Hunting for Chameleons

1. 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

2. 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.

3. 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

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

5. 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

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

7. Effective Potential Energy density in the ith form of matter Equation of motion : Dynamics governed by Effective potential :

8. 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 < RE/RE Corrections of O(1) to Newton’s Constant

9. 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

10. 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

11. 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

12. 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

13. GammeV 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

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

15. 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!

16. 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

17. Supplementary

18. Constraints on Model Parameters +

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

20. Quantum Vacuum Classical Vacuum Quantum Vacuum • Use a magnetic field to disturb the vacuum • Probe the disturbance with photons

21. 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

22. 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?

23. 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!

24. 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