Symmetry breaking and axionic strings in the warped deformed conifold

We interpret D-strings at the bottom of the warped deformed conifold as axionic strings in the dual cascading SU(N + M) × SU(N) gauge theory. The axion is a massless pseudo-scalar glueball which we find in the supergravity fluctuation spectrum and interpret as the Goldstone boson of spontaneously broken U(1) baryon number symmetry. The existence of this massless glueball, anticipated in [1], supports the idea that the cascading gauge theory is on the baryonic branch, i.e. the U(1)s global symmetry is broken by expectation values of baryonic operators. We also find a massless scalar glueball, which is a superpartner of the pseudo-scalar. This scalar mode is a mixture of an NS-NS 2-form and a metric perturbation of the warped deformed conifold of a type first considered in [2].