Detection prospects for solar and terrestrial chameleons

Dark energy models, such as the chameleon, where the acceleration of the expansion of the Universe results from the dynamics of a scalar field coupled to matter, suffer from the potential existence of a fifth force. Three known mechanisms have been proposed to restore general relativity in the solar system and the laboratory, which are the symmetron/Damour-Polyakov effect, the Vainshtein property and the chameleon screening. Here, we propose to probe the existence of chameleons in the laboratory, considering their particle physics consequences. We envisage the resonant and nonresonant production of chameleons in the sun and their back-conversion into X-ray photons in a solar helioscope pipe such as the one used by CAST. We find that the resonant contribution is only present for a very small range of chameleon couplings to matter and occurs in extremely narrow magnetized regions in the sun. However, in the nonresonant case, chameleons are created in all available solar magnetic regions and for a very wide range of chameleon couplings to photons and matter. Such chameleons could leave the sun and be back-converted into X-ray photons above the photosphere or in the magnetic pipe of a helioscope. A detection of these X-rays would indicate the existence of chameleons. The number of produced chameleons and regenerated X-ray photons varies depending on the strength of the solar magnetic field in the production shell, the matter and photon couplings of chameleons, and the magnetic field length and magnitude inside a dipole magnet. We focus on a template model for the solar magnetic field: a constant magnetic field in a narrow shell surrounding the tachocline. The X-ray photons in a helioscope pipe obtained from back-conversion of the chameleons created inside the sun have a spectrum which is peaked in the sub-keV region, just below the actual sensitivity range of the present axion helioscopes. Nevertheless they are detectable by present-day magnetic helioscopes like CAST and Sumico, which were built originally for solar axions. We also propose a chameleon-through-a-wall experiment whereby X-ray photons from a synchroton radiation source could be converted into chameleons inside a dipole magnet, then pass a wall which is opaque to X-rays before being back-converted into X-ray photons in a second magnet downstream. We show that this could provide a direct signature for the existence of chameleon particles.