davide franco for the borexino collaboration milano university & infn
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Measurement of the solar 8B neutrino flux with 246 live days of Borexino …and observation of the MSW vacuum-matter
transition!
Davide Franco for the Borexino CollaborationMilano University & INFN
Neutrino Oscillation WorkshopSeptember 6-13, 2008 – Conca Specchiulla
Davide Franco – Università di Milano & INFNNOW 2008
MSW-LMA theory: neutrino oscillations are dominated
- by resonant matter-enhanced oscillations at higher energies (>5 MeV)
Solar 8B Neutrino Flux
Measured in real time by water Cherenkov experiments (Kamiokande, SuperKamiokande and SNO)
- by vacuum oscillations at low energies (<2 MeV)
- A smooth transition is expected between the two different regimes: NOT YET EXPLORED!
Davide Franco – Università di Milano & INFNNOW 2008
> 5 MeV
> 7 MeV
> 5 MeV
> 5.5 MeV
> 6 MeV
> 7 MeV
*Threshold is defined at 100% trigger efficiency
BS07(GS98)
Water Cherenkov
8B Flux Measured in Elastic Scattering
Borexinoexpected 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
MSW-LMA (arXiv:0806.2649):• m2=7.69×10−5 eV2 • tan2=0.45
8B solar neutrino flux measurements in elastic scattering
Davide Franco – Università di Milano & INFNNOW 2008
The lowest threshold: 2.8 MeV
2.6 MeV ’s from 208Tl on PMT’s and in the buffer
All volume
R < 3 m (100 tons)
Energy spectrum in Borexino (after subtraction)
Expected 8B rate in 100 tons of liquid scintillator above 2.8 MeV:0.26±0.03 c/d/100 tons
> 5 distant from the 2.6 MeV peak
Davide Franco – Università di Milano & INFNNOW 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 & INFNNOW 2008
Residual Cosmic Muon Flux(1.16 hr-1 m-2)
Muons identified by the muon veto (>99% efficiency) in the 8B energy range
At these energies, detected muons cross the buffer only(not the scintillator!)
Reconstructed in the fiducial volume(radius < 3 m, ~100 tons)
Muon energy spectrum
Davide Franco – Università di Milano & INFNNOW 2008
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 s)
• Residual neutron rate: ~10-4 c/d
Count-rate: 4.8 c/d/100 ton
Davide Franco – Università di Milano & INFNNOW 2008
Fiducial Volume Cut(radius < 3 m, ~100 tons)
Count-rate: 2.3 c/d/100 ton
Surface contamination:• 222Rn and 220Rn emanated from the nylon vessel• Effective attenuation length: ~5 cm• Residual contamination: ~10-4 c/d
Davide Franco – Università di Milano & INFNNOW 2008
Muon induced radioactive nuclides
Cosmogenic candidateenergy spectrum
Entire mass
100 tons
Event delay time after a
= 29±2 ms
= 1.1±0.1 s
Random Coincidences
2/ndf = 43.5/45
Davide Franco – Università di Milano & INFNNOW 2008
Cosmogenic cut
Cosmogenic cut: • 5 s veto after each 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 & INFNNOW 2008
10C and 214Bi removal10C 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 s)
• 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 & INFNNOW 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 & INFNNOW 2008
The 8B spectrum
Neutrino oscillation is confirmed at 4.2 , including the theoretical uncertainty (10%) on the 8B flux from the Standard Solar Model
BS07(GS98)
BS07(GS98) + MSW-LMA
Davide Franco – Università di Milano & INFNNOW 2008
Summary of the Cuts and Systematic
Cut Counts2.8-16.3 MeV
Counts5.0-16.3 MeV
None 20449 14304
Muon cut 3363 1135
Neutron cut 3280 1114
FV cut 567 372
Cosmogenic cut 71 26
10C removal 65 26
214Bi removal 62 26
Expected 208Tl 14 + 3 0
Measured 8B- 48 + 8 26 + 5
BS07(GS98) 8B- 50 + 5 25 + 3
BS07(AGS05) 8B- 40 + 4 20 + 2
*MSW-LMA: m2=7.69×10−5 eV2, tan2=0.45
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%)
Davide Franco – Università di Milano & INFNNOW 2008
> 5 MeV
> 7 MeV
> 5 MeV
> 5.5 MeV
> 6 MeV
> 7 MeV > 2.8 MeV
*Threshold is defined at 100% trigger efficiency
8B equivalent flux
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
ESexp [106 cm−2s−1] 2.65±0.44±0.18 2.75±0.54±0.17
ESexp/ES
th 0.96±0.19 1.02±0.23
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 & INFNNOW 2008
Electron Neutrino Survival Probability
Pee is defined such that:R: measured rateE and Te: neutrino and recoiled electron energiesT0 = 2.8 MeV: energy threshold
E0 = 3.0 MeV: minimum neutrino energy at T0
Ne: number of target electronsx (x=e,): elastic cross sections
For the first time, we confirm at 1.8 , 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
Pee(7Be) = 0.56±0.10 (0.862 MeV)
Pee(8B) = 0.35±0.10 (8.6 MeV)
Davide Franco – Università di Milano & INFNNOW 2008
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 3 C.L. the transition upturn, comparing the BX rate with E < 5 MeV and the SK-I one with E > 5 MeV
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
• For the first time, a single detector observed both the vacuum and matter-enhanced dominated oscillation regimes, confirming the transition at 1.8