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Register a new userImproved Search for Solar Chameleons with a GridPix Detector at CAST
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We report on a new search for solar chameleons with the CERN Axion Solar Telescope (CAST). A GridPix detector was used to search for soft X-ray photons in the energy range from 200 eV to 10 keV from converted solar chameleons. No signiffcant excess over the expected background has been observed in the data taken in 2014 and 2015. We set an improved limit on the chameleon photon coupling, $beta_gamma < 5.7times10^{10}$ for $1<beta_mathrm{m}<10^6$ at 95% C.L. improving our previous results by a factor two and for the first time reaching sensitivity below the solar luminosity bound for tachocline magnetic fields up to $12.5,mathrm{T}$.
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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 $10^4 leqbeta_m leq 10^8$, where the coupling to photons is locally bound to $beta_gamma leq 10^{11}$.
We report on the construction, operation experience, and preliminary background measurements of an InGrid detector, i.e. a MicroMegas detector with CMOS pixel readout. The detector was mounted in the focal plane of the Abrixas X-Ray telescope at the CAST experiment at CERN. The detector is sensitive to soft X-Rays in a broad energy range (0.3--10 keV) and thus enables the search for solar chameleons. Smooth detector operation during CAST data taking in autumn 2014 has been achieved. Preliminary analysis of background data indicates a background rate of $1-5times 10^{-5},mathrm{keV}^{-1}mathrm{cm}^{-2}mathrm{s}^{-1}$ above 2 keV and $sim 3times 10^{-4},mathrm{keV}^{-1}mathrm{cm}^{-2}mathrm{s}^{-1}$ around 1 keV. An expected limit of $beta_gamma lesssim 5times 10^{10}$ on the chameleon photon coupling is estimated in case of absence of an excess in solar tracking data. We also discuss the prospects for future operation of the detector.
The CERN Axion Solar Telescope (CAST) searches for $atogamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the X-ray refractive mass $m_gamma$ to the axion search mass $m_a$. After the vacuum phase (2003--2004), which is optimal for $m_alesssim0.02$ eV, we used $^4$He in 2005--2007 to cover the mass range of 0.02--0.39 eV and $^3$He in 2009--2011 to scan from 0.39--1.17 eV. After improving the detectors and shielding, we returned to $^4$He in 2012 to investigate a narrow $m_a$ range around 0.2 eV (candidate setting of our earlier search) and 0.39--0.42 eV, the upper axion mass range reachable with $^4$He, to cross the axion line for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{agamma}< 1.47times10^{-10} {rm GeV}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to vacuum and aim for a significant increase in sensitivity.
In this work we present a search for (solar) chameleons with the CERN Axion Solar Telescope (CAST). This novel experimental technique, in the field of dark energy research, exploits both the chameleon coupling to matter ($beta_{rm m}$) and to photons ($beta_{gamma}$) via the Primakoff effect. By reducing the X-ray detection energy threshold used for axions from 1$,$keV to 400$,$eV CAST became sensitive to the converted solar chameleon spectrum which peaks around 600$,$eV. Even though we have not observed any excess above background, we can provide a 95% C.L. limit for the coupling strength of chameleons to photons of $beta_{gamma}!lesssim!10^{11}$ for $1<beta_{rm m}<10^6$.
We have started the development of a detector system, sensitive to single photons in the eV energy range, to be suitably coupled to one of the CAST magnet ports. This system should open to CAST a window on possible detection of low energy Axion Like Particles emitted by the sun. Preliminary tests have involved a cooled photomultiplier tube coupled to the CAST magnet via a Galileian telescope and a switched 40 m long optical fiber. This system has reached the limit background level of the detector alone in ideal conditions, and two solar tracking runs have been performed with it at CAST. Such a measurement has never been done before with an axion helioscope. We will present results from these runs and briefly discuss future detector developments.
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