The concept of quasi-integrability has been examined in the context of deformations of the defocusing non-linear Schrödinger model (NLS). Our results show that the quasi-integrability concept, recently discussed in the context of deformations of the sineGordon, Bullough-Dodd and focusing NLS models, holds for the modified defocusing NLS model with dark soliton solutions and it exhibits the new feature of an infinite sequence of alternating conserved and asymptotically conserved charges. For the special case of two dark soliton solutions, where the field components are eigenstates of a space-reflection symmetry, the first four and the sequence of even order charges are exactly conserved in the scattering process of the solitons. Such results are obtained through analytical and numerical methods, and employ adaptations of algebraic techniques used in integrable field theories. We perform extensive numerical simulations and consider the scattering of dark solitons for the cubic-quintic NLS model with potential V = ηI 2 − 6 I 3 and the saturable type potential satisfying V [I] = 2ηI − I q 1+I q , q ∈ Z Z + , with a deformation parameter ∈ IR and I = |ψ| 2 . The issue of the renormalization of the charges and anomalies, and their (quasi)conservation laws are properly addressed. The saturable NLS supports elastic scattering of two soliton solutions for a wide range of values of {η, , q}. Our results may find potential applications in several areas of non-linear science, such as the Bose-Einstein condensation.

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We derive a closed expression for the spatial shift experienced by a black soliton colliding with a shallow dark soliton in the context of deformed non-linear Schrödinger models. A perturbative scheme is developed based on the expansion parameter 1/(γv) << 1, where v is the velocity of the incoming shallow dark soliton, γ ≡ 1/ 1 − v 2 /v 2 s and vs is the Bogoliubov sound speed, therefore it is not restricted to small deformations of the integrable NLS model. As applications of our formalism we consider the integrable NLS model and the non-integrable cubic-quintic NLS model with non-vanishing boundary conditions. Extensive numerical simulations are performed in order to verify our results. A variant of the analysis for gray-gray soliton collision is discussed regarding a fast broad soliton and a slow thin soliton.