Searching for Axions A Symposium in Honor of Helen Quinn SLAC April 16, 2010 Leslie J Rosenberg University of Washington.

Report
Searching for
Axions
A Symposium in
Honor of Helen Quinn
SLAC
April 16, 2010
Leslie J Rosenberg
University of Washington
Searching for Axions
Outline
Recap axion properties (from Roberto’s talk)
Selected axion searches
(not enough time to talk about them all)
5th force searches
Photon regeneration and optical rotation
Solar axion searches
RF cavity search for dark-matter axions
Overall status of axion hunting
Axions and axion-like particles (from Roberto’s talk)
You could imagine many kinds of scalars and pseudoscalars, e.g.,
Majoron (from lepton-number breaking…neutrino masses)
Familon (from family-symmetry breaking)
Dilaton (low-energy state in string theory)
Axion (removes CP violation in strong interactions)
Axions are well-motivated and their
phenomenology is well-understood
QCD and CP violation and axions: a very brief history
1973: QCD…a gauge theory of color.
QCD theory respected the observed conservation of C, P and CP.
1975: QCD + “instantons”  QCD is expected to be hugely CP-violating.
“The Strong-CP Problem”
QCD on the lattice:
CP-violating instantons in a slice of spacetime
(sort of)
Invention & Properties of the axion
Helen and Roberto pondered the strong
CP problem.
Hmmm. Add in a continuous symmetry,
spontaneously broken. One of those new
terms containing the vev cancels terms
responsible for CP violation (the PecceiQuinn mechanism). Voila.
And as we all know, a spontaneously
broken continuous symmetry has a
boson: this is the axion.
Simply: The axion is a light pseudoscalar
resulting from the broken “Peccei-Quinn”
symmetry to enforce Strong CP
conservation
fa, the SSB scale the PQ symmetry
breaking, is the one important
parameter of the theory.
Axions and dark matter
“…I'm much more optimistic about the dark matter problem. Here we have the unusual situation
that two good ideas exist – which, according to William of Occam (the razor guy), is one too many.
“The symmetry of the standard model can be enhanced, and some of its aesthetic shortcomings
can be overcome, if we extend it to a larger theory. Two proposed extensions, logically
independent of one another, are particularly specific and compelling.
“One incorporates a symmetry suggested by Roberto
Peccei and Helen Quinn in 1977. Peccei-Quinn symmetry
rounds out the logical structure of quantum
chromodynamics by removing QCD's potential to support
strong violation of time-reversal symmetry, which is not
observed. This extension predicts the existence of a
remarkable new kind of very light, feebly interacting
particle: the axion. …
This axion can be very useful in unexpected places
It was introduced to solve the ‘strong CP problem’ in particle physics
but shows up elsewhere.
One example: Axions may account for large number of spiral galaxies.
(M51) Rector & Ramirez/NOAO
(HDF) Harry Furguson/STSci
The axion’s contribution to particle, nuclear & astrophysics reminds me of
viewing a piontillist painting; when all the bits come into focus, it’s breathtaking.
Back to axion masses and couplings
10–8
The axion is a
ga (GeV–1)
light cousin of
10–10
0:
J= 0–

Horizontal Branch
Star limit
a
10–12
a > 1
Sn1987a
10–14
ma , gaii  fa–1  ga  ma
a  fa7/6  ma > 1 eV
Axion
models
10–16
10–6
10–4 m (eV) 10–2
a
100
Good news – Parameter space is bounded
Bad news – All couplings are extraordinarily weak
Sn1987a  pulse precludes
NNNNa for ma~10–(3–0) eV
Red giant evolution precludes
ga > 10–10 GeV–1
5th force searches for distances less than 100 m
Axions mediate matter-spin couplings
gs
1
a
i5gp
 2
V ~ 1/rer /    rˆ
The special role of axion-photon mixing in sensitive searches
Lint  aga E  B

Laboratory
(“laser”)
Dark matter
Solar
P. Sikivie, PRL 51, 1415 (1983)
See Raffelt & Stodolsky for general treatment of axion-photon mixing – PRD 37, 1237 (1988)
A class of search: Vacuum birefringence & dichroism
Vacuum dichroism
 ~ N  (1/4 gB0L)2

(N = number of passes)
FabryPerot
B0
Laser
Magnet

l
+
Vacuum birefringence
 = N ·(1/96)·(g B0ma)2·L3/
Maiani, Zavattini, Petronzio, Phys. Lett. B 175 (1986) 359
Example: The PVLAS experiment (INFN Legnaro)
E. Zavattini et al., PRL 96 (2006) 110406
Y.Semertzidis et al.,
PRL 64 (1990) 2998
M = 1/ga


Recent PVLAS details & data
PVLAS Schematic
Phase-Amplitude Plot
Rebuilt detector didn’t find signal.
Their early value of ga was ostensibly excluded already by 4 orders of magnitude, by solar searches,
and stellar evolution (stars would live only a few thousand years)
This renewed polarization-rotatation experiments
around the world, and much theoretical work
Photon regeneration (“shining light through walls”)

