Critical Number of Flavours in QED

We demonstrate that in unquenched quantum electrodynamics (QED), chiral symmetry breaking ceases to exist above a critical number of fermion flavours $N_f$. This is a necessary and sufficient consequence of the fact that there exists a critical value of electromagnetic coupling $\alpha$ beyond which dynamical mass generation gets triggered. We employ a multiplicatively renormalizable photon propagator involving leading logarithms to all orders in $\alpha$ to illustrate this. We study the flavour and coupling dependence of the dynamically generated mass analytically as well as numerically. We also derive the scaling laws for the dynamical mass as a function of $\alpha$ and $N_f$. Up to a multiplicative constant, these scaling laws are related through $(\alpha, \alpha_c) \leftrightarrow (1/N_f, 1/N_f^c)$. Calculation of the mass anomalous dimension $\gamma_m$ shows that it is always greater than its value in the quenched case. We also evaluate the $\beta$-function. The criticality plane is drawn in the $(\alpha,N_f)$ phase space which clearly depicts how larger $N_f$ is required to restore chiral symmetry for an increasing interaction strength.