Chiral symmetry breaking in three dimensional QED with Nf flavors

QED in three dimensions (QED3) is studied by computer simulations, Schwinger-Dyson equations and physical arguments to understand its chiral symmetry breaking features for various Nf, the number of dynamical, light 4-component Dirac fermions. We find that chiral symmetry is broken in the continuum limit only if Nf is smaller than Nc=3.5±0.5. Simulations on 63, 83 and more sophisticated simulations on 103 lattices at bare fermion masses of 0.04, 0.03 and 0.02 are used to arrive at this conclusion. Furthermore, we simulate the lattice theory at infinite coupling and find that fermion screening eliminates chiral symmetry breaking for Nf>8. These results confirm recent 1/Nf expansions of the theory's gap equation. We discuss chiral symmetry breaking mechanisms in these theories at low Nf and map the physics of chiral symmetry breaking in QED3 as a function of Nf to chiral symmetry breaking in quenched QED4 as a function of coupling. We suggest that QED3 is a useful guide to chiral symmetry breaking physics in asymptotically free technicolor models of the Grand Unification Theory. (orig.)