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Status and prospects of the Light Dark Matter scenario in view of the 511 keV line. Is Dark Matter light?. Celine Bœhm, Unesco 2005. Confirmation of a 511 keV emission in the centre of the galaxy by INTEGRAL/SPI. e+. e + e - g g. e-. 33deg, 16deg FoV.

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Status and prospects of the Light Dark Matter scenario in view of the 511 keV line

Is Dark Matter light?

Celine Bœhm, Unesco 2005


Confirmation of a 511 keV emission in the centre of the galaxy by INTEGRAL/SPI


e+ e-g g


33deg, 16deg FoV

Narrow line which is the sign of electron-positron annihilations at rest.



In flight annihilations

Eg = me

Eg < me

Eg < Ee

Celine Bœhm, Unesco 2005


Great improvement of the sensitivity which confirms

the origin of the line and its characteristics

Balloon experiments


Satellite experiments


Celine Bœhm, Unesco 2005

comparison between past and new measurements
Comparison between past and new measurements :
  • Detection of 3 components:
  • Bulge
  • Disc
  • PLE
  • (Positive latitude Enhancement)
  • OSSE:
  • Detection of 1 component:
  • Bulge
  • Disc but due to radioactivity
  • Bulge/Disc>0.4-0.8
  • No PLE

Celine Bœhm, Unesco 2005

possible sources of positrons
Possible sources of positrons
  • Stars
    • SNe (Co 56)
    • SNII (Al26, Ti 44)
    • WR (Al 26)
  • Compact sources
    • Pulsars
    • Black holes
    • Low Mass Binaries
  • Cosmic rays
    • p-anti p -> positrons
    • Radioactive isotopes

General problem

(except for old populations/LMB):

Too low Bulge/Disc ratio

Celine Bœhm, Unesco 2005

possible source of low energy e in the gc
Possible source of low energy e+in the GC
  • LMB, old stellar population, orother unknown sources
    • Not clear whether LMB could fit both the observed flux, the line width and the morphology of the emission, but …
    • Maybe new mechanisms are the answer but, in any case, an astrophysical explanation remains to be found
  • New physics (or astrophysics)

Easier in fact since the model already existed for other purposes!

Celine Bœhm, Unesco 2005

new physics at the origin of the emission







New physics at the origin of the emission(?)
  • DM annihilates into electon-positron
  • The positrons lose their energy through ionization
  • Once at rest, the positrons can annihilate with electrons of the medium and form para-positronium
  • The para-positronium states gives 511 keV photons

To avoid an overproduction of low energy gamma rays, the DM mass must be lower than 100 MeV


e+ lose energy

(DM mass must be < the muon threshold, to avoid pion production)

Celine Bœhm, Unesco 2005

are light dark matter particles lighter than a proton possible
Are Light Dark Matter particles (lighter than a proton) possible?
  • Scenario proposed before INTEGRAL
  • The aim was to show that it is possible to evade the Lee-Weinberg limit
  • I.e. DM particles can be lighter than a few GeV but the annihilation cross section nowadays must be reduced compare to its value in the past universe by 5 order of magnitude times mdm2


But are their characteristics compatible with the morphology

of the 511 keV emission in the galactic centre?

Celine Bœhm, Unesco 2005


First results from a model fitting analysis

(modelling the source)

~ 10-3 ph/cm2/s

FWHM ~ 8.5deg

Width is less than 10 keV!

Celine Bœhm, Unesco 2005

na ve comparison with dm prediction assuming a dm halo profile as r 0 r
Naïve comparison with DM prediction!(Assuming a DM halo profile as ρ(r)≈ρ0/r)
  • Full Width Half Maximum (extension)
  • Flux: require cross section of 10-31 cm3/s

Full width

Half maximum

Celine Bœhm, Unesco 2005


Needs to assume a model for the source, e.g.

gaussian, ponctual, halo/bulge model or DM distribution

One ponctual source is excluded!


