Axionic Dark Matter
Mike Glatzmaier
University of Wisconsin-Madison
Physics 801, May, 2010
1
Tuesday, May 11, 2010
Outline
• Dark Matter Puzzles
• Introduction to Axions
• Current Experimental Efforts to Find the Axion (ADMX)
• Concluding Remarks
2
Tuesday, May 11, 2010
Dark Matter Must be there, but what is it?
-Currently, we have still been unable to pinpoint dark matter, but knowing
what dark matter is has far-reaching implications for astrophysics - structure
formation, and particle physics
3
Tuesday, May 11, 2010
Properties Dark Matter :
-They must provide the correct relic density:
-Must be weakly interacting
-Possible candidates include: SUSY, sneutrino, neutralino, and the AXION
-Axions, it turns out, do provide the correct relic density, and are essentially
collisionless
4
Tuesday, May 11, 2010
Outline
• Dark Matter Puzzles
• Introduction to Axions
• Current Experimental Efforts to Find the Axion (ADMX)
• Concluding Remarks
5
Tuesday, May 11, 2010
Some initial problems in QCD:
The QCD lagrangian has a number of symmetries, if we take the quarks to
have vanishing mass,
U (N )V × U (N )A
iαa T2a
qf → e
iγ5 αa T2a
qf ; qf → e
mu , md << ΛQCD
U (2)V = SU (2)V × U (1)V
Isospin
Baryon number
What about the axial symmetries??
Tuesday, May 11, 2010
6
The Axial Symmetries:
• For the axial symmetries, things are different......
< q̄f qf >�= 0
• Dynamically generated quark condensates spontaneously break these
symmetries
• Similar to the Higgs, mechanism, we expect to see nearly massless particles
in the hadronic spectrum due to this spontaneous symmetry breaking
(Goldstone bosons)
• The pions fit well, but there is no good pseudo-scalar candidate for the U(1)symmetry.
U (2)A = SU (2)A × U (1)A
Perhaps not a real symmetry of QCD
Tuesday, May 11, 2010
7
Strong CP-problem:
• It turns out that the divergence of the axial current does not vanish.
µ
∂µ J5
∼
�
dσµ K �= 0
µ
• The gluon configurations which do not vanish contribute to vacuum-vacuum
transitions in QCD
• These vacuum-vacuum effects can be incorporated into the lagrangian by
adding the following term:
QCD
Lθ
g 2 αβµν a a
=θ
�
G
G
µν
αβ
2
32π
• However, this is a CP-non conserving quantity, which is experimentally highly
constrained!
8
Tuesday, May 11, 2010
The Neutron Electric Dipole Moment:
EDM for
is theevidence
component
of thesources of CP or T violation:
Look
of new
electric
dipole
parallel
to the spin.
electric
dipole
moments
(eg, neutron):
S
S
Experimental
Limit:
EDM isCurrent
the component
of the
−25
d
<
3.0
×
10
electric dipole d parallel to e-cm
the spin... odd under P & T
(and hence signal for P, T, CP
violation)
It is odd under P and T
Experimental limit:
Highly Constrains theta term!
d < 0.29 10−25 e-cm
S
vid B. Kaplan
day, April 23, 2009
Tuesday, May 11, 2010
Harvard Loeb Lecture
θ ≤ 10
−9
Puzzle: Why is theta so small???
April
23,CP
2009
(Strong
problem)
9
θ replaced by a dynamical field that relaxes to zero:
θ = 0 minimizes QCD vacuum energy (and makes
Soneutron
how does
the small)
axion play a role?
EDM very
Peccei-Quinn mechanism
• Promote theta to a dynamical field.
trick: a Goldstone boson for spontaneously broken
• When this field relaxes to zero, theta is very small
symmetry with a QCD anomaly will couple just like θ :
• Pecci-Quinn solution.
AXION
Y
A
C
axion
E
D
ON
decay
T
N
A
T
NS constant
plan
2
a(x) g
µν
�
G
G
µν
2
a(x)
fa g32πG2 G˜µν
fa
32π 2
µν
Harvard Loeb Lecture
a(x)
θ→
→0
fa
April 23, 2009
10
Tuesday, May 11, 2010
Axion Phenomenology/Couplings:
• Axion potential:
• Axion Mass:
V (a) ∼
−m2π fπ2 cos
ma ∼ 6 × 10
−6
eV
�
�
a
fa
�
10 GeV
fa
12
�
• Axion Couplings: 2 photons, electron, quark (model dependent)
• Lab Constraints:
fa > 10 GeV
6
Current Searches!
11
Tuesday, May 11, 2010
Outline
• Dark Matter Puzzles
• Introduction to Axions
• Current Experimental Efforts to Find the Axion (ADMX)
• Concluding Remarks
12
Tuesday, May 11, 2010
ADMX: (Axionic Dark Matter Experiment)
• Experiment designed search for cold-dark matter axions using a radio-cavity.
• The mass range ADMX is sensitive to is ~ 10^-2 ev - 10^-6 ev
• Sensitive to axion coupling to two photons via interactions with ~8 T
magnetic field.
Primakoff-Effect
13
Tuesday, May 11, 2010
Basic Idea:
gaγ
1 ∂ 2
E − E · (∇ × B) = gaγ ȧ (E · B)
2 ∂t
Idea: Using a strong B-field, convert axion energy into
electromagnetic energy through the source term above
1
Ea = ma c + ma v 2
2
2
Tuesday, May 11, 2010
14
The Resonant Cavity:
Sensitive to a frequency range 460 MhZ-810 MhZ
15
Tuesday, May 11, 2010
Preliminary Result:
Tuesday, May 11, 2010
FIG. 10: 97.7% confidence level limits for the HR 2-bin search
on the density of any local axion dark matter flow as a function of axion mass, for the DFSZ and KSVZ aγγ coupling
16
strengths. Also shown is the previous ADMX limit using the
MR channel. The HR limits assume that the flow velocity
Outline
• We know dark matter exists, but we still have not found evidence for it within
the standard model
• The axion is one possible dark matter candidate.
• Experiments like ADMX hopefully find evidence for dark matter in the near
future.
• Stay tuned!
17
Tuesday, May 11, 2010