An (abridged) review of axion

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An (abridged) review of axion -or axion-like
particle- searches J.I. Collar, University of
Chicago
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THE AXION
Nambu-Goldstone boson from spontaneous breaking
of PQ (Peccei-Quinn) symmetry (introduced to
explain CP conservation in QCD).
STRONG CP PROBLEM

• pseudoscalar
• and neutral (i.e., much like a p0)
• named after a laundry detergent.
• Like a p0 it can couple to two photons
• (one can be virtual)
• Excellent dark matter candidate for some mass
  ranges
• Can be copiously produced in stellar interiors
  via a variety of processes
• Mimics many g ?couplings, can substitute for one
  in nuclear and plasma processes

KEEP IN MIND
Any boson that couples to charged particles can
couple to two photons via vacuum loops
? more generic framework axion-like particles

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Rich phenomenology
Moody Wilczek, PRD 30 (84) 130
? numerous searches, have spanned 18 orders of
magnitude in ma

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Some historical searches
Telescope search for relic thermal axion decay
(a???)  Good limits in the 3.5-7.5 eV mass
range (Phys. Rev. Lett. 66 (1991) 1398.)

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Some historical searches
Photon regeneration  light shining through
walls  Ruoso et al., Z. Phys. C56 (1992) 505. A
variation of same at CERN shown here IS300 (de
Rujula et al.)

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Some historical searches
Accelerator experiments (luminosity
limited) NOMAD shown here (P. Astier et al, Phys.
Lett. B)

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Some historical searches

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Some historical searches

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Some historical searches

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Some historical searches
First step  correlate each event in detector
with solar position
time-dependence of expected signal as Bragg
angles are struck 

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Some historical searches

• Rich signature energy and Bragg angle related by
  E (keV) 12.4 / l(Å) where
• 2 d sin ?B m l as the Sun moves, the incident
  angle on atomic planes changes ? E for enhanced
  conversion varies accordingly...
• ghostly "threads" of events in E vs.
• Solar position parameter space
• expected (blue lines in figure)...

Unfortunately future limits (e.g., CUORE,
GENIUS) will increase only as exposure 1/8
(astro-ph / 9912491)  

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How to detect dark-matter axions (Sikivie, 1983)
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The parameter space is bounded
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Axion hardware
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Axion hardware (contd)
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Microwave amplifiers
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Limits on axion models
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Local axion halo density excluded
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Breakthrough Microstrip-coupled SQUIDs
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Rydberg atom single-quantum detector (Kyoto)
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Electron spectrum with selective field ionization
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Excluded gAgg vs. mA with all experimentaland
observational constraints
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Solar axions

• Solar axion flux van Bibber PRD 39 (89)
• AXION LUMINOSITY CAN BE UP TO FEW PERCENT OF
  TOTAL

Solar physics Primakoff effect Only one
unknown parameter gagg
Axion-photon conversion in the detector

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Solar axions Principle of detection
AXION PHOTON CONVERSION
axions
L
COHERENCE

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Platform magnet
Picture taken Dec 2001

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Platform magnet
March 2002 First test of platform movement with
magnet
Magnet pointing up 8º
Magnet pointing down -8º

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Platform magnet --cryogenics--
Picture taken 14th May 2002 Cryogenics
installation
He4 flexible line
Magnet Feed Box being connected to the magnet

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Platform magnet --cryogenics--
Picture taken 14th May 2002 Cryogenics
installation
Cold Box
Magnet power supply
Counting room
East-side end of the magnet (looking towards
sunset)

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Platform magnet --cryogenics--
Picture taken 14th Aug 2002 First intentional
magnet quench

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How to align a blind telescope?(present
calculated error 0.007, solar angle subtended
0.5)

• Check your astronomy
• (go real fancy)
• Enroll a team of surveyors able to send a n beam
  from CERN to LNGS (CERN/EST)
• Crush their measurements into mathematical
  submission
• Blend Fortran kernels with LabView code able to
  read encoders and control motors
• Keep fingers crossed (option for the faithless
  install 4 independent levels of safety against
  derailment)

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and be able to sleep at night?

