Neutrino Oscillations: Experimental Results Future Measurements

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Neutrino OscillationsExperimental Results
Future Measurements
Introduction Solar Neutrino Oscillations D(m12
)2, q12 Atmospheric Neutrino
Oscillations D(m23)2, q23 LSND-type
Oscillations? q13, dCP Future Experiments
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(Brief) Introduction

• The well-established oscillations (solar,
  atmospheric) can be described
• by three active, massive neutrinos and the
  MNSP (Maki-Nakagawa-
• Sakata-Pontecorvo) matrix, analogous to the CKM
  matrix
• Mass hierarchy not completely known
• Both solar and atmospheric oscillations are
• well approximated by two-flavor mixing
• because q13 is small, D(m23)2 ?? D(m12)2
• Two-flavor survival probability, e.g. solar ne

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Solar Neutrino Oscillations

• The sun as a neutrino source
• Complicated energy spectrum
• MSW Effect
• propagation through matter
• modifies the ne survival probability

91
7
0.2
0.008

• Below critical energy,
• vacuum oscillations dominate
• Above critical energy
• matter effects dominate
• Critical energy depends on Dm2,
• 1.8 MeV for LMA
• Spectral distortion near critical energy

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Radiochemical Experiments

• ne capture on select radioisotopes
• Chlorine ne 37Cl ? e 37Ar gt 814 keV
• Gallium ne 71Ga ? e 71Ge gt 233 keV
• Detect decays of capture daughters
• Sensitive only to integrated ne flux above
  threshold
• Results
• Homestake (Cl) ?Cl/SSM 0.34 ? 0.03
• SAGEGALLEX/GNO ?Ga/SSM 0.54 ? 0.03
• (SSM is Standard Solar Model,
• BP00 Bahcall/Pinsonneault,Astrophys. J. 555,
  990, 2001)

Ray Davis
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Kamiokande/Super-K

• Water-Cerenkov, detects forward-scattered
• electrons from neutrino-electron elastic
  scattering
• nx e ? nx e
• Only sensitive to most energetic 8B solar
  neutrinos
• Real-time detection of electron energy and
  direction
• Flux result ?SK/SSM 0.465 ? 0.015
• Constant suppression
• no time variation or energy distortion detected

Koshiba Masatoshi
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SNO

• Heavy-water-Cerenkov detector, 5 MeV threshold
• Three different n detection modes
• CC (charged current) ne D ? p p e ne
  only
• NC (neutral current) nx D ? p n nx all
  three flavors!
• ES (elastic scattering) nx e ? nx
  e (same as Super-K)
• Full SSM flux seen
• 5.3s appearance of nm,t in a ne beam
• Ratio of CC to NC strongly
• constrains the mixing angle q12
• CC/NC 0.306 ? 0.026 ? 0.024
• SNO also sees constant suppression

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q12, D(m12)2 from solar data

• SNO 95 allowed regions
• overlaid with previous solar
• neutrino measurements
• In global fits with the most
• recent SNO data, only
• the LMA region survives.
• Maximal mixing ruled out at 5.4s

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KamLAND

• Studies solar-type LMA oscillations in reactor
  antineutrinos
• Reactor baselines on the order of the
  oscillation length
• Source is a time-varying ensemble of reactors
• Liquid scintillator calorimeter, sub-MeV
  threshold
• Inverse b-decay ne p e n
• Coincidence signal
• prompt e annihilation (E En ? 0.8 MeV)
• delayed n capture (190 ms) (E 2.2 MeV)
• No directional information

Detected ne spectrum (no oscillations)
Inverse b-decay cross-section
Reactor ne spectrum
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Antineutrino Rate Analysis

• Observed 54 (145.1 days livetime)
• No-oscillation expectation 86.8 ? 5.6 (syst)
• Background 1 ? 1
• (NobsNBG)/Nno-osc 0.611 ? 0.085 (stat) ?
  0.041 (syst)
• (statistics above on 54 events)
• Probability that 86.8 events would
• fluctuate down to 54 is lt 0.05
• Standard ne propagation
• is ruled out at the
• 99.95 confidence level
• Neutrinos and antineutrinos both see
• effects consistent with the same mixing
• parameters

curve, shaded region global-fit solar LMA
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Rate Shape Analysis

