Hunting for Chameleons

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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
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Plan

• Motivation - Theoretical Observational
• Chameleon idea and thin shell effect
• Predictions for tests in space
• Dark Energy Candidate
• Quantum vacuum polarisation experiments
• The GammeV experiment and Chameleons

See Mota talk
See Mota talk
See Mota talk
We can learn about fundamental physics using low
energy and low cost techniques.
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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

Gravitational strength coupling
Light scalar field
?

Tension between theory and observations
Opportunity! - Connect to Cosmology
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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

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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
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Ingredients
Reduced Planck Mass
Coupling to photons
Matter Fields
Einstein Frame Metric
Conformally Coupled
Potential is of the runaway form
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Effective Potential
Energy density in the ith form of matter
Equation of motion
Dynamics governed by Effective potential
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Predictions for Tests in Space
Different behaviour in space
New Feature !!
Eöt-Wash Bound ? lt 10-13
Tests for UFF
Near- future experiments in space
STEP ? 10-18 GG
? 10-17 MICROSCOPE ? 10-15
We predict
SEE Capsule
lt 10-7
?RE/RE
10-15 lt

Corrections of O(1) to Newtons Constant
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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!
? gtgt 1 ? more likely to satisfy thin shell
condition
? Thin shell possible in space ?
suppress signal
Strong coupling is not ideal for space tests -
loophole
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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

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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
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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
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GammeV
NdYAG laser at 532nm, 5ns wide pulses, power
160mJ, rep rate 20Hz
Glass window
Tevatron dipole magnet at 5T
PMT with single photon sensitivity

• 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
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Afterglow
B 5T, L 6M, E 2.3eV
Transition probability
Flux of photons
Integration time
Afterglow rate
Decay time vs coupling
Afterglow vs time
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Complications

• Not longitudinal motion - chameleons and photons
  bounce
• absorption of photons by the walls
• reflections dont 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!
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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
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Supplementary
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Constraints on Model Parameters

Coincides with Energy scale of Dark Energy
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Fifth Force
5th Force
Range of interaction
Potential
Separation
Hoskins et. Al. ? lt 10-3
Strength of interaction, ?
Require both earth and atmosphere display thin
shell effect
Thin shell ?
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Quantum Vacuum
Classical Vacuum
Quantum Vacuum

• Use a magnetic field to disturb the vacuum
• Probe the disturbance with photons

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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

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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?
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PVLAS
(Polarizzazione del Vuoto con LASer)
ALP interpretation photon splits into
neutral scalar
pseudoscalar
scalar
or
Ellipticity
? Angle betw pol and B
Rotation
Extract information about m? and g and about
parity!
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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