Topological properties of $CP^{N-1}$ models in the large-$N$ limit

TL;DR

This study uses lattice simulations to analyze the $ heta$-dependence of 2D $CP^{N-1}$ models for large N, providing continuum estimates of topological coefficients and comparing with large-N analytical predictions.

Contribution

It offers the first numerical continuum estimates of topological coefficients in $CP^{N-1}$ models for large N and compares them with theoretical large-N predictions.

Findings

Continuum estimates for second and fourth order coefficients in $ heta$ expansion.

Higher order corrections may be significant for N > 9.

Results for $b_4$ are consistent with large-N predictions and include new data for $SU(2)$ Yang-Mills.

Abstract

We investigate, by numerical simulations on a lattice, the $\theta$-dependence of 2$d$ $CP^{N-1}$ models for a range of $N$ going from 9 to 31, combining imaginary $\theta$ and simulated tempering techniques to improve the signal-to-noise ratio and alleviate the critical slowing down of the topological modes. We provide continuum extrapolations for the second and fourth order coefficients in the Taylor expansion in $\theta$ of the vacuum energy of the theory, parameterized in terms of the topological susceptibility $\chi$ and of the so-called $b_2$ coefficient. Those are then compared with available analytic predictions obtained within the $1/N$ expansion, pointing out that higher order corrections might be relevant in the explored range of $N$, and that this fact might be related to the non-analytic behavior expected for $N = 2$. We also consider sixth-order corrections in the $\theta$ expansion, parameterized in terms of the so-called $b_4$ coefficient: in this case our present statistical accuracy permits to have reliable non-zero continuum estimations only for $N \leq 11$, while for larger values we can only set upper bounds. The sign and values obtained for $b_4$ are compared to large-$N$ predictions, as well as to results obtained for $SU(N_c)$ Yang-Mills theories, for which a first numerical determination is provided in this study for the case $N_c = 2$.