Searching for the Axion Leslie J Rosenberg Lawrence Livermore National Laboratory August 2, 2004
Outline What is the axion? Axion properties. The window of allowed axion masses and couplings. Selected current laboratory and astrophysical searches: RF cavity experiments; Radiotelescope; Solar-axion search; 5th force. Overall status. Conclusions.
QCD is expected to have large CP violation 1973: QCD a gauge theory of color. QCD respected the observed C, P and CP conservation. 1975: QCD + instantons QCD has CP-violating interactions. QCD on the lattice: CP-violating instantons in a slice of spacetime
Peccei and Quinn: CP conserved through a hidden symmetry This CP violation should, e.g., give a large neutron electric dipole moment (T + CPT = CP); none is unobserved. (9 orders-of-magnitude discrepancy.) Why doesn t the neutron have an electric dipole moment? This leads to the Strong CP Problem : Where did QCD CP violation go? 1977: Peccei and Quinn: Posit a hidden broken U(1) symmetry 1) A new Goldstone boson (the axion); 2) Remnant axion VEV nulls QCD CP violation.
Properties of the axion
What is the dark matter? The Coma cluster of galaxies The difference between this result and Hubble s value for the average mass of a nebula must remain unexplained until further information becomes available. Zwicky and Smith 1936 They found a huge discrepancy between the visible mass and the dynamical mass. The nature of dark matter is one of the most pressing questions in science
Axions and dark matter Some properties of dark matter (from the earlier lecture): No interactions with normal matter and radiation ( dark ); Gravitational interactions ( matter ); Cold (slow-moving in the early universe); Mostly bosonic (to stuff large quantities into rich clusters). Dark matter properties are those of a low-mass axion: Low mass axions are an ideal dark matter candidate. Plus The axion mass is constrained to 1 or 2 orders-of-magnitude; Select axion couplings are constrained to 1 order-of-magnitude; The axion is doubly-well motivated it solves 2 problems (Occam s razor).
Summary of laboratory searches: A heavy axion is excluded For example: SLAC E137 (Bjorken et al.) lifetime of a γγ (sec) 20 GeV electrons earth shield axions produced here via Primakoff effect a γγ detector f PQ must be considerably greater than the weak scale
Summary of astrophysical bounds: The axion mass is small Example:neutrinos from SN1987A Log {axion luminosity (erg/sec)} Supernova in the LMC. Neutrinos are trapped and diffuse out over timescales of around 10 seconds. Kamiokande and IMB together recorded 19 neutrinos from SN1987A. An axion of mass between 10-3 and 2 ev would take so much energy out that... the length of the explosion would be observably forshortened. Overall summary: Astrophysics (stellar evolution and SN1987A), cosmology, and laboratory experiments leave the invisible CDM axion window 10-6 < m a < 10-3 ev (with large uncertainties)
Bounded window of allowed axion masses Very light axions forbidden: else too much dark matter Dark matter range: axion window Heavy axions forbidden: else new pion-like particle
Current experiments probing the axion mass window Two broad classes of experiments: 1) Detect relic (big bang left-overs) axions; 2) Produce and detect axions; this is in-general harder as there are two factors of small couplings. Selected current experiments: RF Cavity Experiments: ADMX, CARRACK Astrophysical: Radiotelescope, CAST* Short-range forces* *These experiments do not depend on detecting remnant axions
Principle of RF cavity experiments: Axion and electromagnetic fields exchange energy The axion-photon coupling g aγ is a source in Maxwell s Equations ( ) t 2 E 2 /2 E ( B)= g aγ a Ý E B ( ) Imposing a strong external magnetic field B 0 allows the axion field to pump energy into the cavity.
ADMX: Axion Dark Matter Experiment Core team: LLNL: S. Asztalos, C. Hagmann, D. Kinion, L.J Rosenberg, K. van Bibber, D. Yu Univ. Florida: L. Duffy, P. Sikivie, N.S. Sullivan, D.B. Tanner U.C. Berkeley: J. Clarke NRAO: R. Bradley
ADMX hardware (I) Magnet with insert (side view) Magnet arrives
ADMX hardware (II)
The axion receiver
Sample data and candidates
Brief outline of analysis 100 MHz of data
Recent exclusion limits Particle Physics Astrophysics These are interesting regimes of particle and astrophysics: realistic axion couplings and halo densities
The parameter space present experiment Sensitivity in the heart of the axion parameter space
Microwave amplifiers
The world s quietest radio receiver Systematics-limited for signals of 10-26 W ~10-3 of DFSZ axion power.
Gigahertz SQUID amplifiers An old idea from antenna design ( shunt detuned frequency ) applied to quantum electronics.
The target sensitivity Definitive sensitivity over lowest decade in mass (where dark matter axions would be) Plus operations into second decade of mass (where unusual axions might be)
CARRACK: Kyoto RF cavity axion search Their apparatus is similar to that of ADMX, except their receiver is an exotic microwave-photon phototube For any detector of electromagnetic radiation, there s a number-of-quanta, phase-of-radiation uncertainty relation: n φ 1 If you don t measure the electromagnetic phase φ, you can measure the number of quanta n to arbitrarily high precision. This phototube for microwave photons can evade the standard quantum limit of phase-sensitive detectors.
Rydberg-atom single-microwave-quantum detector
Single-microwave-photon counting sensitivity goal GHz level spacing Single-microwave-photon counting Operating a 3 GHz cavity (12 µev axion mass) with calibrations and studies of dark current.
Radio telescope axion search Axions in halos of astrophysical objects spontaneously decay into photons; the lifetime is long (10 50 seconds), but there are a lot of halo axions. γ a γ Synthetic axion line overlaid on power spectrum from dwarf galaxy
Radio telescope search: Current limits and projected sensitivities Projected sensitivities Limits from nearby dwarf galaxies
CAST solar axion search ek 1 c m 2 s e c 1 V 8 10 14 6 10 14 4 10 14 2 10 14 CERN Axion Solar Telescope Axions from the sun become x-rays inside an LHC dipole magnet 0 0 2 4 6 8 10 E(keV)
CAST technology State-of-the-art x-ray detection borrowed from astrophysics Grazing-incidence x-ray optics Micromegas x-ray camera
CAST search range Current the best astrophysical bounds vary He gas pressure to match dispersion relation
5 th force searches Axions mediate matter-spin couplings g s a iγ 5 g p V ~1/r ( )e r / λ σ r ˆ Ni et al. 1999 ψ 1 ψ 2 QuickTime and a TIFF (LZW) decompressor are needed to see this picture.
Overall status Experiments are now sensitive to realistic axions in the allowed mass window SN1987A
Conclusions A Peccei-Quinn symmetry remains a promising solution to the Strong CP Problem; hence axions, and axions are an attractive dark-matter candidate. Current experiments are finally sensitive to realistic axion couplings and masses; they could see an axion at any time. Upgrades are underway for definitive axion searches. These would be sensitive to even the more feeble axion couplings and would either detect or rule-out Peccei-Quinn axions. This is an exciting time for axion searchers.