• Ask for permission to deface CERN property (move
  fast!)
• Rig an alignment scope out of an astronomy finder
  and Taylor-Hobson sphere (saving 10k in process)
• Align it with magnet bore using arcane surveying
  wisdom
• The hardest part Wait for a sunny morning in
  Geneva (not to mention red tape involved in
  attempting to trim trees)

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Why such obsession with precision?
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• Micromegas
• Very good spatial resolution
• 350 mm X-Y strip pitch
• Extremely low threshold
• Low background (high rejection capability from
  spatial resolution)

(Athens/Saclay/CERN)
6.4 keV Calibration spectrum

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Grazing Incidence X-Ray Telescope
(see R. Giacconi, Sci. Amer. 242 (1980) 70)
CAST X-Ray Telescope X-rays from axion photon
conversion in the magnet are focused onto a 1 mm
diameter image at 1.7 m large
increase in signal/noise from reduction of
region where signal is expected
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The CAST X-Ray Telescope a spare unit from
the ABRIXAS Space Mission

• 27 nested shells
• Focal length 1.7 m
• To be fitted asymmetrically on
• CAST magnet opening

Telescope mounted on magnet (mechanical design)
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X-ray focusing device (MPI)

• X-ray focusing
• 50 mm Ø ? 2 mm Ø
• About 35 efficiency due to reflections
• IMPROVEMENT OF SIGNAL TO NOISE RATIO CAN BE UP TO
• 200
• (1 event /mo/10 keV!)
• INSTALLED DECEMBER 2002

x-y image of 6.4 KeV X-ray beam in MicroMegas
chamber (log scale for density)

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CAST experiment PROSPECTS

• Surpass for the first time astrophysical
  constrains
• Probe deep into theoretically-favored band

.
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TPC accumulated exposure
18th CAST Meeting - CERN 08.10.2003
Igor G. Irastorza
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• Analysis problem of background

Hair rising but background not stable with time
Total tracking
Total background
3rd CAST Analysis Meeting - CERN 07.10.2003
Igor G. Irastorza
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• Analysis TPC background depends strongly on
  magnet position (up to 50)

Same plot coarse version
3rd CAST Analysis Meeting - CERN 07.10.2003
Igor G. Irastorza
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• New strategy built combined background spectrum
  using appropriate weights for each cell.

• RESULTS
• The only dependence of the background appreciable
  with the present statistics is with the magnet
  position.
• It can be corrected for
• Error in background bins is appreciable and will
  limit the sensitivity of the final result.
• Careful program of background in several
  positions is needed to minimize these errors.

3rd CAST Analysis Meeting - CERN 07.10.2003
Igor G. Irastorza
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• Analysis c2 minimization 95 C.L. exclusion.

gagg(95)1.2710-10 GeV-1 (c2 39.9)
Analysis constrained to the physical region
c2 min/dof 36.0 / 28
3rd CAST Analysis Meeting - CERN 07.10.2003
Igor G. Irastorza
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CAST experiment extensions

• Axion astronomy?
• (farfetched, but we have the ability)

Looking at points on the sky which could be
potential axion sources

• Pulsars
• Magnetized stars
• Supernovae (axions responsible for dimming?)
• GRBs (BACODINE alarm)
• Galactic center
• Much more realistic Solar nuclear
• reactions and plasma processes
• UoCh detector for Ea lt160 MeV

