Full QCD with milder topological freezing

TL;DR

This paper demonstrates that a parallel tempering algorithm with open and periodic boundary conditions significantly reduces topological charge auto-correlation times in full QCD simulations, enabling finer lattice spacings without topology freezing.

Contribution

The authors adapt and implement a parallel tempering algorithm with boundary conditions to mitigate topological freezing in full QCD simulations at physical parameters.

Findings

Reduces auto-correlation time of topological charge

Enables simulations at lattice spacing as fine as 0.02 fm

Prevents topology freezing in dynamical fermion simulations

Abstract

We simulate $N_f=2+1$ QCD at the physical point combining open and periodic boundary conditions in a parallel tempering framework, following the original proposal by M. Hasenbusch for $2d$ $\mathrm{CP}^{N-1}$ models, which has been recently implemented and widely employed in $4d$ $\mathrm{SU}(N)$ pure Yang-Mills theories too. We show that using this algorithm it is possible to achieve a sizable reduction of the auto-correlation time of the topological charge in dynamical fermions simulations both at zero and finite temperature, allowing to avoid topology freezing down to lattice spacings as fine as $a \sim 0.02$ fm. Therefore, this implementation of the Parallel Tempering on Boundary Conditions algorithm has the potential to substantially push forward the investigation of the QCD vacuum properties by means of lattice simulations.