Search for New Photon Couplings in a Magnetic Field

Of great interest to particle physics is the question of the existence of new, light, pseudoscalar (or scalar) particles. In particular, the existence of a light pseudoscalar boson, known as the axion, would prove a solution to the strong CP problem. These particles, which must be very weakly coupled to ordinary matter, could also be the missing matter in the universe. We attempted to produce axions in the laboratory by shining a laser beam through a transverse magnetic field. Only light polarized parallel to the magnetic field produces axions, so the polarization state of the light was carefully controlled. To increase the production of axions, we constructed a multipass optical cavity that makes the light travel as much as 4 km through the magnetic field region. Using two different methods to detect the production of axions, we set limits on the axion coupling to two photons. In the first experiment, the change in polarization of the light was measured. To do this, we constructed an ellipsometer, which could measure changes in polarization angle as small as 4 times 10^{ -11} rad. From the absence of an optical rotation due to the production of axions, we were able to set a limit on axion coupling to two photons of g_ {rm agammagamma} < 3.6 times 10^{-7 } GeV^{-1}. In the second experiment we attempted to more directly measure the production of axions. In this case the axions were reconverted to photons, and the regenerated photons were counted by a low dark current photomultiplier tube. No photons in excess of the dark current were detected, and the limit on axion coupling to two photons from this experiment is g_{rm agamma gamma} < 6.7 times 10^{-7} GeV^{-1}.