The phase diagram of T(T)over-bar-deformed Yang-Mills theory on the sphere

We study the large-N dynamics of T (T) over bar -deformed two-dimensional Yang-Mills theory at genus zero. The 1/N-expansion of the free energy is obtained by exploiting the associated flow equation and the complete phase diagram of the theory is derived for both signs of the rescaled deformation parameter tau. We observe a third-order phase transition driven by instanton condensation, which is the deformed version of the familiar DouglasKazakov transition separating the weakly-coupled from the strongly-coupled phase. By studying these phases, we compute the deformation of both the perturbative sector and the Gross-Taylor string expansion. Nonperturbative corrections in tau drive the system into an unexplored disordered phase separated by a novel critical line meeting tangentially the Douglas-Kazakov one at a tricritical point. The associated phase transition is induced by the collision of large- N saddle points, determining its second-order character.