• Fit prompt (positron) energy spectrum above 2.6
  MeV with
• full reactor information (power, fuel, flux),
  2-flavor mixing
• Energy spectrum shape provides additional
• constraints on oscillation parameters

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KamLAND mixing parameter constraints
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Future
HolandaSmirnov

• SNO
• Improved CC/NC measurement to
• further constrain tan2q
• Improved day/night ratio better constrains
• Dm2 in solar-only fits
• Possible to see MSW distortions?
• KamLAND
• Better measurements with more data
• Distinguish LMAI vs. LMAII
• Observe shape distortion, oscillation?
• gt published data are consistent
• with constant supression

Scatter of 500-event MC datasets generated
with common mixing parameters
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7Be solar neutrino measurement?
Borexino

• Players Borexino, KamLAND
• Goal is a direct measurement of the
• solar 7Be neutrino flux
• Tough measurement
• single ES event
• need very low background to
• statistically extract the signal
• SSM 7Be prediction is at the 10 level
• gt This measurement is not expected to improve
• the determination of oscillation parameters
• gt Measurement can improve the SSM
• Still an important check
• 7Be neutrino energy is below the MSW transition

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pp neutrinos?

• The pp solar neutrino flux is by far the largest
  (gt90)
• and best-determined (1)
• Measurement of pp neutrinos could give the best
• solar neutrino oscillation parameter constraints
• Even though the rates are high,
• backgrounds are the key issue
• RD efforts on a variety of
• approaches

M.Nakahata
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Atmospheric neutrino oscillation

• Cosmic-ray showers in the earths
• atmosphere produce nm, ne
• In an underground detector,
• atmospheric ns from different
• directions have different baselines
• Detect neutrinos through charged-current
• interactions in the detector ne ? e, nm ? m
• The q23 oscillation causes a deficit of
  upward-going nm
• Best atmospheric oscillation measurement is from
  Super-K,
• though multiple other experiments have seen the
  same effect
• (Kamiokande, IMB, MACRO, Soudan-II, )

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Super-K atmospheric measurement

• Most recent analysis presented at NOON 2004
  (C.Saji)
• Super-K is working on a final analysis of the
  SK-I dataset
• improved analysis, MC, flux predictions,
  calibrations, cross-sections,
• SK-II is taking more data. Results improve as
  (data)1/2

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K2K

• Atmospheric oscillations have also been
  detected with
• accelerator neutrinos
• K2K (KEK-to-Super-K) 250 km baseline
• Data collection ongoing, new near detector

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Super-K L/E analysis

• New, very interesting preliminary analysis of
  Super-K data
• presented at NOON 2004 by M.Ishitsuka
• Analyze data vs. L/E instead of zenith angle
• Uses subset of events with
• good L/E resolution
• Dip at first oscillation minimum,
• later maxima/minima smeared out
• Oscillation preferred to other
• explanations of deficit
• Better Dm2 resolution than
• standard zenith-angle analysis
• (Note added May 26, 2004 preprint hep-ex/0404034
  now available)

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MINOS

• Long-baseline accelerator neutrino experiment
• sending nms from the NuMI beam at Fermilab
• past Madison to the Soudan mine
• Detect neutrinos through CC interaction in
• steelscintillator detector with B-field
• Tunable beam energies
• Beam scheduled for April 2005
• Already taking atmospheric neutrino data
• gt Can distinguish nm from nm, test CPT

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MINOS

• MINOS limits compared to current Super-K values
• Better constraints for higher Dm2 oscillation
  minimum at higher energy

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nt appearance

• Goal observe nt appearance in nm beam
• to confirm that the osciallation is to nt
• CERN to Gran Sasso CNGS beam optimized
• for nt appearance high enough energy
• Detectors
• ICARUS liquid Argon TPC
• OPERA lead emulsion

ICARUS cryostat
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Future

• Improved atmospheric parameter measurements
  from
• MINOS
• Continued Super-K operation
• gt Is mixing really maximal?
• Confirmation of oscillation from Super-K L/E
  analysis
• CPT test MINOS comparison of nm, nm
• Direct observation of nt appearance

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LSND?

