Chameleon foreCAST
  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 non-resonant 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. A detection of these X-rays would
indicate the existence of chameleons. 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.
