Neutrino Oscillations and the Sudbury Neutrino Observatory

1
Results from the Sudbury Neutrino Observatory
Dave Wark Nikhef January 11th, 2002
2
Fusion in the Sun
3
Solar Neutrino Fluxes
4
The Solar Neutrino Problem
5
Helioseismology
6
Helioseismology
7
The Solar Neutrino Problem
8
Neutrino Oscillations

• Let us assume that neutrinos have (different)
  masses - Dm2
• Let us assume that the mass eigenstates are not
  identical to the weak eigenstates
• If we consider 2 flavours the mixing is
  characterized by a single angle q analogous to
  the Cabibbo angle in case of quarks

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Neutrino Oscillations
Consider ? 45?
10
Vacuum Oscillations

• In general this leads to the disappearance of the
  original neutrino flavour
• With the corresponding appearance of the wrong
  neutrino flavour

11
The MSW effect

• ne have an extra diagram for scattering from
  electrons (W as well as Z exchange)
• gives ne an effective mass in matter
  proportional to the electron density Ne
• can lead to an energy dependent resonant
  enhancement of oscillations for both large (LMA)
  and small (SMA) mixing angles ( ??m2/NeE )
• The MSW effect

12
Global fit, 8B Flux a free parameter

• Includes
• Rates
• Homestake
• SAGE
• GALLEX/GNO
• Super-K
• Super-K spectra
• day
• night

From Bahcall, Krastev, and Smirnov hep-ph/0103179
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Sterile n solutions

• Includes
• Rates
• Homestake
• SAGE
• GALLEX/GNO
• Super-K
• Super-K spectra
• day
• night

From Bahcall, Krastev, and Smirnov hep-ph/0103179
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SNO
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The SNO Detector
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n Reactions in SNO

• Good measurement of ne energy spectrum
• Weak directional sensitivity ? 1-1/3cos(q)

17
Cerenkov Light Production
Charged current interaction in D2O
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n Reactions in SNO

• Good measurement of ne energy spectrum
• Weak directional sensitivity ? 1-1/3cos(q)

• Measure total 8B n flux from the sun.

• Low Statistics
• Strong directional sensitivity

19
The enemy..
bs and gs from decays in these chains interfere
with our signals at low energies
And worse, gs over 2.2 MeV cause d g ? n p
Design called for D2O lt 10-15 gm/gm U/Th
H2O lt 10-14 gm/gm U/Th Acrylic lt 10-12
gm/gm U/Th
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Construction
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Water Systems
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SNO Water Assays
Targets for D2O represent a 5 background from
dg ? np
Targets are set to reduce b-g events
reconstructing inside 6m
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Signals in SNO
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Smoking Guns in SNO - 1
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Smoking Guns in SNO - 2
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A Neutrino Event
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Signals in SNO
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Instrumental Backgrounds
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Instrumental Background Cuts
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How do we know this worked ?

• We did it twice.
• Two different semi-independent sets of cuts were
  developed.

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How do we know this worked?
Contamination measured with independent cuts
Signal loss measured with calibration sources
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Solar Neutrino Spectrum
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Current SNO data set

• Data Period 2/11/99 ? 15/01/01
• Livetime 240.9 Days
• Data set 1 Analysis Data
• used to develop the data analysis
• 166 days livetime
• Data set 2 Blind data
• test for statistical bias
• 75 days livetime

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SNO Livetime
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Manipulator
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SNO Energy Calibrations
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Backgrounds from the Data

• External g-ray background
• bg background from the AV
• bg background from the H2O
• bg background from the PMTs

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Acrylic Vessel Assay

• Every piece sampled and tested
• Sample bonds tested
• Direct Assay by Cerenkov light

AV well below (1/10) the target level of 2
ppt U/Th
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Signal Extraction

• Threshold set at Teff 6.75 MeV
• removes most of the neutrons
• further reduces the background
• Fit resulting events to Probability Density
  Functions (pdfs) in
• effective energy Teff
• volume weighted position (R/RAV)3
• angle from the Sun cosq?

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Signal Extraction
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Signal Extraction

• Threshold set at Teff 6.75 MeV
• removes most of the neutrons
• further reduces the background
• Fit resulting events to Probability Density
  Functions (pdfs) in
• effective energy Teff
• volume weighted position (R/RAV)3
• angle from the Sun cosq?

