Experimental
searches for axion
like particles
Gentner day
10/2011, CERN, Geneva
M. Betz
M. Gasior
F. Caspers
M. Thumm
(CERN, Geneva)
(CERN, Geneva)
(CERN, Geneva)
(KIT, Karlsruhe)
Outline
What this talk will be about
• Introduction to Axions
• Existing experimental searches around the
world
• The “microwaves shining through the wall”
experiment at CERN
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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What is an axion?
Introduction
• A hypothetical elementary particle
• Postulated by R. Peccei, H. Quinn, S. Weinberg
and F. Wilczek in 1977 – 1978 to explain the
strong CP-violation
• A candidate for dark matter in our universe
• Also a washing detergent
Some properties
Charge:
None
Mass:
10-6 … > 100 eV/c²
Mean lifetime:
1017 years
No interaction with matter!
M. Betz; Experimental searches for axion like particles,
Geneva 2011
3
What is an axion?
The strong CP problem
• The theory of quantum chromodynamics (QCD) is
explicitly CP-violating if one of its parameters θ>0
• θ was expected to be of order 1
 Experimental verification
QCD neutrons should have an
electrical dipole moment in the
order of
|dN| ≈ θ 10-16 e cm
 The result was puzzling
Current experimental limit:
|dN| < 10-27 e cm
Puzzling questions for QCD-physicists:
• Why is the parameter θ so small? (Fine tuning problem!)
• Why is there apparently no CP-violation?
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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What is an axion?
A solution to the strong CP problem
• What if θ is a dynamical variable?
• It would oscillate around zero like a
pendulum
• This would eliminate CP violating terms
from the QCD-Lagrangian
• The oscillations can be seen as new
particle  The axion
• So far the most elegant and widely
accepted solution to the strong CPproblem
• For theoretical physics:
Problem solved!
• But in experimental physics:
No observation of the axion yet
From: Fermilab Seminar Ultrasensitive Searches for
the Axion Karl van Bibber, LLNL January 30, 2008
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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What is an axion?
Also a candidate for dark matter
Dark matter
(unknown identity),
23%
Dark energy
(unknown identity),
73%
Matter made from
particles we know,
4%
Some puzzling question for astrophysicists:
• Why do clusters of galaxies rotate faster
on their outskirts than they should?
• Why does the cosmic microwave
background radiation appear to be
distorted?
• Why is the gravitational lensing effect
stronger than predicted?
All of those points could be explained by
assuming there is more matter and energy in our
universe than we can see
But, what is this dark matter made of?
Axions are excellent candidates
for dark matter
Note that axions could exist, even if the
dark matter theory would be disproven
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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The Primakoff Effect
Axions couple to photons in a strong magnetic field
γ can be a photon
with energies
between μeV
(microwave photon)
and up to keV and
beyond
(gamma quantum)
 * is representing the virtual
photons of the magneto-static field
From: Fermilab Seminar Ultrasensitive Searches for the Axion Karl van
Bibber, LLNL January 30, 2008
a = axion
All current
experimental
searches are
based on this
effect
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Experimental searches around the world
Overview
Polarization
Looks for changes in light polarization of a laser
beam in a strong magnetic field
Helioscopes
Looks for axions generated in the sun and sent to
earth
Haloscopes
Looks for dark matter axions, uniformly
distributed in our galaxy
Experimental
searches for the
axion
Light
shining
trough
the wall
Looks for photon axion photon conversions in a
strong magnetic field
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Laser polarization experiments
PVLAS (Istituto Nazionale di Fisica Nucleare, Padova, Italy)
The expected effect is tiny
rotation of 3.9 · 10-12 rad
≈ width of mechanical pencil lead
at the distance of the Moon
• Linear polarized laser
beam transverses strong
magnetic field
• The component parallel to
the magnetic field is
converted to hidden
particles (primakoff effect)
 selective absorption
• The polarization is rotated
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Laser polarization experiments
PVLAS (Istituto Nazionale di Fisica Nucleare, Padova, Italy)
