The elusive neutrino

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The elusive neutrino
Fysica 2002 Groningen

• Piet Mulders
• Vrije Universiteit
• Amsterdam

mulders_at_nat.vu.nl http//www.nat.vu.nl/mulders
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What is it all about

• Neutrinos, quantum mechanics, relativity
• What are neutrinos?
• Where do we find neutrinos?
• How to catch neutrinos?
• Neutrino puzzles
• How heavy are neutrinos?
• Solar neutrinos

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What is a neutrino?
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Matter
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The periodic table
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Matter
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Matter
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Atomic nuclei

• Isotopes
• Radioactivity
• alpha
• beta
• gamma

After 15 min.
1930 W. Pauli 1956 Reines Cowan
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Matter
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The buildingblocks of thesubatomicworld
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What is special with neutrinos?

• No mirror image
• (only lefthanded)
• Barely interacting
• (crossing the earth
• without problems)

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Origin of neutrinos ?
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Origin of neutrinos

• Weak decay of atomic nuclei (Sun/reactors)
  n ? p e- ne (righthanded antineutrino)
  p ? n e ne (lefthanded neutrino)
• Cosmic rays (decay of the pion)
  p- ? m- nm (rechtshandig
  antineutrino) p ? m nm
  (linkshandig neutrino)
• Remnants of the big bang
  just as photons (T 2.7 K background)
  one finds about 500 neutrinos per cm3 for all
  three kinds of neutrinos (ne, nm and nt)

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How do we know all of that?
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Brokenmirrorsymmetry
Wu et al. 1957 (looking at Cobalt nuclei)
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From the largest microscope in the world CERN
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Antiparticles
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Standard model

• 3 families of particles
• 4 fundamental forces
• Carriers of the forces

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Weak interactions

• Force particles play a role in
• Interactions
• Pair creation
• Annihilation

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Example neutron decay
Neutron beta-decay
n ? p e- ne
At the quark level
d ? u e- ne
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Three kinds of neutrinos!

• Z0 decay into
• quark pairs
• (except top quarks!)
• lepton pairs
• ee-, mm-, tt-
• neutrino pairs

lifetime is inverse of decay probability 1/t G
G S Gi
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cross sections
GF a/MW2
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Collission lengths of neutrinos
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Neutrino puzzles
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Questions about neutrinos

• How heavy are neutrinos?
• Where are the solar neutrinos? (compared to the
  SSM)

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How can we detect Neutrinos?
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Neutrino detectors
Super Kamiokande
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Super Kamiokande
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Neutrino detection techniques
Detection via cherenkov light emitted by
particles moving faster than light
(from antares experiment)
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Neutrino oscillations in the atmosphere

• Neutrinos from cosmic rays come from decay of
  pions. These are nm neutrinos
• If the nm neutrino is a quantummechanical
  superposition of neutrinos n1
  en n2 one gets
  oscillations

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Vacuum oscillations
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Neutrino oscillations in the atmosphere

• Superkamiokande found oscillations by
  looking at the zenith angle dependence
• Results are consistent with nm ?
  nt oscillations with Dm2 2 - 3 x 10-3 eV2 and
  sin2 2q 1

lV 1250 km
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My first reaction
Interview in Aik door Wilm Geurts en Joost van
Mameren
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What are the consequences

• For particles with mass both righthanded and
  lefthanded species exist!
• This is only possible if the neutrino is its own
  antiparticle (like the photon, but different from
  the electron)
• (I do not discuss sterile neutrinos)

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Dirac and Majorana fermions
Fermion (general)
Dirac neutrino
Majorana neutrino
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Dirac and Majorana fermions
Although it seems as if the Majorana solution
restores mirror symmetry, this is NOT true
Lefthanded neutrino interacts with lefthanded
electron Righthanded neutrino interacts with
righthanded positron
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CP violation
Mixing between mass and weak-interaction
eigenstates for quarks AND neutrinos
Complex phases (at least requiring 3x3 mixing)
leads for both cases to CP violation
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Solar neutrinos
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Solar neutrinos in SNO(Sudbury Neutrino
Observatory)
En lt 15 MeV

• All neutrinos (x e, m, t)
• nx p ? nx p
• nx d ? nx p n
• nx e- ? nx e-
• (via Z0-exchange)
• Electron neutrinos
• ne d ? e- p p
• ne e- ? ne e-
• (via Z0 and W)

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Solar neutrino oscillations

• Matter contains protons, neutrons and electrons.
• Oscillations arise because ne interacts
  differently with matter dan nm

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Basis states ne and nm
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Solar neutrino oscillations

• SNO showed that the
  missing ne appear as
  different type, most
  probably nm
• le 2 x 107 m/(r/rwater) 2 x 105 m
  (for a density of r/rwater 100)
• lV 2.5 x 103 m(EGeV/Dm2eV2)
• Thus for E 1 MeV and Dm2 6 x 10-5 eV2 one
  finds that lV le and thus one can have the
  situation of a resonance with maximal
  oscillations!

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Why not go the easy way?

• Just observa a supernova emitting photons and
  neutrinos and look which arrive first!
• Particles with mass after all move slower than
  light!
• Surprise! Neutrinos from SN 87A arrived first!
• Explanation the velocity of light in matter is
  smaller than the velocity in vacuum
• In spite of a rather low density (in the galaxy
  about 5/cm3) light is slowed down more than that
  neutrinos move slower than light in vacuum!

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• Vlight 1/n 1 2p N f(k,q0)/E2
• Vneutrino 1 mn2/2E2

m2 10-5 eV2 E 1 GeV v 1 10-23 Dx 3 x
10-15 m/yr
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Nevertheless high-energy neutrinos might be the
messengers that help solving cosmological puzzles!
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An underwater laboratory
ANTARES (mediterranean Sea)
Towards huge volumes of the order of a km3
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Event simulation
ANTARES
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Event simulation
AMANDA (South Pole)
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Concluding remarks

• Neutrinos have mass, but its tiny of the order of
  0.05 - 0.001 eV (cf electron with mass of 511,000
  eV)
• Mass eigenstates are different from
  weak-interaction states (oscillations)
• Explanation of solar neutrino puzzle
• No solution for dark matter problem
• New possibilities in astrophysics

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END