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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

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hunting for chameleons
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

  • 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.

  • 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


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 CompactificationKKLT
  • Special points in moduli space - new d.o.f become
  • light Greene, Judes, Levin, Watson & A.W
chameleon effect
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


Reduced Planck Mass

Coupling to photons

Matter Fields

Einstein Frame Metric

Conformally Coupled

Potential is of the runaway form

effective potential
Effective Potential

Energy density in the ith form of matter

Equation of motion :

Dynamics governed by

Effective potential :

predictions for tests in space
Predictions for Tests in Space

New Feature !!

Different behaviour in space

Eöt-Wash Bound  < 10-13

Tests for UFF

Near- future experiments

in space :

STEP  ~ 10-18

GG  ~ 10-17

MICROSCOPE  ~ 10-15

We predict

SEE Capsule

< 10-7

10-15 <


Corrections of O(1) to Newton’s Constant

strong coupling
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
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

(Polarizzazione del Vuoto con LASer)

To explain unexpected birefringence and dichroism results



(g = 1/M)

Conflicts with astrophysical bounds e.g. CAST (solar cooling)



Too heavy to produce  CAST bounds easily satisfied

Chameleons - naturally evade CAST bounds and explain PVLAS

Davis, Brax, van de Bruck


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 “ -

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.



Nd:YAG 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


B = 5T, L = 6M, E = 2.3eV

Transition probability :

Flux of photons :

Integration time:

Afterglow rate:

Decay time vs coupling

Afterglow vs time

  • 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

fifth force
Fifth Force

5th Force:

Range of interaction

Potential :


Hoskins et. Al.  < 10-3

Strength of interaction, 

Require both earth and atmosphere

display thin shell effect

Thin shell 

quantum vacuum
Quantum Vacuum

Classical Vacuum

Quantum Vacuum

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

Quantum vacuum behaves like a birefringent medium

  • Different index of refraction for different components of polarisation

Elliptically polarised


Linearly polarised


Vacuum region

  • Different components of polarisation vector travel with different velocity
  • Result: same amplitude but out of phase
  • Polarisation : Linear elliptical

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?


(Polarizzazione del Vuoto con LASer)

ALP interpretation photon splits into neutral scalar





 : Angle betw pol and B


Extract information about m and g and about parity!

cosmological evolution
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