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What would be the shape of the Milky Way Dark halo profile if DM was light?. Celine Bœhm, Geneva 2005. New physics at the centre of our galaxy?. 1. Detection of a 511 keV emission line in the centre of the Milky Way. INTEGRAL/SPI.

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What would be the shape of the

Milky Way Dark halo profile

if DM was light?

Celine Bœhm, Geneva 2005


New physics at the centre of our galaxy?

1. Detection of a 511 keV emission line in the centre of the Milky Way


2. Interpretation:electron-positron annihilation

(positronium formation)

Celine Bœhm, Geneva 2005






511 keV line signal!

  • Confirmation of a low energy positrons in the centre of the galaxy

~ 95% of the

events detected

2 photon production from e+e- at rest.

Kinematics: 2 me = 2 E(photon)

2. Ortho-positronium

2 photon production from e+e- at rest.

Kinematics: 2 me = 3 E(photon)

3. In flight

2 photon production from energetic e+e-.

Kinematics: 2 E(e)= 2 E(photon)

Celine Bœhm, Geneva 2005

quick reminder on positronium formation
Quick reminder on positronium formation

Possible states:


S=1 so 3 photons


S=1 so 2 photons

Celine Bœhm, Geneva 2005


Past and present observations of the 511 keV line

INTEGRAL is not the first but its sensitivity is very good and it can map the emission.

Celine Bœhm, Geneva 2005

just a simple comparison
Just a simple comparison:
  • Detection of 3 components:
  • Bulge
  • Disc
  • PLE
  • (Positive latitude Enhancement)
  • OSSE:
  • Detection of 1 component:
  • The bulge!
  • Disc absent but B/D>0.4-0.8
  • No PLE
  • (Positive latitude Enhancement)

Celine Bœhm, Geneva 2005


INTErnational Gamma RayLaboratory


Germanium Dectector

Coded mask

Anticoincidence shield

Fully coded FoV: 16deg*16

The aperture system provides the

imaging capabilities of instrument

Celine Bœhm, Geneva 2005



Needs to assume a

model for the source,

e.g. gaussian,


J. Knodlseder et al, Lonjou et al, …

Celine Bœhm, Geneva 2005



Where the

line come


INTEGRAL has large exposure data

but most of the signal comes from only 9 deg, i.e. the inner part of the galaxy.

After reconstruction, they can exclude an unique source (if ponctual) but several could explain the emission.

If the source is gaussian, then it is possible to deduce the Full Width Half Maximum

Celine Bœhm, Geneva 2005

possible sources de positrons p jean http www cesr fr marcowit pierrejean pdf
Possible sources de positrons (P. Jean,

+ Low Mass Binaries

Celine Bœhm, Geneva 2005


But a problem faced by SN, Wolf Rayet stars etc (except LMB, DM):

the ratio bulge-to-disk is generally not large enough

(some sources being mostly in the disc)

Need for an old stellar population or exotic source

The explanation is therefore likely to be a sign of new physics,

whether it is astrophysical or from particle physics.

But one needs to be careful as long as the origin of galactic

positrons is a not properly identified.

Celine Bœhm, Geneva 2005

Can Dark Matter fit the characteristics

of the signal detected and mapped by INTEGRAL/SPI?


1. Results from a model fitting analysis (modelling the source)

1e-3 ph/cm2/s

FWHM ~ 8.5deg

2. DM must fit both the FWHM, the flux and the ratio bulge-to-disk

Celine Bœhm, Geneva 2005


2 E(e) = 2 mdm

DM annihilations into e+ e- can produce the galactic positrons

  • The positrons must be almost at rest
  • They must lose their energy through ionization
  • Once at rest, they form positronium and produce 2 or 3 photons

This requires mDM < 100 MeV (i.e. very light DM particles).

Celine Bœhm, Geneva 2005


A. How light DM can be ? (Astrophysics)

(Boehm, Ensslin, Silk, 2002)

Annihilations of Light DM (<100 MeV) in the centre of the MW will produce too much low energy gamma rays compare to observations.


True only if one considers an annihilation cross section that allows to get the correct relic density.


The annihilation cross section must vary with time for mdm< 100 MeV.

