Status of Neutrino Science

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Status of Neutrino Science

• Hitoshi Murayama
• LBNLnu
• April 11, 2003

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Two Main Directions

• Neutrino Properties
• Neutrino Masses
• Neutrino Mixings
• Nature of Neutrinos
• Neutrino as a Probe
• Parton Distribution Function
• Precision Electroweak Physics
• Neutrino Astrophysics

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Outline

• Introduction
• Current Status of Neutrino Oscillation
• Main Scientific Questions
• Majorana vs Dirac
• The last mixing angle and CP violation
• Conclusion

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Current Status of Neutrino Oscillation
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Rare Effects from High-Energies

• Effects of physics beyond the SM as effective
  operators
• Can be classified systematically (Weinberg)

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Unique Role of Neutrino Mass

• Lowest order effect of physics at short distances
• Tiny effect (mn/En)2(eV/GeV)21018!
• Interferometry (i.e., Michaelson-Morley)!
• Need coherent source
• Need interference (i.e., large mixing angles)
• Need long baseline
• Nature was kind to provide all of them!
• neutrino interferometry (a.k.a. neutrino
  oscillation) a unique tool to study physics at
  very high scales

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MNS matrix

• Standard parameterization of Maki-Nakagawa-Sakata
  matrix for 3 generations

atmospheric
solar
???
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What we learned in 20012

• Atmospheric nm is lost (gt10s), converted most
  likely to nt (gt99CL) (SK, MACRO, SOUDAN-II)
• Solar ne is converted to either nm or nt (gt5s)
  (SNO)
• Reactor anti-ne are lost (99.95CL) (KamLAND)
• Only the LMA solution left for solar neutrinos
• Tiny neutrino mass the first evidence for
  incompleteness of Minimal Standard Model

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SNO Result

• Only ne produced in the Sun
• Wrong Neutrinos nm,t are coming from the Sun!
• Somehow some of ne were converted to nm,t on
  their way from the Suns core to the detector
• ? neutrino oscillation!

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KamLAND result

• First terrestrial expt relevant to solar neutrino
  problem
• KamLAND will exclude or verify LMA definitively

Dec 2002 Expected events 86.85.6 Background
events 0.950.99 Observed events 54
No oscillation hypothesis Excluded at 99.95
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Summary of Neutrino Oscillation
Before SNO and KamLAND
Dec 2002 with SNO and KamLAND
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Three-generation

• Solar, reactor atmospheric n oscillations
  easily accommodated within three generations
• sin22q23 near maximal, Dm2atm 3?103eV2
• sin22q12 large, Dm2solar 520?105eV2
• sin22q13Ue32lt 0.05 from CHOOZ, Palo Verde
• Because of small sin22q13, solar (reactor)
  atmospheric n oscillations almost decouple

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?

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Extended Standard Model

• Massive Neutrinos ? Minimal SM incomplete
• How exactly do we extend it?
• Abandon either
• Minimality introduce new unobserved light
  degrees of freedom (right-handed neutrinos)
• Lepton number abandon distinction between
  neutrinos and anti-neutrinos and hence matter and
  anti-matter
• Dirac or Majorana neutrino
• Without knowing which, we dont know how to
  extend the Standard Model

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Theoretical BiasSeesaw Mechanism

• Why is neutrino mass so small?
• Need right-handed neutrinos to generate neutrino
  mass

, but nR SM neutral
To obtain m3(Dm2atm)1/2, mDmt, M31015GeV
(GUT!) Majorana Neutrinos
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Neutrinoless Double-beta Decay

• The only known practical approach to discriminate
  Majorana vs Dirac neutrinos
• 0nbb nn ? ppee with no neutrinos
• Matrix element ? ltmnegtSimniUei2
• Current limit ltmnegt about 1eV
• m3(Dm223)1/20.05eV looks a promising goal
• However, m3Ue32ltltm3 and we can ignore m3
• Fortunately, Ue12 and Ue22 cannot cancel exactly
  because the maximal angle excluded by SNO
  Ue12Ue22cos22q12gt0.07 (1s)

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Three Types of Mass Spectrum

• Degenerate
• All three around gt0.1eV with small splittings
• Possible even after WMAP2dF mlt0.23eV
• May be confirmed by KATRIN, cosmology
• ltmnegtSimniUei2gtm cos22q12gt0.07m
• Inverted
• m30, m1m2(Dm223)1/20.05eV
• May be confirmed by long-baseline experiment with
  matter effect
• ltmnegtSimniUei2gt(Dm223)1/2 cos22q12gt0.0035eV
• Normal
• m1m20, m3(Dm223)1/20.05eV
• ltmnegtSimniUei2 may be zero even if Majorana

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?

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Now that LMA is confirmed...

• Dream case for neutrino oscillation physics!
• Dm2solar within reach of long-baseline expts
• Even CP violation may be probable
• neutrino superbeam
• muon-storage ring neutrino factory
• Possible only if
• Dm122, s12 large enough (LMA)
• q13 large enough

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Shootout (Lindner)
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q13 decides the future

• The value of q13 crucial for the future of
  neutrino oscillation physics
• Determines the required facility/parameters/baseli
  ne/energy
• Two paths to determine q13
• Long-baseline accelerator neutrino oscillation
• Reactor neutrino experiment with two detectors

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?

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Verify Oscillation

• Even atmospheric neutrino data do not show
  oscillation yet
• ? MINOS, J-PARC
• ? Dm223, q23, mass hierarchy and q13
• KamLAND data is consistent with overall
  suppression
• ? continued running
• ? Dm212

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Low-Energy Solar Neutrinos

• Solar neutrino data suggest energy-dependent
  survival probability
• ? tests MSW effect
• ? q12
• ? Helps interpretation of CP violation, double
  beta decay data

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?

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If LSND right,All hell breaks loose

• Sterile neutrinos are strongly constrained by the
  combination of all existing data and WMAP2dF
• CPT violation is strongly constrained by
  SNOKamLAND
• If LSND correct, all previous measurements need
  to be re-examined by a collection of short-,
  medium- and long-baseline experiments. Possibly
  mini-muon-storage ring.

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Seven Questions

• Dirac or Majorana?
• Absolute mass scale?
• How small is q13?
• CP Violation?
• Mass hierarchy?
• Verify Oscillation?
• LSND? Sterile neutrino(s)? CPT violation?
• ?Specific recommendations to attack these
  questions in the context of LBNL