Venice 080416

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Neutrino spectroscopy using atoms (SPAN)

• Introduction
• How to verify important ingredients of
  leptogenesis experimentally
• Why atoms
• Enhancement mechanism target coherence and
  resonance

hep-ph/0611362 and PRD, hep-ph/0703019, and to
appear
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Leptogenesis requires

• Majorana nature of neutrino masses, implying
  lepton number violation
• Absolute magnitude of neutrino masses
• lt 0.1 eV cf Giudice et al
• CP violation in neutrino sector
• We wish to resolve 1st 2 issues
  using atoms.

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What 3 major methods of neutrino experiments have
achieved

• Oscillation experiments
• with large mixing
• End point spectrum of beta decay
• lt 2 eV
• Neutrinoless double beta decay
• lt 0.5 eV

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Significance of discovering Majorana neutrino

• Neutral fermions follow economic Majorana eq or
  the same Dirac eq as charged fermions ?
• Leading to lepton number violation and
  lepto-genesis
• particle anti-particle
• missing partner for leptogenesis
• Contributing to better understanding of seesaw
  mechanism and GUT

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Attractive mechanism of small neutrino mass
generation

• Seesaw mechanism
• Heavy Majorana type of masses of neutrino
  partner , indicating physics beyond
  standard theory of particle physics, generates a
  tiny left-handed neutrino masses and mixing a la
• Necessarily violates lepton number conservation
• Agent of L-asymmetry generation provided by

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M vs D in 2-component equations
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Majorana vs Dirac equationschirally projected
solutions
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Merits and demerits of atomic process

• Infinitely many small energies are available
• Systematics for mass spectroscopy 6 thresholds
  and M/D distinction
• Small rate of neutrino pair emission
• How to enhance the rate is crucial
• resonance
• target coherence

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Neutrino interaction with atomic electron
4 Fermi interaction (after Fierz transformation)
MNS matrix elements
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Unique signature of Majorana interference of
identical fermions

• Effective only for pair emission
• Appear only (ii) threshold proportional to m_i2
• Can be positive or negative
• Direct test of Majorana nature cf LV in

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New enhancemet mechanism

• Coherent target effect like superradiance
• Neutrino pair emission more favorable in view of
  larger coherence region, determined by pair
  momenta, and not energy difference of levels
• If anticipated large N3 (target number)
  dependence is realized, gas targets are feasible
• Otherwise,
• use of denser target in solid matrix or
  something similar (nano-space)

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Superradiance case of photon emission for 2
level atom
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Radiative neutrino pair emission

• Emitted photon carries neutrino mass information
• Background 2 gamma

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Target examples

• Ba case 2 types of transitions coexist
• lambda ladder

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Radiative neutrino pair emission from collective
excited atoms

• Standard target shape pencil-like cylinder
  (length l) due to laser irradiation for
  preparation of excited states

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Radiative pair emission has critical information
of neutrino mass threshold

• Threshold distance from the background 2 photon
  peak

1 gamma spectrum of 2 gamma background is
discrete and sharply collimated with target
cylinder axis
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Features of events

• 2 photon background
• back to back, energy of half level difference,
  well collimated to cylinder axis perhaps not
  difficult to separate
• Radiative pair emission
• broader structure of photon energy
  distribution
• Ladder-type system has further resonace
  enchancement

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Rough estimate of rates

• Nu-pair
• 2 gamma

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Threshold behavior sensitive probe of mixing
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Observability of relic neutrino w. T. Takahashi

hep-ph/0703019

• Pauli blocking effect

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Threshold reduction 1/2x1/2 1/4 Temperature
measurement possible ?Case of laser irradiated
pair emission

• For m_1 lt 1meV, temperature measurement is not
  difficult

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Experimental strategy

• Verification of superradiance enhancement in 2
  gamma EM transition
• Discovery of neutrino pair emission far away from
  threshold region, with large rates
• Approach to threshold region for neutrino
  spectroscopy
• Detection of relic neutrino at lightest threshold
• SPAN collaboration

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Summary

• Discussed a new possibility of using coherent
  atomic targets
• Excellent for neutrino mass spectroscopy, if the
  rate is large enough
• If works ideally, a long path towards relic
  detection