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Abstract:
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 104 ≤ βm ≤ 108, where the coupling to photons is locally bound to βγ ≤ 1011.
