A gauge-invariant chiral unitary framework for kaon photo- and electroproduction on the protonB. Borasoy, P. C. Bruns, U. -G. Meißner and R. NißlerEur. Phys. J. A, 34 2 (2007) 161-183DOI: https://doi.org/10.1140/epja/i2007-10492-4