•
In 2006 the PVLAS
collaboration published
their results
•
They claimed to have
observed the effect they
were looking for
•
After an update of the
detector, the results could
not be confirmed
Nonetheless the publication in
2006 triggered world wide
interest and inspired many new
experimental activities
http://physicsworld.com/cws/article/news/30423
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Axion helioscopes
The CERN Axion Solar Telescope (CAST)
Magnetic field
converts axions to
X-ray photons
Magnetic field
converts photons to
axions inside the sun
•
•
•
•
•
Prototype LHC magnet, 10 m long, 9 Tesla on a movable platform
Tracks the sun for 3h / day, 50 days / year
X-ray focusing system (prototype from the space based X-ray telescope ABRIXAS)
X-ray detectors (micromegas, CCD) at both ends of the magnet
Has been running since 2003 and is now waiting for an upgrade in 2012
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Axion helioscopes
The Dark Matter eXperiment (ADMX) in Washington
•
•
•
•
•
Assumes: Axions are dark matter, a
relic from the big bang and already
all around us
8 T Magnet converts relic axions to
microwave photons
Tunable cavity 460 – 810 MHz to
“collect” those photons
SQUID amplifier, system noise
temperature TN = 2.5 K, one of the
quietest microwave receivers in the
world
Running since 2006 (at LLNL),
moved to University of Washington
in 2010, upgrade of cryo system this
year
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Laser LSW experiments
LSW = Light shining through the wall
• Some photons
convert to axions
(emitting side)
• axions can pass the
wall
• Some axions
convert back to
photons (detection
side)
• It seems like light is
shining through
the wall!
• Fabry-Perot
cavities allow to
enhance the
probability:
photons make
many passes
1020 photons/s
photons
axions
< 1 photon/s
photons
(Optical resonator cavities)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Laser LSW
A lot of activity around the world
ALPS at DESY (Germany)
GRIM REPR at Fermilab (USA)
OSQUAR at CERN (next door)
XAX at ESRF (France)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Experimental searches around the world
Results so far: No axion has been observed yet
Laser polarization
Laser LSW
Sensitivity
(ADMX)
Mass
Towards a new generation axion helioscope, Igor G Irastorza
7th Patras Workshop on Axions, WIMPs and WISPs
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Microwaves shining through the wall
Cavities become coupled through axions
γ
a
EM.
Photon
Axion
Electromagnetic
Why microwaves resonators?
• High Q-factors around 105
(low loss) are easily achieved
• Easier construction /
alignment
• Homodyne detection
methods can be applied (very
sensitive)
• Instruments and know-how
exists
But:
• The “wall” becomes a faraday
cage  EMI shielding
challenge
M. Betz; Experimental searches for axion like particles,
Geneva 2011
16
The photon conversion cavities
Prototypes after machining (left) and coating (right)
Material: Brass (non magnetic)
Fine thread tuning screw Coupler (β=1)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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The photon conversion cavities
Numerical simulation of the TE011 mode
Tuning screw:
Possible
location of
an inductive
coupling
loop for the
TE011 mode
(The loop
extends on
the XYplane)
(20 mm diameter, fine thread)
TE011 mode, H–field on YZ-plane
TE011 mode, E–field in X-direction
TE011 mode, E–field on XY-plane
M. Betz; Experimental searches for axion like particles,
Geneva 2011
18
Electromagnetic shielding
Splitting the experiment into two parts
Environmental
RF noise
Shielding
Box 1
(Cryo.)
 Experiment is split into a
cryogenic and room
temperature part
Shielding Box 1
Contains the Axion detection cavity and will later be
placed in the cryostat / magnet
Optical Fibre
Carries the weak signal from Axion conversion to the
measurement instruments, unaffected by ambient EM.
noise and without comprising the shielding boxes
Shielding Box 2
Contains instruments for the detection of weak
narrowband microwave signals and will be outside the
cryostat / magnet
Electric /
optical
converter
Optical /
electric
converter
Shielding Box 2
(Room temp.)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
19
Electromagnetic shielding
Some practical aspects
Optical power
converter
VCC
High power
Laser diode
• EM absorbing material
between shielding layers
(non magnetic!)
