BPS Equations of Monopole and Dyon in $SU(2)$ Yang-Mills-Higgs Model, Nakamula-Shiraishi Models, and Their Generalized Versions from The BPS Lagrangian Method

Abstract

We apply the BPS Lagrangian method~\cite{Atmaja:2015umo} to derive BPS equations of monopole and dyon in the $SU(2)$ Yang-Mills-Higgs model, Nakamula-Shiraishi models, and their Generalized versions. We argue that by identifying the effective fields of scalar field, $f$, and of time-component gauge field, $j$, explicitly by $j=\beta f$ with $\beta$ is a real constant, the usual BPS equations for dyon can be obtained naturally. We validate this identification by showing that both Euler-Lagrange equations for $f$ and $j$ are identical in the BPS limit. The value of $\beta$ is bounded to $|\beta|<1$ due to reality condition on the resulting BPS equations. In the Born-Infeld type of actions, namely Nakamula-Shiraishi models and their Generalized versions, we find a new feature that adding the energy density by a constant $4b^2$, with $b$ is the Born-Infeld parameter, will turn monopole(dyon) to anti-monopole(anti-dyon) and vice versa. In all Generalized versions there are additional constraint equations that relate the scalar-dependent couplings of scalar and of gauge kinectic terms; or $G$ and $w$ respectively. For monopole the constraint equation is $G=w^{-1}$, while for dyon is $w(G-\beta^2 w)=1-\beta^2$ which further gives lower bound to $G$ as such $G\geq|2\beta\sqrt{1-\beta^2}|$. We also write down the complete square-forms of all effective Lagrangians.