Chris Allton

In the QCD phase diagram, the dependence of the pseudocritical temperature, T pc , on the baryon chemical potential, μ B , is of fundamental interest. The variation of T pc with μ B is normally captured by κ , the coefficient of the leading (quadratic) term of the polynomial expansion of T pc with μ B . In this work, we present the first calculation of κ using hadronic quantities. Simulating N f = 2 + 1 flavors of Wilson fermions on astsum ensembles, we calculate the O ( μ B 2 ) correction to mesonic correlation functions. By demanding degeneracy in the vector and axial-vector channels we obtain T pc ( μ B ) and hence κ . While lacking a continuum extrapolation and being away from the physical point, our results are consistent with previous works using thermodynamic observables (renormalized chiral condensate, strange quark number susceptibility) from lattice QCD simulations with staggered fermions.

Center-vortex surfaces are mapped out in four dimensions within the framework of SU(3) lattice gauge theory to understand the role of secondary loops that develop in three-dimensional visualizations of center-vortex structure, appearing separate from the percolating cluster. Loops that initially appear disconnected in three-dimensional slices can originate from the same connected surface in four dimensions depending on the surface’s curvature. For the first time, these secondary loops are identified as “connected” or “disconnected” with respect to the vortex sheet, allowing new insight into the evolution of center-vortex geometry through the finite-temperature phase transition. At low temperatures, we find that secondary loops of any length primarily lie in the same sheet percolating the four-dimensional volume. Only a handful of small secondary sheets disconnected from the percolating sheet are identified. Above the phase transition, the vortex structure is still found to be dominated by a single large sheet but one that has aligned with the temporal dimension. With the near absence of any curvature orthogonal to the temporal dimension, connected secondary loops become vanishingly rare. Other novel quantities, such as the four-dimensional density of secondary sheets and the sheet sizes themselves, are analyzed to build a complete picture of center-vortex geometry in four dimensions.

The FASTSUM collaboration has a long-standing project examining hadronic properties using anisotropic lattice QCD. We determine the spectral properties of bottomonia at finite temperature using lattice NRQCD and describe how our newer simulations improve our control over systematic errors. Motivated by these efforts, the temperature dependence of charm hadron masses is determined where it is found that temperature effects can extend into the confining phase and that some species remain stable deep past the pseudo-critical temperature.

Evidence for the existence of a second finite-temperature transition in quantum chromodynamics (QCD) is obtained through the study of center vortex geometry and its evolution with temperature. The dynamical anisotropic ensembles of the Fastsum Collaboration are utilized to conduct a comprehensive analysis at eight temperatures beyond the established chiral transition. Visualizations of the center vortex structure in temporal and spatial slices of the lattice reveal that vortex percolation persists through the chiral transition and ceases at a temperature that is approximately twice the chiral transition temperature T c . This implies that confinement is retained through temperatures up to T ≈ 2 T c , pointing toward a second transition corresponding to deconfinement. The loss of percolation is quantified by the vortex cluster extent, providing a clear signal for the deconfinement transition. Additional vortex statistics, including temporal correlations, vortex and branching point densities, the number of secondary clusters and vortex chain lengths between branching points, are scrutinized as a function of temperature. All ten measures investigated herein show the characteristics of two transitions in QCD, encompassing the chiral transition at T c and the deconfinement transition at T ≈ 2 T c . Performing an inflection point analysis on the vortex and branching point densities produces an estimate of T c that agrees with the known Fastsum value. By the same procedure, a precise estimate of the deconfinement point is extracted as T d = 321 ( 6 ) MeV .