celine bœhm, geneva 2005

Celine Bœhm, Geneva 2005

What would be the shape of the Milky Way Dark halo profile

if DM was light?


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)

INTEGRAL/SPI

Interpretation:

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

e-

e+

1. Para-positronium

2. Ortho-positronium

3. In flight


2 photon production from e+e- at rest. Kinematics: 2 me = 2 E(photon)

~ 95% of the

events detected

2 photon production from e+e- at rest. Kinematics: 2 me = 3 E(photon)

511 keV line signal!

2 photon production from energetic e+e-. Kinematics: 2 E(e)= 2 E(photon)

Quick reminder on positronium formation

Possible states:


Ortho-positronium

Para-positronium

S=1 so 3 photons

S=1 so 2 photons

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.


Just a simple comparison: OSSE:

INTEGRAL:


Detection of 3 components:

•Bulge•Disc•PLE(Positive latitude Enhancement)

Detection of 1 component:

• The bulge!• Disc absent but B/D>0.4-0.8• No PLE(Positive latitude Enhancement)

INTErnational Gamma Ray Laboratory


Cryostat Germanium Dectector

Anticoincidence shield Coded mask

Fully coded FoV: 16deg*16The aperture system provides the imaging capabilities of instrument

J. Knodlseder et al, Lonjou et al, …


Reconstruction

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


r~33deg

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

Where the line come from!

Possible sources de positrons (P. Jean, http://www.cesr.fr/~marcowit/PierreJean.pdf)

+ Low Mass Binaries


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.


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)

FWHM ~ 8.5deg

1e-3 ph/cm2/s

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


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).

2 E(e) = 2 mdm


A. How light DM can be ? (Astrophysics)

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

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

Solution:

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

(Boehm, Ensslin, Silk, 2002)


If DM is a fermion and coupled to heavy particles (Z, W) then it should be heavier than a few GeV.

Lee-Weinberg:

B. 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.

Lee-Weinberg limit:mdm < O(GeV)

Massive neutrinos, Fermi interactions: dm

dm

f

f

• Depends mainly on mdm,

• if mdm too small, dm> 1 !

2dm

4w

m v

m

First calculations to be done: Lee-Weinberg (1977)

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

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

Annihilation cross sections for fermions



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/r

as MW halo profile is still unknown


Improved Results (CB, Y. Ascasibar, 2004)

taking into account more data (16 deg)

Boehm&Ascasibar, 2004

Implementation of the right velocity dispersion profile


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

New results obtained in collaboration with INTEGRAL



Consequences:

Exchange of heavy particles is needed to fit the 511 keV line

NFW profile is THE profile that fits the data!

For mF ~100 GeV For mF ~1 TeV


Fermionic DM seems to be excluded:

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

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

[σ(νμ N -> νμ X) - σ(νμ N -> νμ X)]

--------------------------------------------- = (g l2 –gr

2)

[σ(νμ N -> μ X) - σ(νμ N -> μ X)]

With gl,r 2 = [(gl,ru) 2 + (gl,r

d)2]/4

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

ν

e e

ν


6

du,

qa

qvf

6

du,

qa

qv f

10 . 3.2072ccG:SM

10 0.0288)(3.1507ccG :NuTeV

QED/EW corrections QCD corrections:

perturbative QCDcharged current charm

productionParton distributionsIsospin breakingNuclear effects Experimental effects

Possible solution:Asymmetric strange sea Isospin violation in parton

distribution

Consequences for Particle Physics


S. Davidson et al mentioned that a light Z’ could explain the NuTeV anomalyCB 2004, yes it is true and in agreement with the LDM but tests to make first.


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.

B- Consequences of (heavy fermionic) F particles


Note on Beacom et al, 2004

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!

Mdm < 20 MeV because of the Final State Radiation


Conclusions

Heavy fermions are required but Z’ exchange possible too

NFW profile (consequences for the MW profile if LDM exists)

Scalar DM

Fermionic and decaying DM are ruled out

Look like SUSY but relationship between the couplings and MF,

Possible implication for NuTeV and the alpha value

The coded mask together with the detector plane define an angular resolution of about 2.5° within a fully coded field of view of 16° x 16°. The partially coded field of view is 34° x 34° while the anticoincidence shield defining a hexagonal aperture of ~ 25° FWHM. The point source location is better than 2° and improves with source intensity and exposure time. The example shown here demonstrates SPI’s imaging capabilities by folding the Galactic 511 keV skymap through a detailed model of the spectrometer. The resulting skymap was obtained by simulating a galactic plane survey of SPI with a realistic background which slightly varied with time. The calculations of SPI’s imaging performance have been performed at the University of Birmingham, UK.

celine bœhm, geneva 2005

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