1. David W. Miller APS Apker Award 8 September, 2005
Search for high-energy axions with the CERN Axion Solar Telescope (CAST) calorimeter
David W. Miller
APS Apker Award
8 September, 2005

2. “Who ordered these?” New Particles (?) Theor. / Exp. Motivation
Higgs boson
Mass mechanism
Weakly Interacting Massive Particles (WIMPs)
Galaxy rotations / formation, CMB
Superparticles
(squarks, sleptons, …)
Hierarchy problem
Axions
CP conservation in QCD

3. Already ordered and delivered…
Old “New” Particles
Theor. / Exp. Motivation
Neutron
Nuclear scattering / atomic binding
Neutrino
β-decay spectrum
Vector bosons (W±, Z)
Electroweak charge conservation
Top quark
QCD prediction of 6 quarks
Muon
NONE!

4. “Who ordered these?” New Particles (?) Theor. / Exp. Motivation
Higgs boson
Electroweak symmetry breaking
Weakly Interacting Massive Particles (WIMPs)
Galaxy rotations / formation, CMB
Superparticles
(squarks, sleptons, …)
Hierarchy problem
Axions
CP conservation in QCD
+ Dark Matter
+ GUT models!

5. The story of the axion
A zero neutron electric dipole moment implies lack of CP-violation in QCD (has been measured)
This anomalous result needs a cause, since there is NO reason NOT to have CP-violation in QCD
Roberto Peccei & Helen Quinn proposed a symmetry which gives an origin for the lack of CP-violation in QCD
Wilczek and Weinberg then noticed this symmetry leads to a new pseudoscalar boson: the AXION (named after a laundry detergent)
“One needed a particle to clean up a problem…”
-- Frank Wilczek

6. The Strong-CP Problem… and the Peccei-Quinn Solution

7. These theoretical suggestions have experimental consequences
Axion Phenomenology
These theoretical suggestions have experimental consequences
This new particle can interact with photons
Can even substitute for photons in certain situations
Photon coupling
In a B-field, the axion can convert into a real photon (Primakoff effect), & vice-versa
Can use stellar plasma fields
Nuclear transitions
Axions can be emitted during certain nuclear transitions instead of γ’s
Many stellar nuclear processes

8. Axion production and detection
Use the sun as an active source of both plasma fields and nuclear processes to produce axions
Convert solar axions into detectable photons via Primakoff effect in a laboratory magnetic field

9. The CAST Experiment

10. The CAST Experiment
The Sun as a source of plasma EM fields to convert solar photons to axions
A long (L = 10m) and powerful (B = 10T) LHC dipole magnetic field as the axion-photon conversion medium
Low-background X-ray detectors to search for excess signal above background
BUT!
Since Sun is also a good source of nuclear processes that can produce axions, why not include a gamma-ray detector as well?!

11. The CAST gamma-ray calorimeter
Motivation
A new axion(-like) particle might be emitted in nuclear reactions within the sun
Such particles should convert into real (detectable) photons in the right situations
Goal
Maximize sensitivity to high energy (MeV) axion signal via axion-γ conversions in laboratory magnetic field
Search for other possible new pseudoscalar bosons
Maintain minimalist design due to CAST constraints
Set limits on axion couplings and mass through solar model constraints (avoid problematic nuclear matrix calcs.)

12. Calorimeter installation on LHC magnet platform
MicroMegas X-ray Detector
X-ray Telescope
adjustable platform
for alignment
Chicago calorimeter
Magnet Platform

13. Calorimeter Design and Properties
Large inorganic scintillating crystal (CdWO4)
Low intrinsic background, high g efficiency
Low-background photomultiplier tube (PMT)
Pulse shape discrimination
Env. radon displacement
Plastic scintillator as a 4π active muon veto
Borated thermal neutron absorber
Sub-200 keV threshold
200 MeV dynamic range

14. γ’s Front View Side View Plastic Muon Veto Muon veto PMT Pb shielding
Ultra-low bckg Pb
Incoming gammas
(from magnet bore)
γ’s
CWO
Crystal
light
guide
Characteristic pulse
Thermocouple
placement
50μs
rate~4 Hz
Low-bckg PMT
Brass support tube
Side View

15. Calorimeter Data Processing
Waveform acquisition with digital spectrometer
Rejection of ~95% muon induced events in coincidence with active muon veto
but waveforms preserved for crosschecks: correct pulse shapes for muon events, event rate, etc.
Livetime calculation using LED pulser (~93%)
Pulse Shape Discrimination of spurious PMT events, pulser events, a’s, neutron recoils
Final data sets (background and signal) must account for systematic detector effects
Gain shifted to correct for energy fluctuations
Spectral position dependence

16. Software cuts Use γ calibrations to determine software cuts
Keep 99.7%!!!!!!
Set cuts for:
Energy
Shape of Pulse
PID = pulse identification parameter
Pulse rise time

17. Pulse shape discrimination

18. Pulse shape discrimination

19. Pulse shape discrimination
~50% reduction

20. Compare solar alignment (tracking) spectrum with background
Solar tracking and background energy spectra for the CAST calorimeter
Search for axion signal → look at residual!

21. Look for excess signal buried in data
mono-energetic peaks at low energies
structured energy deposition at high energies
Obtain 95% CL (2σ) allowed counts at each energy
95% CL peak
Best fit (signal)
Best fit (bckg)
Best fit (sig+bckg)

22. From “counts” to “photons”
Use the 95% CL allowed counts at each energy and convolve with
Detector efficiency (ε)
livetime (t)

23. From “photons” to “axions”
Combine:
95% CL allowed photon flux: Φγ
conversion probability: Pa→γ (a constant dep. on mass, after sep. gaγγ)
helioseismology upper limit on axion flux: Φa
Obtain limit on axion-photon coupling: gaγγ
Can fix either quantity to
obtain limits on the other

24. Final results from the CAST calorimeter
Alternatively, can use CAST X-ray limit on gaγγ (PRL 94, , 2005) to set upper limit on solar-axion flux Φa
Use the helioseismology limit on Φa (0.2L) to set upper limit on the axion-photon coupling gaγγ