The CFT Distance Conjecture and Tensionless String Limits in $\mathcal N=2$ Quiver Gauge Theories

Abstract

We initiate the study of infinite-distance limits on (complex) multi-dimensional conformal manifolds of 4d SCFTs and their bulk interpretation as tensionless-string limits in AdS/CFT. In particular, we focus on 4d $\mathcal{N}=2$ $SU$ quiver gauge theories with hypermultiplets in the bifundamental and fundamental representations. In the overall-free limit, we compute the large-$N$ Hagedorn temperature $T_H$, which governs the stringy exponential growth of the density of states at high energies. We argue that this quantity determines the type of stringy ultraviolet completion in the bulk: it captures the type of string theory in which the bulk physics is embedded while remaining insensitive to detailed geometric data. For linear quivers, we find that $T_H$ depends only on the quiver length, which is tied to the number of NS5-branes in the underlying brane construction and, in turn, to the string theory in which the bulk is embedded. For holographic quivers, where we impose that the two central charges $a$ and $c$ coincide in the large-$N$ limit, we show that $T_H$ coincides with that of $\mathcal{N}=4$ SYM, which befits the 10d Type IIB description of their gravitational duals. We also analyze the exponential rate $\alpha$, which controls how the leading tower of higher-spin currents becomes conserved in these limits, as suggested by the CFT Distance Conjecture. In the large-$N$ regime, we derive sharp bounds on the minimal rate, $1/\sqrt{2}\le \alpha_{\min}\le \sqrt{2/3}$, attained in the overall-free limit. Moreover, we prove that the universal lower bound $\alpha\ge 1/\sqrt{2}$ holds, including at finite $N$. Finally, we go beyond the overall-free ray by characterizing the convex hull of the $\vec{\alpha}$-vectors that encode the exponential rate of the higher-spin towers along any (partial) weak-coupling limit.