Disturbing news about the $d=2+\epsilon$ expansion II. Assessing the recombination scenario

TL;DR

This paper investigates whether multiplet recombination can connect the $d=2+psilon$ expansion of the $O(N)$ NLSM to the Wilson-Fisher fixed point, finding that one-loop results suggest recombination is unlikely.

Contribution

The study assesses the multiplet recombination scenario for $N=3,4$ in the $O(N)$ NLSM, providing one-loop anomalous dimensions that challenge its viability.

Findings

One-loop anomalous dimensions increase with psilon.

Recombination would require these dimensions to decrease to N.

Results suggest multiplet recombination is unlikely.

Abstract

In [De Cesare, Rychkov (2025)], we revisited the $d=2+\epsilon$ expansion in the $O(N)$ Non-Linear Sigma Model (NLSM), emphasizing the existence of a protected operator which is a closed form with $N-1$ indices. The scaling dimension of this operator stays exactly equal to $N-1$, independently of $\epsilon$. Its existence is problematic for the identification of the NLSM fixed point in $d=2+\epsilon$ with the Wilson-Fisher fixed point family obtained by analytically continuing from near $d=4$, which does not possess such a protected operator. Multiplet recombination is one scenario discussed in [De Cesare, Rychkov (2025)], which could allow to connect the two families continuously (although not analytically). In this scenario, the protected dimension is lifted at some critical value of $\epsilon$, thanks to the short conformal multiplet of scaling dimension $N-1$ eating a long conformal multiplet of higher scaling dimension. In this followup work, we assess this scenario for the cases $N=3$ and $N=4$. We identify the lowest candidates for the long multiplet which could be eaten, and compute their one-loop anomalous dimensions. We find that at one loop, scaling dimensions of these candidates grow with $\epsilon$, while it should decrease down to $N$ for the recombination to occur. We conclude that multiplet recombination is unlikely.