J. Knodlseder et al, Lonjou et al, 2003

Celine Bœhm, Unesco 2005

a better analysis was needed
A better Analysis was needed
  • Previous results compared the FWHM expected for DM with that obtained assuming a gaussian distribution.
  • That is not what one should do.
  • Instead one has to determine the characteristics that SPI would see if DM was indeed at the origin of the emission
  • So INTEGRAL analysis must start from the positron distribution as produced by DM annihilations!

Celine Bœhm, Unesco 2005

elements for starting a new analysis
Elements for starting a new analysis
  • Cross section depends on:
      • The DM mass (mdm)
      • The DM energy (Edm)
      • The couplings
      • (The mass of the particle that is exchanged)
  • DM non relativistic at annihilations.

Thus, Edm= ½ mdmv2 + mdm c2

  • Therefore the cross section depends on constant terms and v2
  • A convenient decomposition is then given by:

<σv> = a + bv2 where a and b are constants.

Celine Bœhm, Unesco 2005

new analysis based on spi response and background
New analysis based on SPI response and background
  • Testing the a-term and the b-term
  • 4 different models of the DM halo

About the same as the previous

version of the model !!!

Celine Bœhm, Unesco 2005

results consequences for the model
Results/consequences for the model
  • Decaying DM is now excluded (unless perhaps…)
  • An a-term is needed to fit the 511 keV emission but suppressed by 5 o.m
  • So a b-term is needed for the relic density
  • As predicted initially:
  • with

Contribution to a AND b with a=b

so this diagram MUST be suppressed

But fit the 511 keV line

Contribution to b solely. Cannot

explain the 511 keV line but is

required for the relic density

Celine Bœhm, Unesco 2005

consequences for prospects in particle physics
Consequences for/Prospects in Particle Physics
  • No theory but a very successful model perhaps
  • But important checks to do:
    • Collider physics
    • Neutrino physics (NuTeV)
    • G-2

Celine Bœhm, Unesco 2005


NuTeV anomaly

S. Davidson et al,C. Boehm 2004

Celine Bœhm, Unesco 2005

the fine structure constant
The fine structure constant
  • F particles contribution to g-2
  • Deviation from SM
  • Where does the anomaly come from?
      • ath = f(α)
      • impose ath = aQED and found αth
      • Compare it with the experimental measurement Quantum Hall effect
  • Using the LDM model as determined by the 511 keV line:

(prediction also for the muon!)

For mdm~6-7 MeV

  • Scalar
  • Fermionic

Celine Bœhm, Unesco 2005

  • The 511 keV line characteristics are now extremely well determined
  • Light DM fits successfully the morphology of the emission while astrophysical explanations are still to found (but not excluded!)
  • If LDM is the correct explanation, then the profile of the Milky Way should be cuspy (a la NFW)
  • LDM has maybe already manifested in PP experiments (via g-2 experiments, --NuTeV??--). Needs more focus on these aspects now.
  • LDM should be a scalar rather than a fermion. It should annihilate (not decay).
  • Problem though: no theory (except perhaps N=2 SUSY) but so does Lambda in fact..

Celine Bœhm, Unesco 2005


How light DM can be ? (Particle Physics)

  • Lee-Weinberg:

If DM is afermionand coupled toheavyparticles

(Z, W) then it should beheavier than a few GeV.

  • Boehm-Fayet:

If DM is afermionand coupled tolightparticles

then it can belighter than a few GeV.

If DM is ascalarand coupled tolightorheavyparticles

then it can belighter than a few GeV.

Celine Bœhm, Unesco 2005


Light scalars (Boehm&Fayet, 2003):

coupled to heavy particles (F):

v-independent cross section

coupled to light particles (Z’):

v-dependent cross section

  • Light fermions (Fayet 2004):

coupled to light particles (Z’):

v-dependent cross section

Z’ are required to escape the Gamma ray constraints

Celine Bœhm, Unesco 2005