A common explanation for the observed acceleration of the Universe is a scalar field (i.e., scalar particles). However, experiments such as tests of gravity and measurements of fundamental constants place stringent constraints on such particles. At this time there are only three classes of scalar particles that can simultaneously accelerate the Universe and be consistent with these tests. One of these types of particle is the chameleon particle—a particle whose mass, like a chameleon’s color, depends upon the local environment. In high density regions a chameleon will have a large mass while in low density regions it will have a small one.
If these particles couple to photons as well as matter, then they could be produced through photons converting to chameleons inside a magnetic field. The GammeV Chameleon Afterglow Search (CHASE) is an effort to detect chameleon particles if they exist. This experiment uses a laser to produce chameleons inside a vacuum chamber immersed in a magnetic field. When a chameleon in this chamber approaches a wall or window, the chameleon effect causes its mass to grow and its momentum to slow. Eventually, when the chameleon’s mass exceeds the energy of the initial photon, the particle will reflect from the surface—bouncing back into the volume of the chamber.
At the same time, trapped chameleon particles can reconvert to photons through the reverse of the proces that formed them. By turning off the laser and turning on a photo detector, we can look for an afterglow of light as the trapped chameleons reconvert to photons and stream through the windows of the vacuum chamber. Searching for chameleons in a laboratory setting is a way to study dark energy that is complementary to astronomical surveys.
– Jason Steffen