Tau Neutrino Physics Introduction

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Tau Neutrino PhysicsIntroduction

• Barry Barish
• 18 September 2000

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nt the third neutrino
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The Number of Neutrinosbig-bang nucleosynthesis
D, 3He, 4He and 7Li primordial abundances

• abundances range over nine orders of magnitude
• Y lt 0.25 from number of neutrons when
  nucleosynthesis began (Y is the 4He fraction)
• Yobserved 0.238?0.002?0.005
• presence of additional neutrinos would at the
  time of nucleosynthesis increases the energy
  density of the Universe and hence the expansion
  rate, leading to larger Y.
• ?YBBN 0.012-0.014 ?N?

1.7 ? N? ? 4.3
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The Number of Neutrinoscollider experiments

• most precise measurements come from Z ? e? e?
• invisible partial width, ?inv, determined by
  subtracting measured visible partial widths (Z
  decays to quarks and charged leptons) from the Z
  width
• invisible width assumed to be due to N?
• Standard Model value (?? ? ?l)SM 1.991 ?
  0.001 (using ratio reduces model dependence)

N? 2.984 ?0.008
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?? propertiesexistence

• Existence was indirectly established from
  decay data combined with reaction data (Feldman
  81).
• DIRECT EVIDENCE WAS PRESENTED THIS SUMMER FROM
  FNAL DONUT EXPERIMENT

Observe the t and its decays from nt charged
current interactions
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?? propertiesexistence DONUT concept

• calculated number of interactions 1100 ( nm ,
  ne , nt )
• total protons on target 3.6 1017
• data taken from April to September 1997

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?? propertiesexistence DONUT detectors
Spectrometer
Emulsion-Vertex Detectors
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?? propertiesexistence DONUT detectors

• 6.6 106 triggers yield 203 candidate events

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?? propertiesexistence DONUT events/background
4 events observed 4.1 ? 1.4 expected 0.41 0.15
background
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?? properties
J ½

• J 3/2 ruled out by establishing that the ??
  is not in a pure H ? -1 helicity state in ???????

magnetic moment

• expect ?? ? ? for Majorana or chiral massless
  Dirac neutrinos
• extending SU(2)xU(1) for massive neutrinos,

• where m? is in eV and ?B ? eh/2me Bohr
  magnetons.
• using upper bound mt lt 18 MeV ? ?? lt
  0.6 10-11 mB
• Experimental Bound lt 5.4 10-7 mB from
  ??e? ? ??e? (BEBC)

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?? properties
electric dipole moment
lt 5.2 10-17 e cm from ?(Z ? ee) at LEP
nt charge
lt 2 10-14 from Luminosity of Red Giants (Raffelt)
lifetime
gt 2.8 1015 sec/eV Astrophysics (Bludman) for
mn lt 50 eV
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nt properties direct mass measurements

• direct bounds come from reconstruction of ?
  multi-hadronic decays
• LEP (Aleph)
• from 2939 events ?? ? 2?? ?? ?? lt 22.3
  MeV/c2
• and 52 events ?? ? 3?? 2?? (??) ??
  lt 21.5 MeV/c2
• combined limit lt 18.2 MeV/c2

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nt propertiesdirect mass measurements

• method
• two body decay
• t?(Et,pt) ? h? (Eh,ph) nt (En,pn)
• tau rest frame hadronic energy
• Eh (mt2 ? mh2 mn2) / 2mt
• laboratory frame
• Eh ? (Eh ? ph cos?)
• interval bounded for different mn
• Ehmax,min g (Eh ? b ph)

two sample events ?? ? 3?? 2?? (??) ??
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nt propertiesdirect mass measurements
events contours 0 MeV/c2 and 23 MeV/c2
Log-likelihood fit vs mn
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nt propertiesdirect mass measurements
cosmological bounds
Unstable nt

• bounds on mnt from cosmology
• combined with non observation of lepton number
  violating decay and direct mass limits

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nt propertieslepton sector mixing
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nt propertiesoscillation probability
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nt propertiesoscillation phenomena
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n oscillationsallowed regions
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n oscillationsatmospheric neutrinos
Path length from 20km to 12700 km
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atmospheric neutrinosratio of nm events to ne
events

• ratio-of-ratios (reduces systematics)
• R (nm/ne)obs / (nm/ne)pred

hint 1 ratio lower than expected
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atmospheric neutrinosangular distributions
Hint 2 anisotropy up/down and distortion of the
angular distribution of the up-going events
Superkamiokande
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atmospheric neutrinosangular distributions with
n oscillations
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atmospheric neutrinosenergy dependence - n
oscillations
Hint 3 anomalies have been found in a
consistent way for all energies
Detectors can detect internal of external events
produced in the rock below the detector 100 MeV
to 1 TeV
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nt propertiesmass difference neutrino
oscillations
SuperKamiokande
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atmospheric neutrinoshigh energy events upward
muons
MACRO Detector
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atmospheric neutrinosMACRO event types
MACRO at Gran Sasso

