5%. We report the survival probability for solar νe in the transition region between matter-enhanced and vacuum-driven oscillations. The measured day night asymmetry is Adn=0.001±0.012 (stat)±0.007 (syst), in agreement with the predicition of MSW–LMA neutrino [13] oscillations. This result discourages MSW oscillations with mixing parameters in the LOW region at more than 8.5σ, meaning that this region is, for the first time, strongly rejected without the assumption of CPT symmetry. The result can also be used to constrain some neutrino oscillation scenarios involving new physics.