Light DM,An interesting

candidate?

Celine Boehm, 2004

• Heavy Dark Matter TeV string candidates

neutralinos

GeV (proton mass)

• Light Dark Matter MeV ``new’’ particles?

(10-6 eV axions particles) non thermal

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

Damping constraints in a plane! One can define ``WDM’’ particles as being at the edge of the damping limit.

Means also ``Collisional WDM’’!

2 2dm dm dm,e

dn3H n σ v (n n )

dt

3

3 -2

( )

dmdm dm dm

c

dm fo fo dm

fo dm fo

n m

n t a m

Ha m with H a

v

A very important criterion: Relic density

dm

fo

dm

mfactors

TΩ

σ v

2dm dm

the annihilation rate freezes-out when:

H n = σ v n at tfo

The relic density criterion therefore requires:

One value only does the job!

-12-27 3Ωh

v 7 10 cm /s0.1

Independent of the DM mass!

Too much DMThe annihilationCross section is too small

Not enough

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)

Light Dark Matter (mdm < GeV)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 dm

dm

e-

e+

L rr r l (Cl F f C F f ) . .l

h c

2

dm

22 2 2 dm

4F

7 v1+

3

m v (cl +cr )

m

(Dirac dm = field)

F

dm

dm

f+

f-

F

Almost independent of mdm ! Possible to evade Lee-Weinberg when scalars!!

• By just obtaining a cross section independent of mdm!

• Which DM particles then?

– Fermions: always proportional to mdm

– Scalars: yes, some configurations OK!

L rr r l (cl f F C f F ) . .l

h c

2 2 22f l r

dm2 2dm F

m C Cv (1- v )

m m

How to evade Lee-Weinberg?

• 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

Binney Evans profile

Isothermal profile

NFW profile

Binney Evans profile

Isothermal profile

NFW profileth 5 observed dm

-26 3 1

mσv10

10 cm s MeV

-2

2 2c

2c

2 2

G M(r)v M(r) = 4 (r) d

v1 dM(r)(r) = =

4 dr

r

4 G r

r

r

C.B., T. Ensslin, J. Silk

But DM can annihilate into the galactic center…

• No if the cross section ~ a + b v2 (with a << b)

• Why a cross section in v2 saves the scenario?DM velocity in galaxy < 10-3 cDM velocity in primordial U ~ c

• So v2 reduced the flux by a factor 10-6..

Is that possible in particle physics?

Light DM ruled out then?

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?

2 2 22f l r

dm2 2dm F

m C Cv (1- v )

m m

Summary mid-stage

2

dm

22 2 2 dm

4F

7 v1+

3

m v (cl +cr )

m

• Possible if a new gauge boson (U):

Particle Physics models

f+

f-

dm

dm

22 2 2 2dm

dm U Ul Ur4U

m v v C (f + f )

m

Dependence in mdm!

But OK if light U and small couplings!

(U of a few MeV, < GeV)

U

CU fU

1) cross section smaller than at high energy becausefU is much smaller than electroweak couplings!

2) cross section maximal at low energy (a few MeV). Could be promising but dominated by

• small couplings as required by muon and electron g-2!

• Could not be seen in past colliders:

Light DM finally possible because:

e e dm dm

e e

e e

22 2 2 2dm

dm U Ul Ur4U

m v v C (f + f )

m

2E

1

dm dm -> e+ e-e+ e- -> phot phot (phot)

1) e- and e+ at rest, 2) Photons with Ee, 3) Existence of e+ !

A first evidence of Dark Matter annihilations?

INTEGRAL/SPI: observation of a 511 keV line emission

Prospectives (dwarfs galaxies)

In the sensitivity of Integral satellite

If a signal is detected: confirmation of LDMIf not, the LDM scenario is possibly ruled out

Sagittarius Dwarf Elliptical Galaxy

• Light DM could involves a theory that also predicts a heavy and stable particle

• 2 symmetries (R and M-parities): N=2 supersymmetry??

• The relic density will be ensured by the Light DM so no restriction on the cross section

Heavy Dark Matter

• Light Dark Matter (<GeV) is possible!

• 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…

Conclusion

Collisionless WDM

M_WDM > keV