High Temperature QCD Calculations Using Gradient Flow On An Anisotropic Lattice

Abstract

Understanding the evolution of the universe from its earliest configuration to its present state requires an understanding of strongly coupled systems such as QCD. Many interesting components of QCD such as confinement and chiral-symmetry breaking can only be understood using first principles non-perturbative methods such as lattice field theory. The energy-momentum tensor is key within QCD thermodynamics, containing information on the equation of state and transport properties such as specific heat and viscosity. Defining the energy-momentum tensor on the lattice is challenging, however it has been made obtainable recently by using a tool known generally as gradient flow. This has been used in order to compute thermodynamic quantities on the lattice, however due to the isotropy of the involved lattices, obtaining fine temperature sampling can be difficult. This work implements the gradient flow method for obtaining thermodynamic quantities from the energymomentum tensor, by using an anisotropic lattice, which as far as the author is aware is the first example of such, and thus provides the novelty of this work. Another application is to compute the topological susceptibility, which is important for axion cosmology, and related to both the strong CP problem and dark matter, again for the first time using an anisotropic lattice for N f = 2 + 1 QCD.