1. Clare Burrage Université de Genève

2. Axion-like particles (ALPs) are scalars or pseudo-scalars which couple to photons Scalars Pseudo-scalars Assume no relationship between mass and coupling strength In this talk I will concentrate on very light ALPs The current constrains are Scalars (Will 1993) Pseudo-scalars (Hagman et al. 2008) Constraints from dense environments e.g. stars, SNae Axion-Like Particles

3. 70% of the Universe is Dark Energy Extreme fine tuning problems with a cosmological constant Modify gravity, or include new matter? Introduce a new scalar field coupled to matter Screening mechanism to hide the effects of the field in laboratory/solar system/galactic observations Mass depends on environment – chameleon (Khoury, Weltman 2004) Coupling strength depends on environment – Vainshtein, symmetron (Vainshtein 1976, Hinterbichler, Khoury 2010) “Chameleon” particles

5. A coupling to (charged) matter implies a coupling to photons Need new constraints from vacuum where there is no screening EW precision observables (Brax, CB, Davis, Seery, Weltman 2009) Atomic precision measurements (Brax, CB 2010) Starlight polarizations (CB, Davis, Shaw 2008) Galactic luminosities (CB, Davis, Shaw 2009) New Tests

6. Contribution to the width of Z decay Scalar undetectable if Also scalar corrections to boson propagation Scalar undetectable if ElectroWeak Precision Observables

7. The charge and mass of a nucleus perturb a background scalar field Perturbs electron Hamiltonian, and thus energy levels 1s-2s transition Lamb shift Atomic precision measurements

8. A photon travelling through a magnetic field can convert into an ALP (Raffelt & Stodolsky 1988) Photon number is not conserved Now total number of photons + ALPs is conserved Polarization of light is changed ALPs mix with only one component of photon ALPs and photons propagate at different speeds So far no detection of ALPs in the laboratory Can use magnetic fields of Galaxies and Galaxy clusters Mixing in Magnetic fields

9. Look for correlations in the noise in low frequency measurements Best constraints from WUPPE Milky Way starlight polarization observations (Anderson 1996) Starlight Polarization

10. When the mixing is strong, on average 1/3 of photons are converted to scalars The probability distribution is highly skewed Observed correlations between X-ray and optical luminosities of AGN can be used to constrain this effect 2009: possible detection in COMBO-17, ROSAT observations (CB, Davis, Shaw) 2010: Analysis of new data set from Sloan/Xmm-Newton shows no effect (Pettinari, Crittenden) Luminosity of Active Galactic Nuclei

11. Motivation from cosmology to consider new scalars which couple to matter and photons Screened in dense environments Need constraints from near vacuum environments New constraints from EW precision observables Atomic precision measurements Starlight polarizations Galactic luminosities Best constraint on coupling to photons from the luminosity of AGN Conclusions