Hunting for Chameleons Centre for Theoretical Cosmology University of Cambridge Moriond 2008 astro-ph/ PRL J. Khoury and A.W astro-ph/ PRD J. Khoury and A.W astro-ph/ 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 Amanda Weltman

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

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  Motivation Tension between theory and observations Opportunity! - Connect to Cosmology

Solutions? String loop effects Damour & Polyakov Approximate global symmetry Carroll Invoke a potential 1. Suppress the coupling strength : 2. Field acquires mass due to some mechanism : 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 ~ H 0 Field may be a candidate for acc of universe

Ingredients Matter Fields Reduced Planck Mass Einstein Frame Metric Conformally Coupled Potential is of the runaway form Coupling to photons

Effective Potential Equation of motion : Dynamics governed by Effective potential : Energy density in the i th form of matter

Predictions for Tests in Space Different behaviour in space STEP  ~ GG  ~ MICROSCOPE  ~ 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  <  R E /R E < <

Strong Coupling  Thin shell suppression  Remember :  Effective coupling is independent of  !!  independent If an object satisfies thin shell condition - the  force is  independent strong coupling! Lab experiments are compatible with large  - strong coupling!   Thin shell possible in space  suppress signal Mota and Shaw Strong coupling not ruled out by local experiments!   >> 1  more likely to satisfy thin shell condition 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

To explain unexpected birefringence and dichroism results requiresand Conflicts with astrophysical bounds e.g. CAST (solar cooling)  Chameleons - naturally evade CAST bounds and explain PVLAS Davis, Brax, van de Bruck (g = 1/M) (Polarizzazione del Vuoto con LASer) PVLAS  Too heavy to produce  CAST bounds easily satisfied But +

GammeV “ [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 F 2 coupling Turn of the laser Chameleons turn back into photons Observe the afterglow Failing which - at least rule out chunks of parameter space! A. Chou, J. Steffen, A. Uphadye, A.W. and W. Wester See also - Gies et. Al. + Ahlers et. Al. Alps at DESY, LIPSS at JLab, OSQAR at CERN, BMV, PVLAS

GammeV 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

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

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?    Data Analysis is under way! Bounds from Astrophysics and Cosmology A.W and A. Uphadye in progress

Conclusions/Outlook Complementary Complementary tools of probing fundamental physics Space Space tests of gravity cosmological consequences Intriguing cosmological consequences : chameleon accelerated expansion could be causing current accelerated expansion Lab Lab tests can probe a range of parameter space that complementary is complementary to space tests ( qm vacuum and casimir ) Chameleon fields Chameleon fields: Concrete, testable predictions low energy frontier A lot to learn from probes of the low energy frontier high energy frontier using spare parts from the high energy frontier

Supplementary

Constraints on Model Parameters + Coincides with Energy scale of Dark Energy Coincides with Energy scale of Dark Energy

Fifth Force 5 th Force: Separation Strength of interaction,  Potential : Range of interaction  Thin shell  Require both earth and atmosphere display thin shell effect Hoskins et. Al.  < 10 -3

Quantum Vacuum Use a magnetic field to disturb the vacuum Probe the disturbance with photons Classical VacuumQuantum Vacuum

Birefringence Quantum vacuum behaves like a birefringent medium Vacuum region Linearly polarised light Different index of refraction for different components of polarisation Different components of polarisation vector travel with different velocity Result: same amplitude but out of phase Polarisation : Linear elliptical Elliptically polarised light

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

Cosmological 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 Davis, Brax, van de Bruck, Khoury and A.W.