Our results can be found in our preprint (here).


Chameleons are hypothetical particles that have couplings that depend on the environment. The coupling is usually to the stress-energy tensor in a higher energy or more dense environment, the coupling is stronger. Thus if the there axion-like particles that had this chameleon property, it could be one way that the PVLAS anomalous result might have been observed in the laboratory, but not from star cooling limits. That is, the sun doesn’t burn out in 1000 years because it’s losing energy to chameleons because the chameleons can’t escape the sun due to the high energy densities. However, in vacuum laboratory experiments, chameleons might not see high energy densities until they collide with ordinary matter. Chameleons might have relation to theories of dark energy or string theory which sometimes predicts many new fundamental scalar particles.




The GammeV experiment is reconfigured for the “particle in a jar” technique to search for chameleons.


The figure below shows a schematic of the experiment.  Compared with the “light shining through a wall configuration,” this configuration does not have “a wall;” rather, it has a straight through plunger that terminates in the exit window. The removable mirror is used during the chameleon generation phase to reflect incident laser light back through the magnet for a second pass and to have a monitor of the laser power. For the photon regeneration phase, the mirror is removed and the PMT is used to look for afterglow photons.







The GammeV chameleon experiment proceeds in two phases. Phase I is to generate chameleons by shining polarized laser light through the apparatus and back through after bouncing on the removable mirror. There is a small probability that photons will oscillate into chameleon particles in the presence of the magnetic field with similar properties as an axion-like particle in the light shining through wall experiment. The chameleon particles will travel freely in the vacuum of the “jar,” but will bounce with 100% reflection when they hit the vacuum window or the side of the insulating bore. Because the vacuum windows are not perfectly aligned, the chameleons will soon start bouncing on the side of the insulating bore which is not machined to optical quality. Thus, soon the chameleons will become isotropic in their direction within the “jar.” After about 1 hour in Phase I, there will be a “jar” filled with a gas of isotropic chameleons. For Phase II, the laser is turned off and the removable mirror is removed and the PMT is turned-on. The idea is that now, in the magnetic field, chameleons now have a small probability of reconverting back into photons. This would result in an afterglow of light emerging from the “jar”. If such an effect was observed, it could be checked by ramping the magnet up and down and verifying that the afterglow occurs only when the magnetic field is on. There should also be a B^2 dependence on the afterglow. The figures below show this Phase I and Phase II process in a schematic fashion.





GammeV did not observe an afterglow and limits on chameleon production and regeneration can be derived. In practice, our experiment was limited because of the vacuum system that we employed that could have trapped and expelled chameleons from the jar. Thus, our results are valid only for chameleons that can reflect on the higher residual gas densities in the vacuum system (such that they don’t get expelled). Additionally, the results require that the coherence condition within the jar be met even without a perfect vacuum in the jar. The plot below shows our results, but the more interested reader is referred to our preprint, arXiv:0806.2438.