B0
B0
Photon
Detector
a
Magnet
Magnet
L
L
P(a) ~ 1/16 (gB0L)4
ga (GeV-1)
Laser
Wall
Early measurement:
g < 6.7 x 10-7 GeV-1 for ma < 1 meV
G. Ruoso et al., Z. Phys. C. 56, 505 (1992) &
R. Cameron et al., Phys. Rev. D47, 3707 (1993)
ma (eV)
Several photon regeneration efforts around the world
Experiments in various phases of prepation or operation
CERN Courier, Vol. 47 No. 2 (March 2007)
All of them would still be orders of magnitude away from solar & red-giant limits
Resonantly enhanced photon regeneration
Basic concept – encompass the production and regeneration magnet
regions with Fabry-Perot optical cavities, actively locked in frequency
Sikivie et al. PRL (April 27, 2007)
Laser
Photon
Detectors
IO
Magnet
Magnet
Matched Fabry-Perots
P Re sonant (  a   ) 
2
 
 P Simple (  a   ) 
2
2
FF  P Simple (  a   )
where ’ are the mirror transmissivities & F, F’ are the finesses of the cavities
For  ~ 10(5-6), the gain in rate is of order 10(10-12)
and the limit in ga improves by 10(2.5–3)
Solar axion search

a
Produced by a Primakoff interaction, with a
mean energy of 4.2 keV

Ze
solar-axion spectrum
Flux
[1010 ma(eV)2
Tcentral = 1.3 keV, but plasma screening
suppresses low energy part of spectrum
(says G.Raffelt)
cm-2 sec-1 keV-1 ]
16
The total flux (for KSVZ axions) at the
Earth is given by
a  7.44 1011cm2 sec1(ma /1eV)2

0
E [ keV ]
10
The dominant contribution is confined to
the central 20% of the Sun’s radius
Principle of the solar-axion search experiment
Photon
Detector
B0
a
Magnet
l

a
B
x
1
2
2
(a   )  (ga B0L) F(q)
4
where
Sin(qL /2)
,
F(q) 
(qL /2)
F(0) 1
and




q  k  ka  ma /2
2
Example: The CERN Axion Solar Telescope (CAST)
a

Prototype LHC dipole magnet, double bore, 50 tons, L~10m, B~10T
Tracks the Sun for 1.5 hours at dawn & 1.5 hours at dusk
Instrumented with: CCD with x-ray lens; Micromegas; TPC
CAST results and future
CAST has published results
equaling the Horizontal Branch Star
limit (Red Giant evolution)
They are pushing the mass limit up
into the region of axion models, 0.11 eV
CAST JCAP
Plan: Fill the magnet bore with gas
(e.g. helium), and tune the pressure
When the plasma frequency equals
the axion mass, full coherence and
conversion probability are restored:
K. Zioutas et al., Phys. Rev. Lett. 94, 121301 (2005)
 p  (4Ne /me )1/ 2  m
KvB, P. McIntyre, D. Morris, G. Raffelt PRD 39 (1989) 2085
They will go to higher ma with 3He, and a second x-ray optic

RF cavity axion-search experiments:
Axion and electromagnetic fields exchange energy
The axion-photon coupling…
ga
…is a source in Maxwell’s Equations
E2 /2
 E    B  ga aÝE  B
t
 So imposing a strong external magnetic field B
allows the axion field to pump energy into the
cavity.
RF cavity: How to detect dark-matter axions
Important to lower Ts
ADMX: Axion Dark-Matter eXperiment
U of Washington, LLNL, University of Florida, UC Berkeley,
National Radio Astronomy Observatory/University of Virginia
Magnet with insert (side view)
Magnet arrives
ADMX hardware
high-Q cavity
experiment insert
The axion receiver
The world’s quietest radio receiver
Systematics-limited for signals of 10-26 W
~10-3 of DFSZ axion power (1/100 yoctoWatt).
Recent published data
Particle Physics
Ap.J
Astrophysics
These are interesting regimes of particle and astrophysics:
probe realistic axion couplings and halo densities
Better sensitivity (lower Ts): SQUID Amplifiers
IB

The basic SQUID amplifier is a fluxto-voltage transducer
Vo (t)
SQUID noise arises from Nyquist
noise in shunt resistance
scales linearly with T
However, SQUIDs of conventional
design are poor amplifiers above
100 MHz (parasitic couplings).
Flux-bias to here
Noise Temperature (mK)
Quantum-limited gigahertz SQUID amplifiers
4
SQUID A2-5, f = 684 MHz
SQUID L1-3, f = 642 MHz
SQUID K4-2, f = 702 MHz
2
Semiconductor
1000
6
4
TQL
2
100
Clarke and Kinion
6
4
quantum limit
2
2
4
6 8
100
2
4
6 8
1000
2
4
An old idea from antenna design
Physical Temperature (mK)
(“shunt detuned frequency”)
applied to quantum electronics.
SQUID commissioning
calibration
RF-cavity experiment target sensitivity
“Definitive” sensitivity over lowest decade in mass
(where dark matter axions would likely be)
Plus operations into second decade of mass
(where unusual axions might be)
Overall status of axion hunting
CAST
SN1987A
ADMX Upgrade
Conclusions
There are lots of axion searches out there,
it’s the wild west; most search
for “unusual” axion variants.
If axions are the “usual” Peccei-Quinn type (“QCD axion”)
then ADMX will either find it or exclude it at high
confidence. This effort has a 5-year horizon.
Of course, there are escape hatches:
Are anthropic arguments are correct?
Will experiments show WIMPs saturate the local DM halo?
Axion or not, the ‘strong CP problem’ is there; it may even be a
huge problem (for believers in SUSY).
There must be some reason CP violation is suppressed, and I’m
guessing Helen and her cohorts got it right. Thank you Helen.

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