• Chain of lowpass
feedtrough filters for
supply voltage
If we still see leakage:
• Power over optical fibre
– Commercial systems
available (JDSU Photonic
power module)
– Efficiency 50 %
(optical  electric)
• We can always add
another layer of shielding
M. Betz; Experimental searches for axion like particles,
Geneva 2011
20
DC – feedtrough filters
For feeding DC power through the shielding while keeping RF out
Syfer SFJNC2000684MX1
Measurement with a network analyser in transmission
- 95 dB at 3 GHz
M. Betz; Experimental searches for axion like particles,
Geneva 2011
21
Electromagnetic shielding
Shielding box 1 prototype, containing the receiving cavity
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Debugging of the faraday cage
The current status in the laboratory
E.M. leakage
test setup
•
Phase locked RF – Source (3 GHz)
•
Optical receiver for 10 MHz phase lock
signal
•
50 W RF power amplifier
•
Custom made EMI - feed trough filter
for AC power
•
Faraday cage, containing detection part
•
Fibre optical converter for control
signals
•
Multimeter for tuning the cavity
•
Emitting cavity
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Electromagnetic shielding
Shielding box 2 prototype, containing the instrumentation
E.M. leakage
test setup
• Feedtrough
for optical
fibres
• Receiving
cavity
• Spectrum
analyzer
• Low noise
amplifier
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Online diagnostics
Supervising the shielding attenuation with test tones
We need ONLINE diagnostics showing,
that the shielding performance is really
maintained over the full lifetime of the
experiment. Degradation is possible due
to bad and ageing contacts
Test tones (TXn)
 Low power (μW) probe signals
 Injected in laboratory space and
between shielding layers
 Each one has a slightly different
frequency within the cavity
bandwidth
 Monitoring signal power (RXn)
allows to quantify the
attenuation of each shielding
layer
If dynamic range of the receivers is not sufficient, time
multiplexing is an option.
(Sender and receiver in the same shielding shell are
not enabled at the same time)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Online diagnostics
Possible signal-paths
Shieldingbox
• All possible signal paths are represented as
arrows
• Green signals pass one shielding layer and can
be used to quantify its attenuation
• Red signals pass more than one shielding
layer. Observation of a red signal = veto
condition on Axion detection
Attenuation of the
Shieldingbox is measured
twice, giving us redundancy
M. Betz; Experimental searches for axion like particles,
Geneva 2011
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Detecting weak narrowband signals
Homodyne detection with an commercial vector signal analyser
Vector signal analyser (Agilent N9010A EXA)
Common
reference clock
• To detect signals down to -230 dBm we need resolution bandwidths in
the 10 μHz range
• This can be achieved with a FFT on a 24 h time trace
• Frequency drifts are unavoidable!
• But by phase locking source and analyzer we can eliminate relative
frequency errors
M. Betz; Experimental searches for axion like particles,
Geneva 2011
27
Photon regeneration exp. at CERN
Technical specifications and challenges for hidden photon search
Expected signal power from
the receiving cavity
What we want to achieve (for HSPs):
Pem
50 W = 47 dBm
Pdet
10-26 W = -230 dBm
Signal power from receiving cavity
Q
23 000
Quality factor emitting cavity
Q‘
23 000
Quality factor receiving cavity
G
300 dB
Signal power into emitting cavity
≈ 0.5
HSP. geometry factor
mγ’
12 μeV ≈ 3 GHz
Hidden photon mass
ω0
3 GHz
Χ
arXiv:0707.2063v1
F. Caspers, J. Jaeckel, A. Ringwald, A Cavity Experiment to Search for Hidden Sector Photons
1.1 · 10-9
Cavity resonance frequency
Coupling factor (exclusion limit)
M. Betz; Experimental searches for axion like particles,
Geneva 2011
28
Acknowledgements
• The author would like to thank the CERN BE and BI-dept.
management for support as well as R. Jones and R. Heuer for
encouragement
• Many thanks to A. Ringwald, A. Lindner and J. Jäckel for a
large number of hints as well as and K. Zioutas for having
brought the right people in the right moment together as well
as haven given very helpful comments
M. Betz; Experimental searches for axion like particles,
Geneva 2011
29