Production of light pseudoscalars in external electro-magnetic fields by the Schwinger mechanism

We generalize the Schwinger mechanism and calculate the probability of the decay of intense electromagnetic fields to pseudoscalar particles. In the SLAC E144 experiment electron-positron pairs are produced from the interaction of intense laser fields with a high energy electron beam (by a multiphoton Breit-Wheeler process). We show that from the experimental result that the laser field does not decay into pseudoscalars by the Schwinger process, one can constrain the allowed region of pseudoscalar mass and the pseudoscalar-photon coupling. A better constraint on pseudoscalar properties is obtained from the observation that over the lifetime of the universe the atomic electric field has not decayed into pseudoscalars by any significant probability. In particular we show that the stability of the hydrogen atom over the lifetime of the universe rules out the Peccei-Quinn symmetry breaking scale f_a in the range 1.6 x 10^7 GeV < f_a < 1.6 x 10^{11} GeV