• QCD
• lattice-QCD
• theta-dependence

Strong interactions in particle physics are accurately described by Quantum Chromodynamics, which is a gauge theory with group SU(3). The matter fields of the theory are massive Dirac fermions called. In the limit where the quarks have infinite masses (called quenched limit), the theory exhibits a first order phase transition between a deconfined and a confined phase, at a temperature T_c ≃ 290 MeV. To the Lagrangian of the theory can be added an operator q which is local, gauge and Lorentz invariant and has dimension 4. Its integral over the whole 4-dimensional space is an integer called topological charge. The coefficient coupled to q is usually called θ. Since Q is an integer, θ behaves like an angular variable.
θ-dependence cannot be studied perturbatively, because it is related to the existence of field configurations with non-trivial topology, that cannot be obtained from a vacuum with small deformation. Semi-quantitative analytical results can be obtained in the chiral limit (where the lightest quarks are considered massless) and in the large-number-of-colors limit (where SU(N_c), with N_c approaching infinity, is considered as the gauge group). Otherwise, lattice Monte Carlo simulations are the only way to investigate the θ-dependence.
The θ-dependence of the deconfinement temperature has been studied in the case of the SU(3) gauge theory in previous works. We extended the study to SU(4) and SU(6) theories and verify the preexisting predictions in the large N_c limit.