Status of Neutrino Oscillations Solar

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Status of Neutrino Oscillations (Solar)

• Dave Wark
• RAL/ University of Sussex
• NuFact 01
• May 24, 2001

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Neutrino Oscillations (?)

• The Solar Neutrino Problem
• What are neutrino oscillations?
• Where are the smoking guns?
• What will SNO show us?
• What will other experiments show us?

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The Solar Neutrino Problem
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The Solar Neutrino Problem
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The Solar Neutrino Problem
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The Solar Neutrino Problem
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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

• Or
• Recall that
• Consider q 45

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Vacuum Oscillations

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

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The MSW effect

• ne have an extra diagram for scattering from
  electrons
• gives ne an effective mass in matter
• can lead to resonant enhancement of oscillations
  ? the MSW effect

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MSW effects on survival probabilities
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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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Smoking guns?

• n oscillations are consistent with the data, but
  are they the right explanation?
• Other models have been proposed - FCNC, extra
  dimensions, violations of the EP, neutrino decay,
  neutrino magnetic moments, etc.
• What would be proof for oscillations?

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The Super-Kamiokande Detector
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Super-K Elastic Scattering
Directly away from the Sun
Directly towards the Sun
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Smoking guns from solar n?

• The appearance of the wrong-flavour neutrinos in
  the beam - not seen yet for solar neutrinos
• A variation of the rate with L/E
• Spectral distortions
• Seasonal variations
• Regeneration of neutrinos passing through the
  Earth.

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Spectral Data from Super-K
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Seasonal Variations?
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Day/Night variation in Super-K
Taken from Micheal Smys talk, Moriond 2001
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The Dog that Didnt Bark
ne ? nm,t
ne ? nsterile
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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)

• Measure total 8B n flux from the sun.

• Low Statistics
• Strong directional sensitivity

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SNO run sequence

• Neutron Detection Method
• Capture on D
• Capture on Cl
• Capture on 3He
• Event by event separation of CC and NC events

The Three Phases

• Pure D2O
• Good CC sensitivity
• Added Salt in D2O
• Enhanced NC sensitivity
• Neutral Current Detectors
• 3He proportional counters in the D2O

n ? d ? t ? g ? e? (Eg 6.3 MeV)
n ? 35Cl ? 36Cl ? ?g ? e? (E?g 8.6 MeV)
n ? 3He ? p ? t
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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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Signals in SNO
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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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Acrylic Vessel Assay

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

AV well below (1/10) the target level of 2
ppt U/Th
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A Neutrino Event
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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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SNO Energy Calibrations
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SNO Event Reconstruction
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SNO Event Reconstruction
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SNO cos(q)sun distribution
Electron Angle with respect to the direction
from the Sun
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Low threshold cos(q)sun
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Smoking Guns in SNO - 1
ES
Charged-current spectrum is more sensitive to
shape distortions!
CC
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Smoking Guns in SNO - 2
Charged-Current to Neutral Current ratio is a
direct signature for oscillations
Bahcall etal., Phys. Rev. D 54, 5147 (1996)
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Smoking Guns in SNO - 3
CC/ES Could also show significant effects!
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Borexino
ES in 300t superclean scintillator target
Under Construction, data taking end 2001
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KamLAND
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Signals and Backgrounds
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Oscillation Sensitivity
Reactor n disappearance
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Icarus
5 MeV thresh. ES - 800 kt/yr CC - 2100 kt/yr
10 m3 prototype operated in Gran Sasso
600t module under construction!
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Things I should have talked about...

• LENS - pp spectrometer
• Yb loaded Scintillator
• Delayed coincidence
• SIREN - pp spectrometer
• Gd loaded Scintillator
• HELLAZ
• 4He - TPC shielded with dry ice
• HERON
• Simultaneous measurement of rotons/photons in
  liquid 4He
• Many many others...

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Conclusions

• Neutrino Oscillations very strongly indicated
• No Solar n smoking gun (see next talk for
  atmospheric.)
• SNO should say something interesting, and soon
• We live in interesting times...