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Hunting for Chameleons
Amanda Weltman
Centre for Theoretical Cosmology
University of Cambridge
Moriond 2008
astro-ph/0309300 PRL J. Khoury and A.W
astro-ph/0309411 PRD J. Khoury and A.W
astro-ph/0408415 PRD P. Brax, C. van de Bruck, J.Khoury, A. Davis and A.W
hep-ph in progress A. Chou, J. Steffen, W. Wester, A. Uphadye and A.W
astro-ph in progress A. Uphadye and A. W
Plan
• Motivation - Theoretical + Observational
See Mota talk
• Chameleon idea and thin shell effect
See Mota talk
• Predictions for tests in space
• Dark Energy Candidate
See Mota talk
• Quantum vacuum polarisation experiments
• The GammeV experiment and Chameleons
We can learn about fundamental physics using low
energy and low cost techniques.
Motivation
• Massless scalar fields are abundant in String and SUGRA
theories
• Massless fields generally couple directly to matter with
gravitational strength
• Unacceptably large Equivalence Principle violations
• Coupling constants can vary
• Masses of elementary particles can vary
Light scalar field

+
Gravitational strength coupling
Tension
between
theory and
observations
Opportunity!
- Connect
to Cosmology
Solutions?
1. Suppress the coupling strength :
• String loop effects Damour & Polyakov
• Approximate global symmetry Carroll
2. Field acquires mass due to some mechanism :
• Invoke a potential
• Chameleon Mechanism Khoury & A.W
• Flux Compactification KKLT
• Special points in moduli space - new d.o.f become
light Greene, Judes, Levin, Watson & A.W
Chameleon Effect
Mass of scalar field depends on local matter density
In region of high density  mass is large  EP viol suppressed
In solar system  density much lower  fields essentially free
On cosmological scales  density very low  m ~ H0
Field may be a candidate for acc of universe
Ingredients
Reduced Planck Mass
Coupling to photons
Matter Fields
Einstein Frame Metric
Conformally Coupled
Potential is of the runaway form
Effective Potential
Equation of motion :
Dynamics governed by
Effective potential :
Energy density in the
ith form of matter
Predictions for Tests in Space
New Feature !!
Different behaviour in space
Tests for UFF
Eöt-Wash Bound  < 10-13
Near- future experiments
in space :
STEP
GG
MICROSCOPE
 ~ 10-18
 ~ 10-17
 ~ 10-15
We predict
SEE Capsule
10-15 <RE/RE < 10-7
Corrections of O(1) to Newton’s Constant
Strong Coupling
Strong coupling not ruled out by local experiments! Mota and Shaw
Thin shell suppression

Remember :

Effective coupling is independent of !!
If an object satisfies thin shell condition - the  force is  independent
Lab experiments are compatible with large  - strong coupling!
 >> 1  more likely to satisfy thin shell condition
 Thin shell possible in space  suppress signal
Strong coupling is not ideal for space tests - loophole
Coupling to Photons
Introduces a new mass scale :
Effective potential :
We can probe this term in quantum vacuum experiments
• Use a magnetic field to disturb the vacuum
• Probe the disturbance with photons
• Expect small birefringence
• Polarisation : Linear
elliptical
PVLAS
(Polarizzazione del Vuoto con LASer)
To explain unexpected birefringence and dichroism results
requires
and
(g = 1/M)
Conflicts with astrophysical bounds e.g. CAST (solar cooling)

But
+
Too heavy to produce  CAST bounds easily satisfied
Chameleons - naturally evade CAST bounds and explain PVLAS
Davis, Brax, van de Bruck
GammeV
A. Chou, J. Steffen, A. Uphadye, A.W. and W. Wester
“ [Photon]-[dilaton-like chameleon particle] regeneration using a
"particle trapped in a jar" technique “ - http://gammev.fnal.gov
Idea :
• Send a laser through a magnetic field
• Photons turn into chameleons via F2 coupling
• Turn of the laser
• Chameleons turn back into photons
• Observe the afterglow
Failing which - at least rule out chunks of parameter space!
See also - Gies et. Al. + Ahlers et. Al.
Alps at DESY, LIPSS at JLab, OSQAR at CERN, BMV, PVLAS
GammeV
Nd:YAG laser at 532nm, 5ns wide pulses, power 160mJ, rep rate 20Hz
Tevatron dipole magnet at 5T
Glass window
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
PMT with single photon sensitivity
a) Chameleon production phase: photons propagating through a region
of magnetic field oscillate into chameleons
• Photons travel through the glass
• Chameleons see the glass as a wall - trapped
b) Afterglow phase: chameleons in chamber gradually decay
back into photons and are detected by a PMT
Afterglow
B = 5T, L = 6M, E = 2.3eV
Transition probability :
Flux of photons :
Integration time:
Afterglow rate:
Decay time vs coupling
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see thi s picture.
Afterglow vs time
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Complications
• Not longitudinal motion - chameleons and photons bounce
• absorption of photons by the walls
• reflections don’t occur at same place
• Photon penetrates into wall by skin depth
• Chameleon bounces before it reaches the wall
Phase difference at each reflection. V dependent
• Other loss modes. Chameleon could decay to other fields?
• Fragmentation?   
• Bounds from Astrophysics and Cosmology
A.W and A. Uphadye in progress
Data Analysis is under way!
Conclusions/Outlook
• Chameleon fields: Concrete, testable predictions
• Space tests of gravity
• Lab tests can probe a range of parameter space that
is complementary to space tests (qm vacuum and casimir)
• Intriguing cosmological consequences : chameleon
could be causing current accelerated expansion
• Complementary tools of probing fundamental
physics
A lot to learn from probes of the low energy frontier
using spare parts from the high energy frontier
Supplementary
Constraints on Model
Parameters
+
Coincides with Energy scale of Dark Energy
Fifth Force
5th Force:
Range of interaction
Potential :
Separation
Strength of interaction, 
Hoskins et. Al.
 < 10-3
Require both earth and atmosphere
display thin shell effect
Thin shell 
Quantum Vacuum
Classical Vacuum
Quantum Vacuum
• Use a magnetic field to disturb the vacuum
• Probe the disturbance with photons
Birefringence
Quantum vacuum behaves like a birefringent medium
• Different index of refraction for different components of polarisation
Elliptically polarised
light
Linearly polarised
light
Vacuum region
http://www.ts.infn.it/physics/experiments/pvlas/
• Different components of polarisation vector travel with different velocity
• Result: same amplitude but out of phase
• Polarisation : Linear
elliptical
Dichroism
Differential absorption of polarisation components
• One component of polarisation vector preferentially absorbed
• Result: same phase but different amplitude
• Polarisation : Rotation in polarisation plane
In vacuum : • Birefringence expected to be v. small
• No dichroism expected
PVLAS : anomolous signals for both rotation and ellipticity
New Physics?
PVLAS
ALP interpretation
(Polarizzazione del Vuoto con LASer)
photon splits into neutral scalar
pseudoscalar
scalar
or
Ellipticity:
 : Angle betw pol and B
Rotation:
Extract information about m and g and about parity!
Cosmological Evolution
Davis, Brax, van de Bruck, Khoury and A.W.
What do we need?
• attractor solution
If field starts at min, will follow the min

•  must join attractor before current epoch

•  Slow rolls along the attractor

• Variation in m  is constrained to be less than ~ 10%.
Constrains BBN  the initial energy density of the field.
Weaker bound than usual quintessence