First Results from the Borexino Solar Neutrino Experiment

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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 Searches Summary 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 1950s by
  inverse beta decay ?ep-gtne.
• 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/4?r2) 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 -gt 37Ar e-
• Detect 37Ar e- -gt 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
  suns 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 -gt p p e- (only ?e detected)
• ?x d -gt 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(sensiti
ve 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?
Friedland, Lundardini Peña-Garay
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 e Where is the relative
effect of new physics the largest? At resonance!
Blue Standard ? oscillations Red 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

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The Borexino Detector
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Detection Principles

• Detect ?-e scattering via scintillation light
• Features
• Low energy threshold (gt 250 keV to avoid 14C)
• Good position recostruction by time of flight
• Good energy resolution (500 pe/MeV)
• Drawbacks
• No directional measurements
• ? induced events cant be distinguished from
  other ß/? 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
pep
Mono-energetic 7Be and pep neutrinos produce
a Box-like electron recoil energy spectrum
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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 a/ß 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)
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Expected interaction rate in absence of
oscillations 754 cpd/100 tons for LMA-MSW
oscillations 494 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 232Th
Specs 232Th 1. 10-16 g/g 0.035
cpd/ton
Assuming secular equilibrium, 232Th is measured
with the delayed coincidence
212Bi-212Po
?42342 ns
Time (ns)
Events are mainly in the south vessel surface
(probably particulate)
z (m)
Only few bulk candidates
R (m)
R(m)
From 212Bi-212Po correlated events in the
scintillator 232Th lt 6 10-18 g(Th)/g (90
C.L.)
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Background 238U
Specs 238U 1. 10-16 g/g
Assuming secular equilibrium, 238U is measured
with the delayed coincidence
214Bi-214Po
?2408?s
Time ?s
Setp - Oct 2007
214Bi-214Po
z (m)
lt 2 cpd/100 tons 238U 6.6 1.710-18 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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Background 85Kr
85Kr ? decay (b decay has an energy spectrum
similar to the 7Be recoil electron )
85Kr is studied through

• 85Kr came from a small leak during a short part
  of filling.
• Important background to be removed in future
  purification.

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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
n Capture
11C
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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 muonneutron tag to
  reduce 11C background.
• Similar methods could be applied to neutrinoless
  ?? decay and other low background exps..

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Borexino Collaboration
Princeton University
Virginia Tech. University
APC Paris
Kurchatov Institute (Russia)
Jagiellonian U. Cracow (Poland)
Dubna JINR (Russia)
Munich (Germany)
Heidelberg (Germany)
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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
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Vacuum Oscillation Length for 2-state mixing
masses m1,m2
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