Solitary waves in a Two Parameter Family of Generalized Nonlinear Dirac Equations in $1+1$ Dimensions

Abstract: We obtain exact solutions of the nonlinear Dirac equation in 1+1 dimension of the form $\Psi(x,t) = \Phi(x) e^{-i \omega t}$ where the nonlinear interactions are a combination of vector-vector (V-V) and scalar-scalar (S-S) interactions with the interaction Lagrangian given by $L_I= \frac{g^2}{(\kappa+1)}(\bar{\psi} \psi)^{\kappa+1} -\frac{g^2}{p(\kappa+1)}[\bar{\psi} \gamma_{\mu} \psi \bar{\psi} \gamma^{\mu} \psi]^{(\kappa+1)/2}$. This generalizes the model of ABS (N.V. Alexeeva, I.V. Barashenkov and A. Saxena, Annals Phys. {\bf 403}, 198, (2019)) by having the arbitrary nonlinearity parameter $\kappa>0$ and by replacing the coefficient of the V-V interaction by the arbitrary positive parameter $p>1$ which alters the relative weights of the vector-vector and the scalar-scalar interactions. We show that the solitary wave solutions exist in the entire allowed $(\kappa,p)$ plane for $\omega/m > 1/p^{1/(\kappa+1)} $, for frequency $\omega$ and mass $m$. These solutions have the property that their energy divided by their charge is $\it {independent} $ of the coupling constant $g$. As $\omega$ increases, there is a transition from the double humped to the single humped solitons. We discuss the regions of stability of these solutions as a function of $\omega,p,\kappa$ using the Vakhitov-Kolokolov criterion. Finally we discuss the non-relativistic reduction of the 2-parameter family of generalized ABS models to a modified nonlinear Schr\"odinger equation (NLSE) and discuss the stability of the solitary waves in the domain of validity of the modified NLSE.