Large N Volume Independence and an Emergent Fermionic Symmetry

Large-N volume independence in circle-compactified QCD with adjoint Weyl fermions implies the absence of any phase transitions as the radius is dialed to arbitrarily small values. This class of theories are believed to possess a Hagedorn density of hadronic states. It turns out that these properties are in apparent tension with each other, because a Hagedorn density of states typically implies a phase transition at some finite radius. This tension is resolved if there are degeneracies between the spectra of bosonic and fermionic states, as happens in the Nf = 1 supersymmetric case. Resolution of the tension for Nf > 1 then suggests the emergence of a fermionic symmetry at large N , where there is no supersymmetry. We can escape the Coleman-Mandula theorem since the N = ∞ theory is free, with a trivial S-matrix. We show an example of such a spectral degeneracy in a non-supersymmetric toy example which has a Hagedorn spectrum.