• LSND reported a significant excess of ne in a nm
  beam (3.8s)
• So far, this result has not been reproduced by
  any other experiment
• Some parameter space is still left

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LSND?

• LSND cant fit into the MNSP picture along with
  the solar and
• atmospheric oscillations as the third
  oscillation the mass differences
• dont add up
• If LSND is right, we need another oscillation
  somehow
• CPT violation?
• gt Consistent results for neutrinos, antineutrinos
  in solar sector
• gt Two different mass differences in atmospheric
  sector strongly
• disfavored by Super-K
• Additional sterile neutrinos?
• gt 31 (one sterile) disfavored
• gt 32, etc. possible
• If LSND is correct, things get very interesting
• gt Fully testing LSND is an experimental priority

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MiniBooNE

• MiniBooNE is a short-baseline accelerator
  neutrino experiment
• running at Fermilab that can definitely confirm
  or rule out LSND
• See Sam Zellers talk!

two-ring event
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q13, dCP

• In the MNSP picture, we should have a third
  oscillation,
• characterized by q13, D(m13)2 ? D(m23)2
• How big is q13?
• Experiment Currently only have limits
• Theory No firm predictions
• gt Some models suggest q13 should be within the
  range of the
• next generation of experiments
• gt Global three-flavor fits favor q13 just below
  current limits
• If q13 is small, why? Symmetry?
• If q13 is large enough, we can also try to
  measure CP violation
• in the lepton sector, characterized in MNSP by
  dCP
• gt CP violation could be large can it explain
  the
• matter/antimatter asymmetry through leptogenesis?

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Reactor q13 limits

• The best limits on q13 come from short (1km)
  baseline
• reactor neutrino measurements that saw no flux
  deficit.
• High-precision measurement 2.7 systematic
  error at CHOOZ

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Reactor measurement of q13

• Goal is to try to measure the subdominant q13
  oscillation in
• ne disappearance
• Measurement requires control of systematics to
  the 1 level
• How do we do better than CHOOZ?
• Use two detectors to cancel flux uncertainties
• The disappearance effect depends only on q13
• no matter effects, ambiguities from dCP

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Reactor measurement of q13

• Double-CHOOZ approved
• Fast, cheap approach
• sin22q13 down to 0.03
• Options in the US?
• Braidwood (Illinois)
• gt flat site, vertical access
• Diablo Canyon (California)
• gt horizontal access, more overburden
• Need reactor company agreement
• Other sites Daya Bay (China),
• Kashiwazaki (Japan), Krasnoyarsk (Russia),
• Angra dos Reis (Brazil)

Braidwood
Diablo Canyon
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Sensitivity to q13
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Off-axis accelerator q13

• With the right accelerator beam, we can measure
  q13 , dCP
• in nm ? ne appearance
• Rate depends on multiple unknown parameters
• q13, dCP, and matter effects (hierarchy)

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Off-axis accelerator q13

• A single measurement doesnt
• unambiguously measure q13
• Parameter degeneracies can
• be resolved by
• Running in both neutrino
• and antineutrino modes
• Running at different baselines
• multiple experiments are
• complementary

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Off-axis accelerator q13
Two experiments (not a complete list of
proposals!) T2K Tokai-to-Super-K high
priority in Japan Proposed NuMI off-axis
experiment
Soudan
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Longer term neutrino superbeams

• Example Brookhaven-to-Homestake
• A wideband, high-flux, on-axis beam
• allows parameter measurement by
• observing multiple oscillation peaks
• Potential for very good measurements
• Improved detectors, beams necessary

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Conclusions

• Neutrino Oscillations is a rich experimental
  field, with a variety
• of sources (sun, atmosphere, reactors,
  accelerators) and detection
• techniques
• Two different oscillation effects are
  well-established
• gt Experiments ongoing to improve the parameter
  measurements
• And verify that the oscillation interpretation is
  correct
• LSND signal MiniBooNE is key
• gt If LSND signal is real, things get very
  interesting
• Big goals
• gt Measure q13 oscillation
• gt Measure CP violation in neutrino sector
• Other questions
• gt Which hierarchy?
• gt Absolute mass scale?
• gt Are neutrinos Dirac or Majorana?
• Non-oscillation experiments are also a big part
  of the field
• neutrinoless 2b decay, cosmological limits, etc.