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Davide Franco for the Borexino Collaboration Milano University & INFN

Measurement of the solar 8 B neutrino flux with 246 live days of Borexino … and observation of the MSW vacuum-matter transition!. Neutrino Oscillation Workshop September 6-13, 2008 – Conca Specchiulla. Davide Franco for the Borexino Collaboration Milano University & INFN.

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Davide Franco for the Borexino Collaboration Milano University & INFN

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  1. Measurement of the solar 8B neutrino flux with 246 live days of Borexino …and observation of the MSW vacuum-matter transition! Neutrino Oscillation Workshop September 6-13, 2008 – Conca Specchiulla Davide Franco for the Borexino Collaboration Milano University & INFN

  2. MSW-LMA theory: neutrino oscillations are dominated - by resonant matter-enhanced oscillations at higher energies (>5 MeV) - by vacuum oscillations at low energies (<2 MeV) - A smooth transition is expected between the two different regimes: NOT YET EXPLORED! Solar 8B Neutrino Flux Measured in real time by water Cherenkov experiments (Kamiokande, SuperKamiokande and SNO) Davide Franco – Università di Milano & INFN

  3. BS07(GS98) > 7 MeV > 7 MeV > 5.5 MeV > 5 MeV > 5 MeV > 6 MeV Water Cherenkov *Threshold is defined at 100% trigger efficiency 8B n Flux Measured in Elastic Scattering Borexino expected rate in 100 tons of liquid scintillator in the entire energy spectrum: 0.49±0.05 c/d/100 tons Above 5 MeV: 0.14±0.01 c/d/100 tons 8B solar neutrino flux measurements in elastic scattering • MSW-LMA (arXiv:0806.2649): • Dm2=7.69×10−5 eV2 • tan2=0.45 Davide Franco – Università di Milano & INFN

  4. The lowest threshold: 2.8 MeV Expected 8B n rate in 100 tons of liquid scintillator above 2.8 MeV: 0.26±0.03 c/d/100 tons 2.6 MeV g’s from 208Tl on PMT’s and in the buffer All volume R < 3 m (100 tons) Energy spectrum in Borexino (after m subtraction) > 5s distant from the 2.6 MeV g peak Davide Franco – Università di Milano & INFN

  5. Count-rate: 30 c/d/100 ton S/B ratio < 1/100!!! Background in the 2.8-16.3 MeV range • Cosmic Muons • High energy gamma’s from neutron captures • 208Tl and 214Bi from radon emanation from nylon vessel • Cosmogenicisotopes • 214Bi and 208Tl from 238U and 232Th bulk contamination Raw Spectrum live-time: 246 days Davide Franco – Università di Milano & INFN

  6. m Residual Cosmic Muon Flux(1.16 hr-1 m-2) Muons identified by the muon veto (>99% efficiency) in the 8B energy range Muon energy spectrum At these energies, detected muons cross the buffer only (not the scintillator!) Reconstructed in the fiducial volume (radius < 3 m, ~100 tons) Davide Franco – Università di Milano & INFN

  7. Muon and neutron cuts • Muon cut: • All events detected by the outer detector are rejected • Residual muon rate: <10-3 c/d • Neutron cut: • 2 ms veto after each muon detected by the outer detector, in order to reject induced neutrons (mean capture time ~250 ms) • Residual neutron rate: ~10-4 c/d Count-rate: 4.8 c/d/100 ton Davide Franco – Università di Milano & INFN

  8. Fiducial Volume Cut(radius < 3 m, ~100 tons) • Surface contamination: • 222Rn and 220Rn emanated from the nylon vessel • Effective attenuation length: ~5 cm • Residual contamination: ~10-4 c/d Count-rate: 2.3 c/d/100 ton Davide Franco – Università di Milano & INFN

  9. Event delay time after a m t = 29±2 ms c2/ndf = 43.5/45 t = 1.1±0.1 s Cosmogenic candidate energy spectrum Random Coincidences Entire mass 100 tons Muon induced radioactive nuclides Davide Franco – Università di Milano & INFN

