Davide Franco for the Borexino Collaboration Milano University & INFN 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

Solar 8B Neutrino Flux - A smooth transition is expected between the two different regimes: NOT YET EXPLORED! 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) Measured in real time by water Cherenkov experiments (Kamiokande, SuperKamiokande and SNO) Davide Franco – Università di Milano & INFN NOW 2008

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 > 5 MeV > 7 MeV > 5.5 MeV > 6 MeV *Threshold is defined at 100% trigger efficiency BS07(GS98) Water Cherenkov 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 NOW 2008

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 NOW 2008

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 Cosmogenic isotopes 214Bi and 208Tl from 238U and 232Th bulk contamination Raw Spectrum live-time: 246 days Count-rate: 30 c/d/100 ton S/B ratio < 1/100!!! Davide Franco – Università di Milano & INFN NOW 2008

Residual Cosmic Muon Flux (1.16 hr-1 m-2) Muons identified by the muon veto (>99% efficiency) in the 8B energy range m 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 NOW 2008

Muon and neutron cuts Muon cut: Neutron 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 NOW 2008

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 NOW 2008

Muon induced radioactive nuclides Cosmogenic candidate energy spectrum Entire mass 100 tons Event delay time after a m t = 29±2 ms t = 1.1±0.1 s Random Coincidences c2/ndf = 43.5/45 Davide Franco – Università di Milano & INFN NOW 2008

Cosmogenic cut Cosmogenic cut: Count-rate: 0.4 c/d/100 ton 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 NOW 2008

10C and 214Bi removal 10C rejection: 214Bi 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 10C spectrum Count-rate: 0.3 c/d/100 ton Davide Franco – Università di Milano & INFN NOW 2008

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 NOW 2008

The 8B n spectrum BS07(GS98) + MSW-LMA 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 NOW 2008

Summary of the Cuts and Systematic Counts 2.8-16.3 MeV 5.0-16.3 MeV None 20449 14304 Muon cut 3363 1135 Neutron cut 3280 1114 FV cut 567 372 Cosmogenic cut 71 26 10C removal 65 214Bi removal 62 Expected 208Tl 14 + 3 Measured 8B-n 48 + 8 26 + 5 BS07(GS98) 8B-n 50 + 5 25 + 3 BS07(AGS05) 8B-n 40 + 4 20 + 2 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 NOW 2008

8B equivalent n flux Equivalent unoscillated 8B neutrino flux, as derived from the electron scattering rate 2.8-16.3 MeV 5.0-16.3 MeV Rate [c/d/100 tons] 0.26±0.04±0.02 0.14±0.03± 0.01 FESexp [106 cm−2s−1] 2.65±0.44±0.18 2.75±0.54±0.17 FESexp/FESth 0.96±0.19 1.02±0.23 > 5 MeV > 7 MeV > 5.5 MeV > 6 MeV > 2.8 MeV *Threshold is defined at 100% trigger efficiency Good agreement with the SK-I and SNO D20 measurements (same threshold at 5 MeV) Davide Franco – Università di Milano & INFN NOW 2008

Electron Neutrino Survival Probability R: measured rate En and 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 NOW 2008

Conclusion Future: 278 tons analysis 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 NOW 2008

(submitted to PRL) Davide Franco – Università di Milano & INFN NOW 2008

Spares Davide Franco – Università di Milano & INFN NOW 2008

8B Spectral Distortion ratio Davide Franco – Università di Milano & INFN NOW 2008