Theta dependence in holographic QCD

Abstract

Abstract:We study the effects of the CP-breaking topological θ-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 θ regime, the related holographic instantonic soliton solutions. We find that while the baryon spectrum does not receive O(θ) corrections, this is not the case for observables like the electromagnetic form factor of the nucleons. In particular, it exhibits a dipole term, which turns out to be vector-meson dominated. The resulting neutron electric dipole moment, which is exactly the opposite as that of the proton, is of the same order of magnitude of previous estimates in the literature. Finally, we compute the CP-violating pion-nucleon coupling constantḡ πN N , finding that it is zero to leading order in the large N c limit.

Theta angle in holographic QCD

“…We stress, however, that V-QCD is more general, producing a model for the full m q dependence as these observables, for example. Similar results as those presented below have also been found recently in the Witten-Sakai-Sugimoto model [24]. The topological susceptibility χ can be defined as the second derivative of the free energy with respect toθ.…”

“…In this talk the construction of V-QCD and its application to the physics of axial anomaly and the θ-angle [22] will be reviewed. For description of CP-odd physics using top-down holography see [23,24] and references therein. For a brief, more generic introduction to V-QCD, see [25].…”

Theta Dependence in Yang-Mills from Holography

“…Moreover, as it is shown in [10], holographic tools allow to extract the θ-dependence of observables like the 't Hooft loop, the baryon mass (vertex) and the entanglement entropy, which are not under control using other techniques. It is also possible to introduce massive quarks and study the physics of the associated QCD-like holographic model, obtaining interesting results concerning for example the vacuum structure of the theory or the calculation of the neutron electric dipole moment [11].…”

Deuteron electric dipole moment from holographic QCD

“…Note that this contribution is consistently suppressed with respect to the isovectorial one, that scales as λ −2 [15], but not strongly, which allows to obtain the correct order of magnitude with extrapolation to phenomenological λ.…”

“…On the theory side instead, the holographic model of Witten-Sakai-Sugimoto (WSS) [12][13][14] has been used to successfully compute the electric dipole moments of the neutron and the proton [15,16]: Despite the computation leading to the old chiral Lagrangian cancellation issue (D n = −D p ), the result was obtained at leading order in few parameters of the theory, in particular neglecting time derivatives, leaving open the possibility of the appearance of a splitting in the magnitudes of the electric dipole moments of the nucleons at the next-to-leading order or beyond.…”