Light DM,

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Light DM, An interesting candidate?
Celine Boehm, 2004
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• Heavy Dark Matter
• TeV
  string candidates
• 
  neutralinos
• GeV
  (proton mass)
• Light Dark Matter
• MeV
  new particles?
• (10-6 eV
  axions particles)
• non thermal

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Weakly Interacting Massive Particles

• Silk (1968), Gunn et al (1978), Davis
  et al (1980), Peebles (1982) etc..
• No electromagnetic interactions Weakly
  Interacting
• Not neutrino-like Massive
• Weakly Interacting Massive Particles (WIMPs)
• But in fact more complicated..damping.ppt

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Damping constraints in a plane! One can define
WDM particles as being at the edge of the
damping limit. Means also Collisional WDM!
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A very important criterion Relic density
at tfo
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The relic density criterion therefore requires
Independent of the DM mass!
Too much DM The annihilation Cross section is too
small
One value only does the job!
Not enough
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First calculations to be done Lee-Weinberg
(1977)
Massive neutrinos, Fermi interactions

• Depends mainly on mdm,
• if mdm too small, Wdm 1 !

Lee-Weinberg limit mdm O(GeV)
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Light Dark Matter (mdm Forbidden by Lee-Weinberg because the cross
section is too small!
But they considered massive neutrinos and with a
cross section proportional to mdm!
Extension to more general DM candidates!
Examples fermions
F
(Dirac dm d field)
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How to evade Lee-Weinberg?

• By just obtaining a cross section independent of
  mdm!
• Which DM particles then?
• Fermions always proportional to mdm
• Scalars yes, some configurations OK!

Almost independent of mdm ! Possible to evade
Lee-Weinberg when scalars!!
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But DM can annihilate into the galactic center
C.B., T. Ensslin, J. Silk

• Gamma rays Expected between MeV and GeV depending
  on mdm
• But already lot of observations in this range
  (notably OSSE!).
• No indications for non standard physics so
  predictions have to be compatible.
• Predictions depend on the DM halo profile

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Light DM ruled out then?

• No if the cross section a b v2 (with a
• Why a cross section in v2 saves the scenario?
• DM velocity in galaxy
• DM velocity in primordial U c
• So v2 reduced the flux by a factor
  10-6..
• Is that possible in particle physics?

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Summary mid-stage

• Light DM OK but needs for a
• v2 annihilation cross section!
• Fermionic DM cannot do that!
• Scalar DM?
• if exchange fermions
• If exchange gauge bosons?

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Particle Physics models

• Possible if a new gauge boson (U)

U
CU
fU
Dependence in mdm! But OK if light U and small
couplings! (U of a few MeV,
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Light DM finally possible because

• small couplings as required by muon and electron
  g-2!
• Could not be seen in past colliders

• cross section smaller than
  at high energy because
• fU is much smaller than electroweak couplings!
• 2) cross section maximal at low energy (a few
  MeV).
• Could be promising but dominated by

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INTEGRAL/SPI observation of a 511 keV line
emission
A first evidence of Dark Matter annihilations?
dm dm - e e- e e- - phot phot (phot)

• e- and e at rest,
• Photons with Ee,
• Existence of e !

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Prospectives (dwarfs galaxies)
In the sensitivity of Integral satellite If a
signal is detected confirmation of LDM If not,
the LDM scenario is possibly ruled out
Sagittarius Dwarf Elliptical Galaxy
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Heavy Dark Matter

• Light DM could involves a theory that also
  predicts a heavy and stable particle
• 2 symmetries (R and M-parities) N2
  supersymmetry??
• The relic density will be ensured by the Light DM
  so no restriction on the cross section

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Conclusion

• Light Dark Matter (
• Need for v2 cross section
• possible with light scalars exchanging a light
  gauge boson
• but other solutions may work.
• LDM explain the detection of a 511 keV line in
  the centre of the galaxy very well.
• Possibility of having heavier stable particles

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Collisionless WDM
M_WDM keV