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

“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

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!

“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!

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

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

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

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

The CAST Experiment

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?!

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.)

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

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

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

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

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

Pulse shape discrimination

Pulse shape discrimination

Pulse shape discrimination ~50% reduction

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

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)

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

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

Final results from the CAST calorimeter Alternatively, can use CAST X-ray limit on gaγγ (PRL 94, 121301, 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