Testing Chameleon Dark Testing Chameleon Dark EnergyEnergy

Amanda WeltmanAmanda Weltman

University of University of CambridgeCambridge

Portsmouth June 2008Portsmouth June 2008

University of Cape TownUniversity of Cape Town

MotivationMotivation• 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 observationsOpportunity! - Connect to Cosmology

ObservationsObservations

• Dark Energy p < 0

• Cosmological Constant,

• Dynamical e.o.s w -1

Accelerated expansion of the Universe

Quintessence Need light scalar fieldlight scalar field

Chameleon EffectChameleon 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

astro-ph/0309300 PRL J. Khoury and A.W astro-ph/0309411 PRD J. Khoury and A.W

IngredientsIngredients

Matter Fields

Reduced Planck Mass

Einstein Frame MetricConformally Coupled

Potential is of the runaway form

Coupling to photons

astro-ph/0408415 PRD P. Brax, C. van de Bruck, J.Khoury, A. Davis and A.W

Effective Effective PotentialPotential

Equation of

motion :

Dynamics governed by Effective potential :

Energy density in the ith form of matter

Predictions for Tests in Predictions for Tests in SpaceSpace

Different behaviour in space

STEP ~ 10-18

GG ~ 10-17

MICROSCOPE ~ 10-15

Tests for UFF

Near- future experiments in space :

We predict

New Feature !!

SEE Capsule

Corrections of O(1) to Newton’s Constant

Eöt-Wash Bound < 10-13

RE/RE< 10-710-15 <

Cosmological EvolutionCosmological Evolution

What do we need?

• attractor solution

If field starts at min, will follow the min

• Slow rolls along the attractor

• must join attractor before current epoch

• Variation in m is constrained to be less than ~ 10%. Constrains BBN the initial energy density of the field.

Weaker bound than usual quintessence

astro-ph/0408415 PRD P. Brax, C. van de Bruck, J.Khoury, A. Davis and A.W

Strong CouplingStrong Coupling

Thin shell suppression

Where :

Effective coupling is independent of !!

If an object satisfies thin shell condition - the force is independent independent

Mota and ShawStrong coupling not ruled out by local experiments!

>> 1 thin shell more likely suppresses space signal

LabLab tests on earth can probe a range of parameter space that is complementarycomplementary to space tests.

Opportunity?Opportunity?Loophole!

Coupling to PhotonsCoupling to PhotonsRemember :

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

Test the F2 term

To explain unexpected birefringence and dichroism results

requires and

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

Chameleons - naturally evade CAST bounds and explain PVLASBrax, Davis, van de Bruck

(g = 1/M)

(Polarizzazione del Vuoto con LASer)

PVLAS and CASTPVLAS and CAST

Too heavy to produce CAST bounds easily satisfied

But +

(CERN Axion Solar Telescope)

Particles Trapped in a JarParticles Trapped in a Jar

See also - Gies et. Al. + Ahlers et. Al. (DESY) Alps at DESY, LIPSS at JLab, OSQAR at CERN, BMV

A. Chou et. Al. 0806.2438 [hep-ex]

“ [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 - rule out chunks of parameter space!

http://gammev.fnal.gov

GammeVGammeV

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a) Chameleon production phase: photons propagating through a region of magnetic field oscillate into chameleons

Nd:YAG laser at 532nm, 5ns wide pulses, power 160mJ, rep rate 20Hz

Tevatron dipole magnet at 5T

PMT with single photon sensitivity

Glass window

b) Afterglow phase: chameleons in chamber gradually decay back into photons and are detected by a PMT

• Photons travel through the glass

• Chameleons see the glass as a wall - trapped

A. Uphadye

http://gammev.fnal.gov

SchematicSchematic

GammeVGammeV

AfterglowAfterglow

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Observing window

Stronger coupling decays too fast

Excluded RegionExcluded Region

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PseudoscalarScalar

Fast afterglow decay rates prevent excluding large coupling

Excluded regions

Using minimum afterglow predictions, the sensitivity at low coupling is determined by the PMT noise rate.

ResultsResultsFixing = 2.3 meV, m = 1013V() = 4 exp(n/n)

Testable

Ruled out

ResultsResultsFixing =5e11

m must be in this region for the constraint to be valid

ComplicationsComplications• 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?

• Vacuum design is ineficient for constraining models • Roughing pump decreases Pgas 10-3 Torr • Turbo molecular pump decreases to 10-7 Torr but removes gas volume. I.e. can remove chameleons.

Parameter Space Parameter Space EstimatesEstimates

PVLAS

PRELIMIN

ARY

PRELIMIN

ARY

Conclusions/OutlookConclusions/Outlook

Complementary Complementary tools of probing fundamental physics

• SpaceSpace tests of gravity

• Dark Energy Dark Energy candidate

• LabLab tests can probe a range of parameter space that is complementarycomplementary to space tests (qm vacuum and casimir)

• Chameleon fieldsChameleon fields: Concrete, testable predictions

• New bounds from AstrophysicsAstrophysics and CosmologyCosmology

• First results now out• Potential to dramatically improve these constraints in next generation experiment

• ChameleonsChameleons weaken bounds on f(R) modelsf(R) models(Hu and Sawicki 2007)