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Register a new userA Proposed Experimental Search for Chameleons using Asymmetric Parallel Plates
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Light scalar fields coupled to matter are a common consequence of theories of dark energy and attempts to solve the cosmological constant problem. The chameleon screening mechanism is commonly invoked in order to suppress the fifth forces mediated by these scalars, suficiently to avoid current experimental constraints, without fine tuning. The force is suppressed dynamically by allowing the mass of the scalar to vary with the local density. Recently it has been shown that near future cold atoms experiments using atom-interferometry have the ability to access a large proportion of the chameleon parameter space. In this work we demonstrate how experiments utilising asymmetric parallel plates can push deeper into the remaining parameter space available to the chameleon.
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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 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.
Despite the remarkable success of the $Lambda$Cold Dark Matter ($Lambda$CDM) cosmological model, a growing discrepancy has emerged (currently measured at the level of $sim 4-6 sigma$) between the value of the Hubble constant $H_0$ measured using the local distance ladder and the value inferred using the cosmic microwave background and galaxy surveys. While a vast array of $Lambda$CDM extensions have been proposed to explain these discordant observations, understanding the (relative) success of these models in resolving the tension has proven difficult -- this is a direct consequence of the fact that each model has been subjected to differing, and typically incomplete, compilations of cosmological data. In this review, we attempt to make a systematic comparison of sixteen different models which have been proposed to resolve the $H_0$ tension (spanning both early- and late-Universe solutions), and quantify the relative success of each using a series of metrics and a vast array of data combinations. Owing to the timely appearance of this article, we refer to this contest as the $H_0$ Olympics; the goal being to identify which of the proposed solutions, and more broadly which underlying mechanisms, are most likely to be responsible for explaining the observed discrepancy (should unaccounted for systematics not be the culprit). This work also establishes a foundation of tests which will allow the success of novel proposals to be meaningful benchmarked.
We describe mts, which is a generic framework for parallelizing certain types of tree search programs, that (a) provides a single common wrapper containing all of the parallelization, and (b) minimizes the changes needed to the existing single processor legacy code. The mts code was derived from ideas used to develop mplrs, a parallelization of the reverse search vertex enumeration code lrs. The tree search properties required for the use of mts are satisfied by any reverse search algorithm as well as other tree search methods such as backtracking and branch and bound. mts is programmed in C, uses the MPI parallel environment, and can be run on a network of computers. As examples we parallelize two simple existing reverse search codes: generating topological orderings and generating spanning trees of a graph. We give computational results comparing the parallel codes with state of the art sequential codes for the same problems.
We point out that chameleon field theories might reveal themselves as an afterglow effect in axion-like particle search experiments due to chameleon-photon conversion in a magnetic field. We estimate the parameter space which is accessible by currently available technology and find that afterglow experiments could constrain this parameter space in a way complementary to gravitational and Casimir force experiments.In addition, one could reach photon-chameleon couplings which are beyond the sensitivity of common laser polarization experiments. We also sketch the idea of a Fabry-Perot cavity with chameleons which could increase the experimental sensitivity significantly.
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