Searching for the AxionLeslie J RosenbergLawrence Livermore National LaboratoryAugust 2, 2004

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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.

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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.

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QCD on the lattice: CP-violating instantonsin a slice of spacetime

Peccei and Quinn:CP conserved through a hidden symmetry

This CP violation should, e.g., give a large neutron electric dipolemoment (T + CPT = CP); none is unobserved.(9 orders-of-magnitude discrepancy.)

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.

Why doesn’t the neutron havean electric dipole moment?

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Properties of the axion

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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

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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).

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Summary of laboratory searches:A heavy axion is excludedFor example: SLAC E137 (Bjorken et al.)

detector

a→γγ

20 GeV electrons

earth shield

axions produced herevia Primakoff effectlif

etim

e of

a(s

ec)

→γγ

fPQ must be considerably greater thanthe weak scale

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Summary of astrophysical bounds:The axion mass is smallExample:neutrinos from SN1987A

Log

{axi

on lu

min

osity

(erg

/sec

)}

Supernova in the LMC.Neutrinos are trapped and diffuse outover timescales of around 10 seconds.

Kamiokande and IMB together recorded19 neutrinos from SN1987A.

An axion of mass between10-3 and 2 eV would takeso much energy out that...

the length of theexplosion wouldbe observablyforshortened.

Overall summary: Astrophysics (stellar evolution and SN1987A), cosmology, and laboratory experiments leave the invisible CDM axion window 10-6 < ma < 10-3 eV (with large uncertainties)

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Bounded window of allowed axion masses

Very light axions forbidden:else too much dark matter

Heavy axions forbidden:else new pion-like particle

⇐Dark matter range:“axion window”

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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, CARRACKAstrophysical: Radiotelescope, CAST*Short-range forces*

*These experiments do not depend on detecting remnant axions

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Principle of RF cavity experiments:Axion and electromagnetic fields exchange energy

The axion-photon coupling

gaγ

is a source in Maxwell’s Equations

∂ E2 /2( )∂t 2 −E ⋅ ∇ ×B( )= gaγ Ý a E ⋅B( )

Imposing a strong external magnetic field B0 allows the axion field to pump energy into the cavity.

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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

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ADMX hardware (I)

Magnet arrivesMagnet with insert (side view)

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ADMX hardware (II)

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The axion receiver

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Sample data and candidates

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Brief outline of analysis — 100 MHz of data

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Recent exclusion limits

Particle Physics Astrophysics

These are interesting regimes ofparticle and astrophysics: realistic axioncouplings and halo densities

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The parameter space

presentexperiment

Sensitivity in the heart of the axion parameter spaceSLACSI-02aug04-ljr

Microwave amplifiers

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The world’s quietest radio receiver

Systematics-limited for signals of 10-26 W~10-3 of DFSZ axion power.

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Gigahertz SQUID amplifiers

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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)

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CARRACK: Kyoto RF cavity axion search

Their apparatus is similar to that of ADMX, excepttheir receiver is an exotic “microwave-photon phototube”

∆n ⋅ ∆φ ≥1

For any detector of electromagnetic radiation, there’s anumber-of-quanta, phase-of-radiation uncertainty relation:

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 thestandard quantum limit of phase-sensitive detectors.

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Rydberg-atom single-microwave-quantum detector

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Single-microwave-photon counting

Single-microwave-photoncounting

GHz levelspacing

sensitivity goal

Operating a 3 GHz cavity (12 µeV axion mass)with calibrations and studies of “dark” current.

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Radio telescope axion search

a

γ

γAxions in halos of astrophysical objectsspontaneously decay into photons;the lifetime is long (1050 seconds),but there are a lot of halo axions.

Synthetic axion line overlaid onpower spectrum from dwarf galaxy

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Radio telescope search:Current limits and projected sensitivities

Projected sensitivities

Limits from nearbydwarf galaxies

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CAST solar axion search

CERN Axion Solar Telescope

0 2 4 6 8 10E(keV)

0

2×1014

4×1014

6×1014

8×1014

mc2

c es

1V

e k1 Axions from the sun…

…become x-rays inside an LHC dipole magnet

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CAST technologyState-of-the-art x-ray detection borrowed from astrophysics

Micromegasx-ray camera

Grazing-incidencex-ray optics

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CAST search range

Current the bestastrophysical bounds

vary He gas pressure tomatch dispersion relation

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5th force searchesAxions mediate matter-spin couplings

ψ1

V ~ 1/r( )e−r /λ σ ⋅ ˆ r

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Ni et al. 1999

ψ2

ags iγ5gp

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Overall status

SN1987AExperiments are nowsensitive to realisticaxions in the allowedmass window

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Conclusions

A Peccei-Quinn symmetry remains a promisingsolution to the Strong CP Problem; hence axions,and axions are an attractive dark-matter candidate.

Current experiments are finally sensitive to realistic axioncouplings 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 couplingsand would either detect or rule-out Peccei-Quinn axions.

This is an exciting time for axion searchers.

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