Higher-Loop Calculations of the Ultraviolet to Infrared Evolution of a Vectorial Gauge Theory in the Limit $N_c \to \infty$, $N_f \to \infty$ with $N_f/N_c$ Fixed

We consider an asymptotically free vectorial SU($N_c$) gauge theory with $N_f$ fermions in the fundamental representation and analyze higher-loop contributions to the evolution of the theory from the ultraviolet to the infrared in the limit where $N_c \to \infty$ and $N_f \to \infty$ with $r=N_f/N_c$ a fixed, finite constant. We focus on the case where the $n$-loop beta function has an infrared zero, at $ξ=ξ_{IR,n\ell}$, where $ξ=αN_c$. We give results on $ξ_{IR,n\ell}$, the anomalous dimension of the fermion bilinear evaluated at $ξ_{IR,n\ell}$, denoted $γ_{IR,n\ell}$, and certain structural properties of the beta function, $β_ξ$. The approach to this limit is investigated, and it is shown that the leading correction terms are strongly suppressed, by the factor $1/N_c^2$. This provides an understanding of a type of approximate universality in calculations for moderate values of $N_c$ and $N_f$, namely that $α_{IR,n\ell}N_c$, $γ_{IR,n\ell}$, and structural properties of the beta function are similar in theories with different values of $N_c$ and $N_f$ provided that they have similar values of $N_f/N_c$. We give results up to four loops for nonsupersymmetric theories and up to three loops for supersymmetric theories.

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