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Celine Bœhm, Unesco 2005
Is Dark Matter light?
Status and prospects of the Light Dark Matter scenario in view of the 511 keV line
Celine Bœhm, Unesco 2005
Confirmation of a 511 keV emission in the centre of the galaxy by INTEGRAL/SPI
E = me
Narrow line which is the sign of electron-positron annihilations at rest.
Para-positronium Ortho-positroniume-
e+
In flight annihilations
e+ e-
E < me E < Ee
33deg, 16deg FoV
Great improvement of the sensitivity which confirms the origin of the line and its characteristics
Balloon experiments(HEAO3)
Satellite experiments(OSSE, INTEGRAL)
Celine Bœhm, Unesco 2005
Comparison between past and new measurements :
OSSE:
INTEGRAL:
Celine Bœhm, Unesco 2005
Detection of 3 components:
•Bulge•Disc•PLE(Positive latitude Enhancement)
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
Stars SNe (Co 56) SNII (Al26, Ti 44) WR (Al 26)
Compact sourcesPulsarsBlack holesLow Mass Binaries
Cosmic raysp-anti p -> positronsRadioactive isotopes
General problem (except for old populations/LMB):
Too low Bulge/Disc ratio
Possible source of low energy e+ in the GC
LMB, old stellar population, or other 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(?)
1) DM annihilates into electon-positron
2) The positrons lose their energy through ionization
3) Once at rest, the positrons can annihilate with electrons of the medium and form para-positronium
4) 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-
e+
edm
dm
e-
e+
Celine Bœhm, Unesco 2005
e+ lose energy
(DM mass must be < the muon threshold, to avoid pion production)
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 mdm
2
But are their characteristics compatible with the morphology of the 511 keV emission in the galactic centre?
Nowadays:
Celine Bœhm, Unesco 2005
Celine Bœhm, Unesco 2005
First results from a model fitting analysis (modelling the source)
FWHM ~ 8.5deg
~ 10-3 ph/cm2/s
Width is less than 10 keV!
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
J. Knodlseder et al, Lonjou et al, 2003
Celine Bœhm, Unesco 2005
Reconstruction
Needs to assume a model for the source, e.g. gaussian, ponctual, halo/bulge model or DM distribution
One ponctual source is excluded!
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
Cross section depends on:1. The DM mass (mdm)2. The DM energy (Edm)3. The couplings4. (The mass of the particle that is exchanged)
DM non relativistic at annihilations. Thus, Edm= ½ mdm v2 + mdm c2
Therefore the cross section depends on constant terms and v2
A convenient decomposition is then given by:<σv> = a + b v2 where a and b are constants.
Celine Bœhm, Unesco 2005
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
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
Celine Bœhm, Unesco 2005
Contribution to b solely. Cannot explain the 511 keV line but is required for the relic density
Contribution to a AND b with a=b so this diagram MUST be suppressedBut fit the 511 keV line
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
Celine Bœhm, Unesco 2005
S. Davidson et al, C. Boehm 2004
NuTeV anomaly
The fine structure constant F particles contribution to g-2
Deviation from SM
Where does the anomaly come from?
1. ath = f(α)2. impose ath = aQED and found αth3. Compare it with the experimental measurement Quantum Hall effect
Using the LDM model as determined by the 511 keV line:
For mdm~6-7 MeV
(prediction also for the muon!)
Colliders
Scalar
Fermionic
Celine Bœhm, Unesco 2005
Conclusions
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
If DM is a fermion and coupled to heavy particles (Z, W) then it should be heavier than a few GeV.
Lee-Weinberg:
How light DM can be ? (Particle Physics)
Boehm-Fayet:
If DM is a fermion and coupled to light particles then it can be lighter than a few GeV.
If DM is a scalar and coupled to light or heavy particles then it can be lighter than a few GeV.
Celine Bœhm, Unesco 2005
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
22 2 2 2dm
dm U Ul Ur4U
m v v C (f + f )
m