ETD

• charm effects
• finite temperature qcd
• lattice qcd
• Roberge-Weiss endpoint
• Roberge-Weiss transition
• θ-dependence

In this work, the QCD phase diagram at the physical point with Nf = 2 + 1 + 1 flavours is investigated through lattice simulations on the T–μB plane, focusing on the Roberge–Weiss (RW) phase transition that arises at imaginary chemical potential. Simulations employ stout-improved staggered fermions and the tree-level Symanzik gauge action for Nt = 8, 10, along a line of constant physics. Previous studies with Nf = 2 + 1 locate the RW endpoint at TRW = 208(5) MeV, within the 3D Ising universality class. Here we assess the impact of the charm quark and find no significant corrections, confirming compatibility with earlier results in both the transition temperature and order. We also explore the dependence of the QCD free energy on the topological theta parameter across the RW transition. Topological observables such as the charge, susceptibility, and fourth-order cumulant were measured and compared with the Dilute Instanton Gas Approximation (DIGA). For the first time with statistical significance, we observe that in the presence of dynamical fermions the fourth cumulant drops below the DIGA prediction just after the transition and approaches it from below at higher temperatures.