Theta dependence in Holographic QCD

Understand

We study the effects of the CP-breaking topological $\theta$-term in the large $N_c$ QCD model by Witten, Sakai and Sugimoto with $N_f$ degenerate light flavors.

• We first compute the ground state energy density, the topological susceptibility and the masses of the lowest lying mesons, finding agreement with expectations from the QCD chiral effective action.
• Then, focusing on the $N_f=2$ case, we consider the baryonic sector and determine, to leading order in the small $\theta$ regime, the related holographic instantonic soliton solutions.
• We find that while the baryon spectrum does not receive ${\cal O}(\theta)$ corrections, this is not the case for observables like the electromagnetic form factor of the nucleons.