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!

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!


> 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


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


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!!!


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


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


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


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


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



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



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


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


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%)


> 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)


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)


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


(submitted to PRL)


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8B Spectral Distortionra

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Davide Franco for the Borexino Collaboration Milano University & INFN