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TOPIC: Chameleon particles


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Title: How to Avoid a Swift Kick in the Chameleons
Author: Antonio Padilla, Emma Platts, David Stefanyszyn, Anthony Walters, Amanda Weltman, Toby Wilson

Recently, it was argued that the conformal coupling of the chameleon to matter fields created an issue for early universe cosmology. As standard model degrees of freedom become non-relativistic in the early universe, the chameleon is attracted towards a "surfing" solution, so that it arrives at the potential minimum with too large a velocity. This leads to rapid variations in the chameleon's mass and excitation of high energy modes, casting doubts on the classical treatment at Big Bang Nucleosynthesis. Here we present the DBI chameleon, a consistent high energy modification of the chameleon theory that dynamically renders it weakly coupled to matter during the early universe thereby eliminating the adverse effects of the `kicks'. This is done without any fine tuning of the coupling between the chameleon and matter fields, and retains its screening ability in the solar system. We demonstrate this explicitly with a combination of analytic and numerical results.

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Chameleon fields
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Title: Designing dark energy afterglow experiments
Authors: Amol Upadhye, Jason H. Steffen, Aaron S. Chou

Chameleon fields, which are scalar field dark energy candidates, can evade fifth force constraints by becoming massive in high-density regions. However, this property allows chameleon particles to be trapped inside a vacuum chamber with dense walls. Afterglow experiments constrain photon-coupled chameleon fields by attempting to produce and trap chameleon particles inside such a vacuum chamber, from which they will emit an afterglow as they regenerate photons. Here we discuss several theoretical and systematic effects underlying the design and analysis of the GammeV and CHASE afterglow experiments. We consider chameleon particle interactions with photons, Fermions, and other chameleon particles, as well as with macroscopic magnetic fields and matter. The afterglow signal in each experiment is predicted, and its sensitivity to various properties of the experimental apparatus is studied. Finally, we use CHASE data to exclude a wide range of photon-coupled chameleon dark energy models.

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Title: Chameleon effect and the Pioneer anomaly
Authors: John D. Anderson, J.R. Morris

The possibility that the apparent anomalous acceleration of the Pioneer 10 and 11 spacecraft may be due, at least in part, to a chameleon field effect is examined. A small spacecraft, with no thin shell, can have a more pronounced anomalous acceleration than a large compact body, such as a planet, having a thin shell. The chameleon effect seems to present a natural way to explain the differences seen in deviations from pure Newtonian gravity for a spacecraft and for a planet, and appears to be compatible with the basic features of the Pioneer anomaly, including the appearance of a jerk term. However, estimates of the size of the chameleon effect indicate that its contribution to the anomalous acceleration is negligible. We conclude that any inverse-square component in the anomalous acceleration is more likely caused by an unmodelled reaction force from solar-radiation pressure, rather than a chameleon field effect.

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Ed ~ Conventional forces can explain the anomalous acceleration



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Title: A chameleon helioscope
Authors: Keith Baker, Axel Lindner, Amol Upadhye, Konstantin Zioutas

Chameleon particles, which could explain dark energy, are in many ways similar to axions, suggesting that an axion helioscope can be used for chameleon detection. The distinguishing property of chameleon particles is that, unlike Standard Model particles, their effective masses depend upon the ambient matter-energy density. The associated total internal reflection of chameleons up to keV energies by a dense layer of material, which would occur at grazing incidence on the mirrors of an X-ray telescope, lead to new experimental techniques for detecting such particles. We discuss here when this total internal reflection can happen and how it can be implemented in existing or future state-of-the-art chameleon telescopes. Solar Chameleons would be emitted mainly with energies below a few keV suggesting the X-ray telescope as the basic component in chameleon telescopy. The implementation of this idea is straightforward, but it deserves further scrutiny. It seems promising to prepare and run a dark energy particle candidate detection experiment combining existing equipment. For example, large volumes and strong solenoid magnetic fields, which are not appropriate for solar axion investigations, are attractive from the point of view of chameleon telescopy.

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Cosmologists don't usually take their lead from the animal kingdom. But a model that postulates the existence of a 'chameleon' particle - which would change its mass depending on its surroundings - is gaining attention.
A new paper claims to have spotted signs of this elusive particle, whose existence was first postulated in 20032 to explain the accelerating expansion of the Universe, which has been attributed to some unknown 'dark energy'. The changing mass of a chameleon particle would modify the range at which its force can act, thus possibly explaining why whatever causes the Universe's acceleration hasn't been detected on Earth. On Earth, the chameleon would be too heavy to create any noticeable force, but in the tracts of empty space, its effect would be huge.

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Call them the Energiser bunny of particle physics: the GammeV collaboration reins in costs, works fast and just keeps going, and going
The collaboration of 10 people formed in April 2007 to look for candidates for dark matter and dark energy. Although several members work on other experiments as well, they were drawn to GammeVs unique attributes. Its small collaboration size allows for large individual roles in building and analysis plus a chance to search for exotic particles while exploring areas of physics often overlooked by larger collaborations.

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