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

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.

20 pages, 2 figures, comments welcome; v2: fig.2 and corresponding discussion modified, main conclusion unchanged;