  10. Cosmogenic cut • Cosmogenic cut: • 5 s veto after each m crossing the buffer • Rejection efficiency cut: 99.7% • Residual short-lived cosmogenic rate: 3x10-3 c/d • Dead-time: 23.4% • Effective detector live-time: 188 days Count-rate: 0.4 c/d/100 ton Davide Franco – Università di Milano & INFN

  11. 10C and 214Bi removal 10C spectrum • 10C rejection: • Only 6% of the 10C spectrum falls in the 8B energy range • Three-fold coincidence with the muon parent and the following neutron emission • Efficiency 95% (MC) • Invisible channel, 12C(p,t)10C, contribution estimated (MC) in 2x10-3 c/d • Overall residual rate: 4x10-3 c/d • 214Bi rejection: • 214Bi-Po delayed coincidence (234 ms) • Rejection efficiency: 89% • Residual 214Bi rate: 2x10-3 c/d Count-rate: 0.3 c/d/100 ton Davide Franco – Università di Milano & INFN

  12. 208Tl subtraction • 208Tl: • from 232Th contamination in the scintillator • can not be tagged event by event • estimated with the branching competitor 212Bi-Po • 232Th contamination, assuming secular equilibrium: • (6.8±1.5)x10-18 g/g Davide Franco – Università di Milano & INFN

  13. BS07(GS98) BS07(GS98) + MSW-LMA The 8B n spectrum Neutrino oscillation is confirmed at 4.2 s, including the theoretical uncertainty (10%) on the 8B flux from the Standard Solar Model Davide Franco – Università di Milano & INFN

  14. Summary of the Cuts and Systematic • Systematic errors: • 6% from the determination of the fiducial mass • 3% (2%) uncertainty in the 8B rate above 2.8 MeV (5.0 MeV) from the determination of the light yield (1%) *MSW-LMA: Dm2=7.69×10−5 eV2, tan2=0.45 Davide Franco – Università di Milano & INFN

  15. > 7 MeV > 7 MeV > 2.8 MeV > 5.5 MeV > 5 MeV > 5 MeV > 6 MeV *Threshold is defined at 100% trigger efficiency 8B equivalent n flux Equivalent unoscillated 8B neutrino flux, as derived from the electron scattering rate Good agreement with the SK-I and SNO D20 measurements (same threshold at 5 MeV) Davide Franco – Università di Milano & INFN

  16. Electron Neutrino Survival Probability R: measured rate Enand Te: neutrino and recoiled electron energies T0 = 2.8 MeV: energy threshold E0 = 3.0 MeV: minimum neutrino energy at T0 Ne: number of target electrons sx (x=e,m-t): elastic cross sections Pee is defined such that: Pee(8B) = 0.35±0.10 (8.6 MeV) Pee(7Be) = 0.56±0.10 (0.862 MeV) For the first time, we confirm at 1.8 s, using data from a single detector, the presence of a transition between the low energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillations, in agreement with the prediction of the MSW-LMA solution for solar neutrinos Davide Franco – Università di Milano & INFN

  17. Conclusion • Borexino has detected, for the first time, 8B solar neutrinos in liquid scintillator • The Borexino energy threshold (2.8 MeV) is the lowest so far among spectral measurements of 8B neutrinos • Neutrino oscillation is confirmed at 4.2 s • For the first time, a single detector observed both the vacuum and matter-enhanced dominated oscillation regimes, confirming the transition at 1.8 s • Future: 278 tons analysis • Feasibility of a new analysis on the entire active mass (278 tons) is under study • Advantages: • systematic from the determination of the fiducial mass eliminated • higher statistics • Disadvantage: • statistical subtraction of the surface contamination • Lowering the threshold down to 2 MeV? • In 5 years we can observe at 3s C.L. the transition upturn, comparing the BX rate with E < 5 MeV and the SK-I one with E > 5 MeV Davide Franco – Università di Milano & INFN

  18. (submitted to PRL) Davide Franco – Università di Milano & INFN

  19. Spares Davide Franco – Università di Milano & INFN

  20. 8B Spectral Distortion ratio Davide Franco – Università di Milano & INFN

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