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First Results from the Borexino Solar Neutrino Experiment
Celebrating
F.Avignone, E.Fiorini & P. Rosen
University of South Carolina
May 16, 2008
Frank Calaprice
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First Contact with Frank Avignone
65Zn source given by Ray Davis
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Axion SearchesSummary of Texono Coll. 2006
65Zn
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Science with Borexino
The Neutrino The Sun The Earth Supernovae
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Basic Neutrino Facts
Postulated in 1931 by Pauli to preserve energy conservation in -decay.
First Observed by Cowan and Reines in 1950’s by inverse beta decay: e+p->n+e+.
Electric charge: 0; Spin: 1/2; Mass: very small Like other fermions, comes in 3 flavors:
e, ,
Interactions: only via the weak force (and gravity)
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Solar Neutrino Production
Occurs in two cycles: pp and CNO (mostly pp)
In each pp cycle: 26.7 MeV released 2 neutrinos created 4 protons are converted to 4He
Total Flux constrained by luminosity: =( 2’s/26.7MeV) (L/4r2) ~ 6.6x1010/cm2/s.
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Solar Neutrino Energy Spectrum
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Birth of Solar Neutrino Experiments
1965-67: Davis builds 615 ton chlorine (C2Cl4) detector
Deep underground to suppress cosmic ray backgrounds.
Homestake Mine (4800 mwe depth)
Low background proportional detector for 37Ar decay.
37Cl + e -> 37Ar +e-
Detect 37Ar +e- -> 37Cl + e (t
1/2 ~ 37 d)
Detected ~1/3 of expected rate.
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Chlorine Data 1970-1994
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Neutrino Oscillations
The Solar Neutrino Problem was explained by neutrino oscillations, the possibility of which was first suggested by Pontecorvo in 1967. An electron neutrino that oscillates into a muon
neutrino would not be detected in the chlorine reaction.
Experimental proof of oscillations came decades later from experiments on atmospheric neutrinos (SuperK), solar neutrinos (SNO), and reactor anti-neutrinos (Kamland).
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Neutrino Vacuum Oscillations
In 1967 Pontecorvo showed that non-conservation of lepton charge number would lead to oscillations in vacuum between various neutrino states.
In 1968 Gribov and Pontecorvo suggested this could explain the low result of Davis.
The neutrino rate is 2 times smaller if the oscillation length is smaller than the region where neutrinos are formed. The vacuum oscillation length is smaller than the sun’s
core for the observed mass value. Matter enhancement was needed to get the full deficit
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Matter Enhanced Oscillations 1978 Wolfenstein shows that neutrino
oscillations are modified when neutrinos interact with matter.
1985 Mikhaev and Smirnow show that neutrinos may undergo a resonant flavor conversion if the density of matter varies, as in the sun.
The MSW theory describes the enhanced oscillation in matter.
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The Sudbury Neutrino Observatory (SNO)
SNO is water Cherenkov detector with heavy (deuterated) water.
Detects 8B neutrinos Two reactions enable charged and
neutral currents to be observed e+ d -> p + p +e- (only e detected) x+ d -> p + n + x (all ’s; x = e,
) Observed that e oscillated to x
Total rate of neutrinos agrees with predictions
Oscillations proven to be cause of deficit!
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SNO Results Clinch Neutrino Oscillations
SNO First Results: 2001
Neutral current interactions(sensitive to all neutrinos equally)
Elastic scattering interactions(sensitive to all neutrinos, enhanced sensitivity for electron neutrinos)
Charged current interactions(sensitive only to electron neutrinos)
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The SNO Mixing Parameters
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The Kamland Detector
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Kamland Results 2003
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KamLAND Results 2005 Neutrinos from 53 Reactors
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The Vacuum-Matter Transition Above about 2 MeV solar
neutrino oscillations are influenced by interactions with matter, the MSW effect.
Below ~ 2 MeV neutrino oscillations are vacuum-like.