Wang et al., Nature 415 (2001)
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A high-energy axion detector for CAST ?Goal
extend sensitivity of CAST to axion-induced
gammas from few tens of keV to 150
MeV ?Motivation If new boson couples to
nucleons, it can substitute for ?s in plasma and
nuclear processes 1. Solar luminosity via axion
emission can be as high as few of total. Search
with helioscope has not been performed
before. ? Weak experimental limits already
exist from observed solar ? flux below 5.5 MeV
(a ? ?? following p d ? He a) 2. ?
Other reactions of interest exist (e.g., 2.2
MeV from p n ? d a, 511 keV from e e- ? a
?, 477 keV from 7Bee-?7Li?e 3, etc.) ? A
generic search should not be limited to M1
transitions 4. Should surpass sensitivity of
searches for anomalous production of single ?s
in accelerators 5. May surpass sensitivity to
small branching ratios (lt10-5-10-6) in laboratory
searches 6. ?Due to space and weight
limitations in CAST platform, it must be
extremely compact and non-intrusive, yet reach
the highest efficiency and lowest possible
sea-level background ? Careful design and
selection of detector and shielding materials ?
Use of Pulse-shape particle identification in
lieu of additional shielding 1 G. Raffelt,
"Stars as laboratories for fundamental physics",
University of Chicago Press, Chicago and London
(1996). 2 G. Raffelt and L. Stodolsky, Phys.
Lett. B119, 323 (1982). 3 M. Krcmar et al.,
Phys. Rev. Lett. (hep-ex/0104035) 4 G. Raffelt,
Priv. Comm.. 5 C. Hearty et al., Phys. Rev. D
39(1989)3207. 6 A. V. Derbin et al., Phys. At.
Nucl. 65 (2002)1335 M. Minowa Phys. Rev. Lett.
71(1993)4120.
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Detector installation Mar. 2004 (support platform
design allows alignment and access to other
detectors)
Exact Micromegas cross-section determines
low-energy threshold (300 keV expected)
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DAQ XIA Polaris DSPEC digital spectrometer (shoeb
ox-sized package offers event-by-event waveform
capture for pulse-shape discrimination (PSD),
muon veto input and all power supplies)
Minimalist detector and DAQ (space between mMs
and CCD very limited, also must not add to
platform burden. Must be sturdy to withstand
frequent activity in its vicinity)
Front View
4p Plastic Muon Veto (doubles as protection)
Borated thermal-n absorber (3 mm)
Pb shielding
Thin brass endcap (Tedlar seals against Rn)
thermocouple (gain monitoring)
Ultra-low bckg Pb (lt0.02 Bq 210Pb/kg)
collimated axion-induced gs from magnet bore
PMT
CWO Crystal
light guide
long tunnel design reduces bckg from open end by
reducing solid angle (good alignment
essential to accept 100 of signal)
Low-bckg PMT (Electron tubes, lt 20 40K gamma /
day)
Fiber optic and LED (to pulser, for dead time
monitoring)
protected PMT low-voltage (5V) power bases
brass support canister
Rn displacement (N2 purge gas)
Side View (total length 60 cm, weight 25 kg)
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Decisions, decisions Monte Carlo of inorganic
crystal response reduces realistic choices to BGO
or CWO (PWO has too low a light yield) Choice of
optimal crystal length and radius via Monte Carlo
of collimated signal and isotropic backgrounds.
Crystal must be well-aligned with magnet bore
(only slightly larger than it).
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Crystal selection in EFI low-background lab (6-60
m.w.e)
REJECTED
Comparison of standard BGO and CAST CWO (muon
veto, low Pb-210 shielding, 6 m.w.e. depth)
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CWO has the radiopurity, efficiency and
excellent PSD. Calibrations with different sources
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PSD can reject non gamma-like backgrounds while
keeping shielding to a minimum
Achieved PSD surpasses that of CWOs used in
bb-decay searches (T. Fazzini et al. NIM
410(98)213 F.A. Danevich et al. PLB
344(95)72 F.A. Danevich et al. nucl-ex/0003001)
CAST CWO
98.8 g signal acceptance for 95 a-background
rejection
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Also at UoC new search for a monochromatic solar
axion line(in collab. with F.T. Avignone (USC)
and W. Haxton (UW))
Use isotopically enriched Kr-83 (9.4 keV M1
level) as target (Kr-85 is a bad actor) Use of
Co-57 source to mimic complex axion signature
Kr-83 a 5.0keV ?(147 ns delay) ? 9.4
keVCo-57 6.4keV ?(107 ns delay) ? 14.4 keV
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Also at UoC new search for a monochromatic solar
axion line(in collab. with F.T. Avignone (USC)
and W. Haxton (UW))
A zero-background experiment? (results expected
end of 2004)