Search for chameleons with an InGrid based X-ray detector at the CAST experiment

The CERN Axion Solar Telescope (CAST) searches for axions and also other exotic particles emerging from the Sun. Chameleons, for example, are part of Dark Energy theories. Like Axions they can be converted into soft X-ray photons in a high magnetic field and should result in an X-ray spectrum peaking below 1 keV. Because of their low energy and weak coupling, detectors with low energy threshold and low background rates are mandatory. Both requirements are met by an X-ray detector based on the combination of a Micromegas gas amplification stage with a highly integrated pixel chip which allows to make full use of the Micromegas structure's granularity. It has been demonstrated that these devices can detect even single electrons. Thus, allowing for a topological background suppression as well as for detection of low energy X-ray photons. After the detection threshold had been evaluated to be low enough to allow for the detection of the carbon K_α line at 277 eV, the detector was mounted at one of CAST's X-ray telescopes in 2014. During data taking until end of 2015 background rates of less than 10^-4 /keV/cm²/s have been achieved below 2 keV. Preliminary results of the chameleon analysis and possibly an improved limit for solar chameleons are presented as well as prospects for improvements followed in the development of a new detector for 2016.