Low-energy limit of Yang-Mills with massless adjoint quarks: Chiral Lagrangian and Skyrmions

If the fundamental quarks of QCD are replaced by massless adjoint quarks, the pattern of the chiral symmetry breaking drastically changes compared to the standard one. It becomes SU(N_f) → SO(N_f). While for N_f ≤ 2, the chiral Lagrangian describing the 'pion' dynamics is well known, this is not the case at N_f > 2. We outline a general strategy for deriving chiral Lagrangians for the coset spaces and study in detail the case of N_f ≤ k ≤ 3. We obtain two- and four-derivatives terms in the chiral Lagrangian on the coset space ≤ SU(3)/SO(3), as well as the Wess-Zumino-Novikov-Witten term, in terms of an explicit parameterization of the quotient manifold. Then we discuss stable topological solitons supported by this Lagrangian. Aspects of relevant topological considerations scattered in the literature are reviewed. The same analysis applies to SO(N) gauge theories with N_f Weyl flavours in the vector representation.