• Use maximum likelihood to extract components

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SNO cosq? distribution
ES strongly peaked
CC 1-1/3cosq?
Neutrons isotropic
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SNO energy spectrum from unconstrained fit
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SNO energy spectrum from an unconstrained fit
(Adding syst. bin by bin in quadrature give c2 of
12 for 11 D.O.F.)
45
SNO energy spectrum from an unconstrained fit
(Adding syst. bin by bin in quadrature give c2 of
12 for 11 D.O.F.)
46
SNO energy spectrum from an unconstrained fit
(Adding syst. bin by bin in quadrature give c2 of
12 for 11 D.O.F.)
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Solar Neutrino Fluxes

• Absolute fluxes from constrained fit

NB All fluxes quoted are in units
106/cm2/sec
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Systematic Flux Uncertainties

• Error Source
• Energy scale
• Energy resolution
• Non-linearity
• Vertex shift
• Vertex resolution
• Angular resolution
• High Energy ?s
• Low energy background
• Instrumental background
• Trigger efficiency
• Live time
• Cut acceptance
• Earth orbit eccentricity
• 17O, 18O
• Experimental uncertainty
• Cross-section
• Solar Model

• ES error ()
• -3.5, 5.4
• 0.3
• 0.4
• 3.3
• 0.4
• 2.2
• -1.9, 0.0
• -0.2, 0.0
• -0.6, 0.0
• 0.0
• 0.1
• -0.6, 0.7
• 0.2
• 0.0
• -5.7, 6.8
• 0.5
• -16, 20

CC error () -5.2, 6.1 0.5 0.5 3.1 0.7 0.5 -
0.8, 0.0 -0.2, 0.0 -0.2, 0.0 0.0 0.1 -0.6,
0.7 0.2 0.0 -6.2, 7.0 3.0 -16, 20
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Solar Neutrino Fluxes

• Absolute fluxes from constrained fit

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Flux Differences
The hypothesis that this is a downward statistical
fluctuation is ruled out at 99.96
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Ftotal vs. Fe
You can extract the total neutrino flux from
these results
This can be compared to the SSM prediction
John Bahcall has been right all these years!
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Using the 8B flux to constrain G and WIMPS
See Lopes and Silk, astro-ph/0112310
See Lopes and Silk, astro-ph/0112390
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Allowed Solutions for Neutrino Oscillations
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Equalizing SNO/SK n response
CC and ES have different En response
However, choosing different thresholds can
compensate
For the current analysis, Tthresh 6.75 MeV for
SNO and 8.6 MeV for Super-K equalize response to
few
For these thresholds the fluxesstill differ by
0.53 0.17
Sterile oscillations ruled out by this test at gt
3s
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Allowed Solutions for Neutrino Oscillations
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Oscillation Analyses including SNO

• Barger, Marfatia and Whisnant hep-ph/0106207
• Oscillations to partially sterile neutrinos still
  allowed
• Fogli et al. hep-ph/0106247
• Purely sterile oscillations ruled out at gt3s
• SMA flavoured oscillations also ruled out

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Allowed regions from Fogli et al.
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Oscillation Analyses including SNO

• Barger, Marfatia and Whisnant hep-ph/0106207
• Oscillations to partially sterile neutrinos still
  allowed
• Fogli et al. hep-ph/0106247
• Purely sterile oscillations ruled out at gt3s
• SMA flavoured oscillations also ruled out

• Bahcall, Gonzalez-Garcia, Pena-Garay
  hep-ph/0106258
• No, at 3s everything still allowed
• Bandyopadhyay et al. hep-ph/0106264
• Includes SNO energy spectrum
• SMA ruled out
• Berezinsky hep-ph/0108166
• SNO was right about everything.
• Krastev and Smirnov hep-ph/0108177
• No they werent, but neither are the others..

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What is going on?

• Why do these analyses differ?
• Different handling of spectral oversampling
• Slightly differences in the methods
• Which one is right?
• Beats me..
• As Rutherford said, if you need a statistical
  test, you did the wrong experiment.

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The Right Experiment

• SNO Neutral Current Measurement
• Pure D2O - data in can, results soon
• Salt data now being taken
• Higher efficiency for neutron capture
• Capture signal at higher E, clear of background
• Independent test for NC/CC discrimination
• Different systematics
• Discrete NCDs to be deployed next
• Stringent test for non-electron neutrino
  appearance

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Conclusions

• The SNO detector is working and taking beautiful
  data.
• The CC rate measured in SNO is incompatible with
  the Super-K ES rate.
• This is strong evidence (gt99.8 c.l.) for the
  appearance of m or t neutrinos from the Sun.
• Sterile and Just-So2 oscillations are excluded by
  these results at gt99.8 c.l.
• The 8B n flux from the Sun is now measured to be
  in agreement with the predictions of the Standard
  Solar Model.
• Super-KT2 b decay ? 0.001 lt Wn lt 0.18

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Outlook

• These results are just the first of what SNO will
  produce.
• The conclusions listed on the preceeding slide
  are systematics dominated. They will be severely
  tested by new measurements
• NC measurements in pure D2O
• Day/night in pure D2O
• The same measurements with NaCl added
• The same measurements with the NCDs
• Borexino and KamLAND will give additional
  information in the near future
• The future - SIREN, LENS, etc.

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Outlook

• Solar Neutrinos have demonstrated (confirmed?)
  that neutrinos have mass and undergo flavour
  oscillations
• Now we must understand why..