Chameleon particle

Chameleon

Composition	{{{composition}}}
Interactions	Gravity, Electroweak
Status	Hypothetical
Mass	variable, depending on environment
Electric charge	0
Spin	0

The "chameleon" is a postulated scalar particle with a non-linear self-interaction which gives the particle an effective mass that depends on its environment: the presence of other fields. It would have a small mass in much of intergalactic space, but a large mass in terrestrial experiments, making it difficult to detect. The chameleon is a possible candidate for dark energy and dark matter, and may contribute to cosmic inflation.

Hypothetical properties

In most theories, chameleons have a mass that scales as some power of the local energy density: m_eff∼ρ^α, where $\alpha \simeq 1$.

Chameleons also couple to photons, allowing photons and chameleons to oscillate between each other in the presence of an external magnetic field.

Chameleons can be confined in hollow containers because their mass increases rapidly as they penetrate the container wall, causing them to reflect. One strategy to search experimentally for chameleons is to direct photons into a cavity, confining the chameleons produced, and then to switch off the light source. Chameleons would be indicated by the presence of an afterglow as they decay back into photons.

Experimental searches

A number of experiments have attempted to detect chameleons along with axions.

The GammeV experiment is a search for axions, but has been used to look for chameleons too. It consists of a cylindrical chamber inserted in a 5T magnetic field. The ends of the chamber are glass windows, allowing light from a laser to enter and afterglow to exit.

References

Journal entries

• Khoury, J.; Weltman, A. (2004). "Chameleon fields: awaiting surprises for tests of gravity in space". Physical Review Letters 93 (17): 171104. arXiv:astro-ph/0309300. Bibcode 2004PhRvL..93q1104K. doi:10.1103/PhysRevLett.93.171104. PMID 15525066. 
• Khoury, J.; Weltman, A. (2004). "Chameleon cosmology". Physical Review D 69 (4): 044026. arXiv:astro-ph/0309411. Bibcode 2004PhRvD..69d4026K. doi:10.1103/PhysRevD.69.044026. 
• Brax, P.; van de Bruck, C.; Davis, A.-C.; Khoury, J.; Weltman, A. (2004). "Detecting dark energy in orbit: The cosmological chameleon". Physical Review D 70 (12): 123518. arXiv:astro-ph/0408415. Bibcode 2004PhRvD..70l3518B. doi:10.1103/PhysRevD.70.123518.