Cosmic neutrino absorption spectroscopy

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Cosmic neutrino absorption spectroscopy
Gabriela Barenboim
University of Valencia
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According to standard cosmology, neutrinos should
be the most abundant particles in the Universe,
after CMB photons.
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The CMB neutrino is the oldest relic !!!
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Resonant annihilation of extremely high neutrinos
on background neutrinos through the reaction
n n Z
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The relic neutrino background
The annihilation of e e- g g at T
? 2 me 1 MeV transferred entropy from e e- to
g s and so heated g s leaving
Tn (4/11)1/3 Tg
Neutrinos are fermions while photons are bosons,
so at a common temperature
nn 3/4 ng rn 7/8 rg
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Today
nn0 ¾ (Tn / Tg)3 ng0 112 cm-3
The temperature of a massless decoupled species
scales as T0 (1z) The relic neutrino number
density will be red-shifted as nn0 (1z)3
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However, the effective relic neutrino density
that an UHE neutrino would encounter while
traversing the expanding Universe we live in, is
the neutrino density per unit red shift
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Cosmic neutrino spectroscopy
The location of the absorption lines in the UHEn
spectrum points to neutrino masses !!!
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Good news !!!
It is possible to detect the absolute masses and
the flavor composition of the mass eigenstates
through the detection of the dips
It could be, assuming perfect energy resolution
and flavour tagging
Bad news !!!
Integration over cosmic time (red-shift),
unconventional neutrino histories, thermal
history of the Universe
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The ideal(ized) experiment
Ideal laboratory we neglect the expansion of
the Universe and relic neutrino
temperature
Ideal target the cosmic neutrino attenuator is a
very long uniform column of length L with the
current relic density
Ideal beam the UHE neutrino beam is originated
gt100 Mpc and contains all neutrino flavors in
sufficient numbers
Ideal detector detector with perfect energy
resolution and flavour tagging
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Normal hierarchy
Inverted hierarchy
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Annihilation cross section (eV-2)
Resonant absorption energy (eV)
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Normal hierarchy
Inverted hierarchy
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Absorption lines in an expanding Universe

• Evolution of the relic neutrino density changes

• The energy of the UHE neutrino is red-shifted

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neutrino mass 0.1 eV
0 ? z ? 4
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neutrino mass 0.1 eV
4 ? z ? 8
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neutrino mass 0.1 eV
8 ? z ? 12
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neutrino mass 0.1 eV
12 ? z ? 16
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neutrino mass 0.1 eV
16 ? z ? 20
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neutrino mass 0.1 eV
0 ? z ? 20
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neutrino mass 0.1 eV
0 ? z ? 20
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Absorption lines in an expanding Universe
Normal hierarchy
Inverted hierarchy
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Alternative thermal histories
and the effect grows with red-shift !!!
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Neutrino temperature
neutrinos are moving targets
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Neutrino temperature
neutrinos are moving targets
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when considering cosmic evolution, the thermal
motion has to be considered for m lt .1 eV
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Normal hierarchy
Inverted hierarchy
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Diagnostic potential of cosmic spectroscopy

• The detection of relic neutrinos would be a major
  discovery
• As a byproduct, in principle, cosmic neutrino
  spectroscopy could provide the value of the
  individual neutrino masses and mixings,
  unveiling neutrino properties or unconventional
  neutrino stories.
• The red-shift dependence is both a blessing and a
  curseit shifts the dips to lower energies but
  gets them distorted, compromising the
  determination of neutrino masses.
• With external information, neutrino spectroscopy
  can tell us about the thermal history of the
  Universe.