1. 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

2. 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

3. 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: ):
Dm2=7.69×10−5 eV2
tan2=0.45
Davide Franco – Università di Milano & INFN
NOW 2008

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
NOW 2008

5. 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

6. 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

7. 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

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
NOW 2008

9. 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

10. 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

11. 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

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
NOW 2008

13. 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

14. Summary of the Cuts and Systematic
Counts
MeV
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

15. 8B equivalent n flux
Equivalent unoscillated 8B neutrino flux, as derived from the electron scattering rate
MeV
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

16. 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

17. 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

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

19. Spares
Davide Franco – Università di Milano & INFN
NOW 2008

20. 8B Spectral Distortion ratio
Davide Franco – Università di Milano & INFN
NOW 2008