• Detector mass 5.3 kton
• Event Rate
• up throughgoing m
• (ToF) 160 /y
• (2) internal upgoing m
• (ToF) 50/y
• (3) internal downgoing m
• (no ToF) 35/y
• (4) upgoing stopping m
• (no ToF) 35/y

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atmospheric neutrinosMACRO high energy events
MACRO results
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atmospheric neutrinosMACRO evidence for
oscillations
Probabilities of nm ? nt oscillations (for
maximal mixing)

• the peak probability from the angular
  distribution agrees with the peak probability
  from the total number of events
• probability for no-oscillation 0.4

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atmospheric neutrinosagreement between
measurements and experiments
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atmospheric neutrinososcillation to sterile or
tau neutrino??
SuperKamiokande
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atmospheric neutrinososcillation to sterile or
tau neutrino??
MACRO

• ratio (Lipari- Lusignoli, Phys Rev D57 1998) can
  be statistically more powerful than a c2 test
• 1) the ratio is sensitive to the sign of the
  deviation
• 2) there is gain in statistical significance
• disadvantage the structure in the angular
  distribution of data can be lost.
• nm ? nt oscillation favoured with large mixing
  angle?m2 2.5x10-3 eV2
• sterile n disfavoured at 2 s level

test of oscillations the ratio vertical /
horizontal
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atmospheric neutrinososcillation to sterile or
tau neutrino??
SuperKamiokande

• excluded regions using combined analysis of low
  energy and high energy data
• Sobel n2000 stated .

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nt future speculations - supernovae
SN1987a
What can be learned about the nt from the next
supernovae .??
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nt future speculations - supernovae

• direct eV scale measurements of m(nm) and m(nt)
  from Supernovae neutrinos
• early black hole formation in collapse will
  truncate neutrino production giving a sharp
  cutoff
• allows sensitivity to m(ne) 1.8 eV for SN at
  10 kpc in Superkamiokande detector
• (Beacom et al hep-ph/0006015)

Events in SK Low 0 lt E lt 11.3
MeV mid 11.3 lt E lt 30 MeV High 30 lt E lt ?
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nt future speculations - supernovae

• rate in OMNIS, a proposed supernovae detector
• tail 6.1 eV ? 2.3 events

OMNIS delayed counts vs mass nt
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nt the ultra high energy neutrino universe
OWL - Airwatch
GZK cutoff neutrinos ??
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nt the ultra high energy neutrino universe

• OSCILLATIONS
• ?
• FLUXES OF nt AND nm
• ARE EQUAL

• neutrinos from interactions of ultrahigh energy
  cosmic rays with 3 K cosmic backgrond radiation
• neutrinos from AGNs, GRBs, etc
• Z?bursts relic neutrinos from big bang
  cosmology

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nt the ultra high energy neutrino universe
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nt future speculations cosmic nts

• high energy ns E gt 106 GeV
• neutrinos from proton acceleration in the cores
  of active galactic nuclei
• vacuum flavor neutrino oscillations enhance nt
  / nm ratio
• detectable in under water / under ice detectors
• (Athar et al hep-ph/0006123)

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nt future speculations cosmic nts

• nt identified by characteristic double shower
  events
• charged currect interaction tau decay into
  hadrons and nt
• second shower has typically twice as much
  energy as first
• double bang

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nt future speculations cosmic nts

• shower size vs shower separation
• identified events will clearly result from
  vacuum neutrino oscillations, since without
  enhancement expect nt / nm lt 10-5
• nt events can be identified in under water/ice
  detectors

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Acceleratorslong baseline nm ? nt oscillations
MINOS
K2K
CERN ? GS
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Acceleratorslong baseline nm ? nt oscillations
nt appearance
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Acceleratorsneutrino factory neutrinos from
muon collider
muon collider
Example 7400 km baseline Fermilab ? Gran
Sasso world project
neutrino beams select nms or anti nms
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Acceleratorsneutrino factory neutrinos from
muon collider

• accurately determine n mixing matrix
• perhaps even measure CP violation in n sector

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Conclusions

• direct observation of the tau neutrino by
  DONUT is an important milestone
• properties of tau neutrino like other neutrinos
  ne, nm, nt
• neutrino oscillations open up a variety of new
  future possibilities for nt in cosmology,
  astrophysics and future accelerators