May 25 – 27, 2011
Rettorato, University Roma TRE, Roma, Italy
Europe/Rome timezone
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New Solar 7Be rate and day night effect measurements in Borexino

May 26, 2011, 5:05 PM
Rettorato, University Roma TRE, Roma, Italy

Rettorato, University Roma TRE, Roma, Italy

Viale Ostiense 159 00148, Roma Italy Room: Aula Magna


Gemma Testera (GE)


Two new experimental data about solar 7Be neutrinos have been released in the middle April 2011 by the Borexino collaboration: the new high precision interaction rate and the absence of the day night effect. The two results will be discussed together with their physical consequences about low energy neutrino oscillations.


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 +1.6-1.5 syst counts/(day100 ton). This 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 4.9 % 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 neutrino in vacuum, and permits to probe with unprecedented sensitivity the transition between the matter-enhanced and vacuum-dominated neutrino

I will in addition report on a search for the day-night asymmetry of the 7Be solar neutrino rate measured by Borexino at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The measured value, Adn = 0.001 +- 0.012 (stat) +- 0.007 (syst), shows the absence of a significant asymmetry. This result alone rejects the so-called LOW solution at more than 8.5 sigma. Combined with the other solar neutrino data, it isolates the Large Mixing Angle LMA-MSW solution at chi2 > 190 without relying on the assumption of CPT symmetry in the neutrino sector. We also show that including the day-night asymmetry, data from Borexino alone restricts the MSW neutrino oscillations to the LMA solution at 90% CL.

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