1 / 20

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.

denton-santos
Download Presentation

Davide Franco for the Borexino Collaboration Milano University & INFN

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related