Lawrence Livermore National Laboratory August 2, 2004
Transcript of Lawrence Livermore National Laboratory August 2, 2004
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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
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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
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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.
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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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