# Safety versus triviality on the lattice

## TL;DR

This study investigates the ultraviolet behavior of non-asymptotically free SU(2) gauge theories with many fermions using lattice simulations, exploring whether they develop a trivial or safe fixed point.

## Contribution

First numerical analysis applying gradient flow to non-asymptotically free theories, connecting lattice results with perturbative predictions and exploring the existence of safe fixed points.

## Key findings

- Gradient flow method effectively determines running coupling.
- Results align with perturbative Gaussian fixed point.
- Difficulty in reaching large renormalized couplings suggests a need for more flavors or limitations of current lattice actions.

## Abstract

We present the first numerical study of the ultraviolet dynamics of non-asymptotically free gauge-fermion theories at large number of matter fields. As testbed theories we consider non-abelian SU(2) gauge theories with 24 and 48 Dirac fermions on the lattice. For these number of flavors asymptotic freedom is lost and the theories are governed by a gaussian fixed point at low energies. In the ultraviolet they can develop a physical cutoff and therefore be trivial, or achieve an interacting safe fixed point and therefore be fundamental at all energy scales. We demonstrate that the gradient flow method can be successfully implemented and applied to determine the renormalized running coupling when asymptotic freedom is lost. Additionally, we prove that our analysis is connected to the gaussian fixed point as our results nicely match with the perturbative beta function. Intriguingly, we observe that it is hard to achieve large values of the renormalized coupling on the lattice. This might be an early sign of the existence of a physical cutoff and imply that a larger number of flavors is needed to achieve the safe fixed point. A more conservative interpretation of the results is that the current lattice action is unable to explore the deep ultraviolet region where safety might emerge. Our work constitutes an essential step towards determining the ultraviolet fate of non asymptotically free gauge theories.