The 0.86 MeV 7Be neutrino provides a data point in the vacuum region
The Predicted Vacuum-Matter transition is being tested by Borexino.
p-p, 7Be, pep
8B
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Non-Standard Neutrino-MatterInteractions?
Exploring the vacuum-matter transition is sensitive to new physics.
New neutrino-matter couplings (either flavor-changing or lepton flavor violating) can be parametrized by a new MSW-equivalent term ε
Where is the relative effect of new physics the largest? At resonance!
Friedland, Lundardini & Peña-Garay
Blue: Standard oscillationsRed: Non-standard interactions tuned to agree with experiments.
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Borexino Historical Highlights
1989-92: Prototype CTF Detector started 1995-96: Low background in CTF achieved 1996-98: Funding INFN,NSF, BMBF, DFG 1998-2002: Borexino construction August 16 2002: Accidental release of ~50 liter of
liquid scintillator shuts down Borexino and LNGS 2002-2005: Legal and political actions: Princeton 2005 Borexino Restarts Fluid Operations August 16, 2007 First Borexino Results on Web.
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John Bahcall-Martin Deutsch
Borexino Mishap August 16 2002
Martin Deutsch January 29, 1917
August 16, 2002.
John Bahcall December 30, 1934 August 17, 2005
Borexino First Results Paper August 16 2007
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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The Borexino Detector
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Detection Principles
Detect -e scattering via scintillation light Features:
Low energy threshold (> 250 keV to avoid 14C) Good position recostruction by time of flight Good energy resolution (500 pe/MeV)
Drawbacks: No directional measurements ν induced events can’t be distinguished from
other β/γ due to natural radioactivity
Experiment requires extreme ssuppression of all radioactive contaminants
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Solar Neutrino Science Goals
Test MSW vacuum solution of neutrino oscillations at low energy.
Look for non-standard interactions. Measure CNO neutrinos- metallicity
problem. Compare neutrino and photon luminosities
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Neutrinos and Solar Metallicity A direct measurement of the CNO neutrinos rate could
help solve the latest controversy surrounding the Standard Solar Model.
One fundamental input of the Standard Solar Model is the metallicity of the Sun - abundance of all elements above Helium
The Standard Solar Model, based on the old metallicity derived by Grevesse and Sauval (Space Sci. Rev. 85, 161 (1998)), is in agreement within 0.5% with the solar sound speed measured by helioseismology.
Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a metallicity lower by a factor ~2. This result destroys the agreement with helioseismology
Can use solar neutrino measurements to help resolve!7Be (12% difference) and CNO (50-60% difference)
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Low Energy Neutrino Spectrum
Mono-energetic 7Be and pep neutrinos produce aBox-like electron recoil energy spectrum
pep
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The Underground Halls of the Gran Sasso Laboratory Halls in tunnel off A24
autostrada with horizontal drive-in access
Under 1400 m rock shielding (~3800 mwe)
Muon flux reduced by factor of ~106 to ~1 muon/m2/hr
BX in Hall C ~20mx20mx100m
To Rome ~ 100 km
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Special Methods Developed
Low background nylon vessel fabricated in hermetically sealed low radon clean room (~1 yr)
Rapid transport of scintillator solvent (PC) from production plant to underground lab to avoid cosmogenic production of radioactivity (7Be)
Underground purification plant to distill scintillator components. Gas stripping of scintlllator with special nitrogen, free of
radioactive 85Kr and 39Ar from air. All materials electropolished SS or teflon, precision
cleaned with a dedicated cleaning module Vacuum tightness standard: 10-8 atm-cc/s
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Purification of Scintillator
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Assembly of Distillation Column in Princeton Cleanroom
100
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Assembly of Columns
Installing sieve trays in distillation column
Installing structured packing in stripping column
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Fabrication of Nylon Vessel
John Bahcall
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Raw Spectrum- No cuts
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Expected Spectrum
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Data with Fiducial Cut (100 tons)Kills gamma background from PMTs
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Data: α/β Statistical Subtraction
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Data with Expected pep & CNO
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Published Data on 7Be Rate Phys Lett B 658 (2008) 101
Expected interaction rate in absence of oscillations:
75±4 cpd/100 tons
for LMA-MSW oscillations:
49±4 cpd/100 tons
Measured:47± 7± 12 cpd/100ton
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Matter-VacuumBefore Borexino
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After Borexino
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Future Possibilities?Borexino could possibly measure pep, 8B, and pp
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Background: 232ThAssuming secular equilibrium, 232Th is measured with the delayed
coincidence:
