First Results on the Search for Chameleons with the KWISP Detector at
  CAST
  We report on a first measurement with a sensitive opto-mechanical force
sensor designed for the direct detection of coupling of real chameleons to
matter. These dark energy candidates could be produced in the Sun and stream
unimpeded to Earth. The KWISP detector installed on the CAST axion search
experiment at CERN looks for tiny displacements of a thin membrane caused by
the mechanical effect of solar chameleons. The displacements are detected by a
Michelson interferometer with a homodyne readout scheme. The sensor benefits
from the focusing action of the ABRIXAS X-ray telescope installed at CAST,
which increases the chameleon flux on the membrane. A mechanical chopper placed
between the telescope output and the detector modulates the incoming chameleon
stream. We present the results of the solar chameleon measurements taken at
CAST in July 2017, setting an upper bound on the force acting on the membrane
of $80$~pN at 95\% confidence level. The detector is sensitive for direct
coupling to matter $10^4 \leq\beta_m \leq 10^8$, where the coupling to photons
is locally bound to $\beta_\gamma \leq 10^{11}$.