Particle Physics requirement:

The annihilation cross section must be dominated by a velocity-dependent

Celine Bœhm, Geneva 2005


B. 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, Geneva 2005

first calculations to be done lee weinberg 1977





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)

the phenomenology of the model
The phenomenology of the model
  • Scalar DM:
  • Fermionic DM:

Annihilation cross sections for scalars

  • scalars coupled to heavy particles (F): v-independent cross section
  • scalars coupled to light particles (Z’): v-dependent cross section

Celine Bœhm, Geneva 2005

annihilation cross sections for fermions
Annihilation cross sections for fermions
  • Fermions coupled to heavy particles (F): v-independent cross section

Depends on whether Majorana or Dirac. Here Majorana (Boehm&Fayet 2003)

  • fermions coupled to Z’: v-dependent cross section

MeV fermions/scalars: Z’ are required to escape the Gamma ray constraints

Celine Bœhm, Geneva 2005


First results (CB, D. Hooper, J. Silk et al)

  • Flux OK with observations:
  • the cross section must be about five order of magnitude
  • lower than the annihilation cross section for the relic density
  • Z’ favoured!
  • Halo density profile:

Assumptions: 1/rg

as MW halo profile is still unknown

Celine Bœhm, Geneva 2005


Improved Results (CB, Y. Ascasibar, 2004)

  • taking into account more data (16 deg)

Boehm&Ascasibar, 2004

  • Implementation of the right velocity dispersion profile

Celine Bœhm, Geneva 2005


New (Preliminary) Results:

  • Implementation of the e+ distribution for realistic halo profiles
  • (NFW, Moore, Binney-Evans, Isothermal) in INTEGRAL analysis
  • (the source!)
  • Implementation of the right velocity dispersion profile
  • More data, including Dec 2004

Celine Bœhm, Geneva 2005



  • NFW profile is THE profile that fits the data!
  • Exchange of heavy particles is needed to fit the 511 keV line

For mF ~100 GeV

For mF ~1 TeV

Celine Bœhm, Geneva 2005


Fermionic DM seems to be excluded:

  • Decaying DM is excluded (unless ??? the profile is extremely cuspy):

Celine Bœhm, Geneva 2005

a consequences for particle physics





A. Consequences for Particle Physics
  • Z’ changes the neutrino-electron elastic scattering cross section.

[σ(νμ N -> νμ X) - σ(νμ N -> νμ X)]

--------------------------------------------- = (gl2 –gr2)

[σ(νμ N -> μ X) - σ(νμ N -> μ X)]

With gl,r2 = [(gl,ru) 2 + (gl,rd)2]/4

and gl,rf = 2 (T3(fl,r) - Q(f) Sin ΘW on shell2)

  • QED/EW corrections
  • QCD corrections:

perturbative QCD

charged current charm production

Parton distributions

Isospin breaking

Nuclear effects

Experimental effects

  • Possible solution:

Asymmetric strange sea

Isospin violation in parton distribution

Celine Bœhm, Geneva 2005

consequences for particle physics
Consequences for Particle Physics

S. Davidson et al mentioned that a light Z’ could explain the NuTeV anomaly

CB 2004, yes it is true and in agreement with the LDM but tests to make first.

Celine Bœhm, Geneva 2005


B- Consequences of (heavy fermionic) F particles

  • The measured value of alpha is not in agreement with

the value obtained from the g-2 of electrons.

  • Generally the discrepancy is disregarded because

there is no simple explanation but with LDM (F particles)

one change the expression of the g-2 of electrons and

one obtains a perfect agreement for mdm < 20 MeV.

Celine Bœhm, Geneva 2005


Note on Beacom et al, 2004

Mdm < 20 MeV

because of the Final State Radiation

  • But they do not compute the process.
  • They use the result of e+ e- into mu+ mu- valid for gamma exchange which is factorizable and also at high energy.
  • However, the F exchange is not factorizable.
  • The final result could change!

Celine Bœhm, Geneva 2005



  • NFW profile (consequences for the MW profile if LDM exists)
  • Scalar DM
  • Fermionic and decaying DM are ruled out
  • Heavy fermions are required but Z’ exchange possible too
  • Look like SUSY but relationship between the couplings and MF,
  • Possible implication for NuTeV and the alpha value

Celine Bœhm, Geneva 2005