212Bi 212Po 208Pb
= 432.8 ns
2.25 MeV ~800 KeV eq.
From 212Bi-212Po correlated events in the scintillator: 232Th: < 6 ×10-18 g(Th)/g (90% C.L.)
Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton
Only fewbulk candidates
212Bi-212Po
Time (ns)
=423±42 ns
Events are mainly in the south vessel surface (probably particulate)
z (m
)
R (m) R(m)
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Background: 238U Assuming secular equilibrium, 238U
is measured with the delayed
coincidence:
214Bi 214Po 210Pb
= 236 s
3.2 MeV ~700 KeV eq.
214Bi-214Po=240±8s
Time s
214Bi-214Po
z (m
)
Setp - Oct 2007
Specs: 238U: 1. 10-16 g/g
< 2 cpd/100 tons
238U: = 6.6 ± 1.7×10-18 g(U)/g
R(m)
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Background: 210Po Big background!60 cpd/1ton
Not in equilibrium with 210Pb and 210Bi. But how???
210Po decays as expected. Where it comes from is not
understood at all! It is also a serious problem
for other experiments- dark matter, double beta decay
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85Kr came from a small leak during a short part of filling.
Important background to be removed in future purification.
Background: 85Kr
85Kr is studied through :
85Kr decay :(decay has an energy spectrum
similar to the 7Be recoil electron )
85Kr
85Rb
687 keV
= 10.76 y - BR: 99.56%
85Rb85Kr 85mRb
= 1.46 s - BR: 0.43%
514 keV
173 keV
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Removal of 11C Produced by muons: 25 cpd/100ton Obscures pep (2 cpd/100ton) Muon rate too high and half-life too long
to veto all events after each muon. Strategy suggested by Martin Dentsch Look for muon-neutron coincidence and
veto events near where the neutron is detected.
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μ Track
11Cn Capture
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Conclusions Methods developed for Borexino successfully
achieved for the first time, a background low enough to observe low energy solar neutrinos in real time.
Preliminary results on 7Be favor neutrino oscillations in agreement with the MSW Large Mixing Angle solution.
Backgrounds may be low enough to measure pep and CNO neutrinos using the muon+neutron tag to reduce 11C background.
Similar methods could be applied to neutrinoless decay and other low background exps..
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Borexino Collaboration
Kurchatov Institute(Russia)
Dubna JINR(Russia) Heidelberg
(Germany)
Munich(Germany)
Jagiellonian U.Cracow(Poland)
Perugia
Genova
APC Paris
Milano
Princeton University
Virginia Tech. University
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Rejection of 11C Background
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Muon induced 11C Beta Background & pep neutrinos
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PP Cycle: Branches 1 and 2
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PP cycle Branch 3
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CNO Cycle: Neutrinos from -decay of 13N, 15O and 17F
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Neutrino Mixing
€
e = cos(θ) ν 1 + sin(θ) ν 2
ν μ = −sin(θ) ν 1 + cos(θ) ν 2
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Vacuum Oscillation Length for 2-state mixing: masses m1,m2
€
λ(E) = 4πEh /((m22 −m1
2)c 3)
=2.47E /MeV
(m22 −m1
2)c 3 /eV 2meters
≈ 30 km E /MeV
for (m22 −m1
2)c 4 = 8 ×10−5eV 2
Radius of sun's core where neutrinos are produced :
R ≈ 0.2Ro =1.4 ×105 km.
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THE GRAN SASSO NATIONAL LABORATORIES
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