Abstract
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 1 0 4 ≤ β m ≤ 1 0 8 , where the coupling to photons is locally bound to β γ ≤ 1 0 11 .
Highlights
Chameleons are Weakly Interacting Slim Particles (WISPs) that, due to their scalar nature and density-dependent effective mass, are viable dark energy candidates [1]
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
Chameleons can couple both to photons, in analogy with the Sikivie coupling of axions [2], and directly to matter, with the special property that their effective mass is dependent on the surrounding matter density
Summary
Chameleons are Weakly Interacting Slim Particles (WISPs) that, due to their scalar nature and density-dependent effective mass, are viable dark energy candidates [1]. Note, reflected off a boundary surface chameleons deposit momentum, resulting in a net force being exerted on the surface itself [5] It is this effect that the KWISP [8] (Kinetic WISP) force sensor attempts to exploit by searching for the force exerted on a thin Si3N4 membrane by chameleons produced in the Sun. Tiny membrane displacements in response to applied forces are sensed with optical interferometry, in this particular case using a Michelson-type interferometer with a balanced homodyne readout. With these settings the perturbation to the membrane due to the light radiation pressure is minimal This was the case in the analysed run, where the dependence on the phase quadrature of the local oscillator is eliminated, and the limiting factor of the measurement becomes the shot noise of the unknown beam. The results can be exported and displayed in various ways, while overlaying the independently measured chopper frequency over the signal spectrum for quick online analysis and diagnostics
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