Transcript Measurement of the solar B neutrino …

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
Measured in real time by water
Cherenkov experiments
(Kamiokande, SuperKamiokande
and SNO)
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!
NOW 2008
Davide Franco – Università di Milano & INFN
8B
n Flux Measured in Elastic Scattering
BS07(GS98)
Borexino
expected rate in 100 tons
of liquid scintillator in the
entire energy spectrum:
0.49±0.05 c/d/100 tons
> 7 MeV
> 7 MeV
Above 5 MeV:
> 5.5 MeV
> 5 MeV
> 5 MeV
> 6 MeV
0.14±0.01 c/d/100 tons
solar neutrino
flux measurements in
Water
Cherenkov
8B
elastic scattering
*Threshold is defined at 100% trigger efficiency
MSW-LMA (arXiv:0806.2649):
• Dm2=7.69×10−5 eV2
• tan2=0.45
NOW 2008
Davide Franco – Università di Milano & INFN
The lowest threshold: 2.8 MeV
Expected 8B n rate in
100 tons of liquid
scintillator above 2.8
MeV:
2.6 MeV g’s from 208Tl on
PMT’s and in the buffer
All volume
0.26±0.03 c/d/100 tons
R<3m
(100 tons)
Energy spectrum in Borexino
(after m subtraction)
> 5s distant from the 2.6 MeV g peak
NOW 2008
Davide Franco – Università di Milano & INFN
Background in the 2.8-16.3 MeV range
• Cosmic Muons
Raw Spectrum
• High energy gamma’s from
neutron captures
• 208Tl and 214Bi from radon
emanation from nylon vessel
• Cosmogenic isotopes
live-time: 246 days
• 214Bi and 208Tl from 238U and 232Th
bulk contamination
Count-rate: 30 c/d/100 ton
S/B ratio < 1/100!!!
NOW 2008
Davide Franco – Università di Milano & INFN
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!)
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Reconstructed in the fiducial volume
(radius < 3 m, ~100 tons)
Davide Franco – Università di Milano & INFN
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
NOW 2008
Davide Franco – Università di Milano & INFN
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
NOW 2008
Davide Franco – Università di Milano & INFN
Muon induced radioactive nuclides
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
Entire mass
Random Coincidences
100 tons
NOW 2008
Davide Franco – Università di Milano & INFN
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
NOW 2008
Davide Franco – Università di Milano & INFN
10C
10C
and 214Bi removal
10C
rejection:
spectrum
• 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
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Count-rate: 0.3 c/d/100 ton
Davide Franco – Università di Milano & INFN
208Tl
subtraction
208Tl:
• from 232Th contamination in the
scintillator
• can not be tagged event by
event
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• 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
The 8B n spectrum
BS07(GS98)
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
NOW 2008
Davide Franco – Università di Milano & INFN
Summary of the Cuts and Systematic
Cut
Counts
2.8-16.3 MeV
Counts
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
65
26
62
26
Expected 208Tl
14 + 3
0
Measured 8B-n
48 + 8
26 + 5
BS07(GS98) 8B-n
50 + 5
25 + 3
BS07(AGS05) 8B-n
40 + 4
20 + 2
removal
214Bi
removal
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
NOW 2008
Davide Franco – Università di Milano & INFN
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
Good agreement with the
SK-I and SNO D20
measurements (same
threshold at 5 MeV)
> 7 MeV
> 7 MeV
> 2.8 MeV
> 5.5 MeV
> 5 MeV
> 5 MeV
> 6 MeV
*Threshold is defined at 100% trigger efficiency
NOW 2008
Davide Franco – Università di Milano & INFN
Electron Neutrino Survival Probability
Pee is defined such that:
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(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
NOW 2008
Davide Franco – Università di Milano & INFN
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 matterenhanced 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
NOW 2008
Davide Franco – Università di Milano & INFN
(submitted to PRL)
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Davide Franco – Università di Milano & INFN
Spares
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Davide Franco – Università di Milano & INFN
Spectral Distortion
ratio
8B
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Davide Franco – Università di Milano & INFN