Chiral Susceptibilities in noncompact QED: a new determination of the $\gamma$ exponent and the critical couplings

We report the results of a measurement of susceptibilities in noncompact $QE{D}_4$ in $8^4,{10}^4$ and ${12}^4$ lattices. Due to the potentialities of the $MFA$ approach, we have done simulations in the chiral limit which are therefore free from arbitrary mass extrapolations. Our results in the Coulomb phase show unambiguously that the susceptibility critical exponent $\gamma =1$ independently of the flavour symmetry group. The critical couplings extracted from these calculations are in perfect agreement with previous determinations based on the fermion effective action and plaquette energy, and outside the predictions of a logarithmically improved scalar mean field theory by eight standard deviations.