On internal mechanical properties of Electroweak Magnetic Monopoles and their effects on stability

Abstract: By considering properties of the energy-momentum tensor of the electroweak magnetic monopole and its Born-Infeld extension, we attempt to make comments on the stability of these configurations. Specifically, we perform a study of the behaviour of the so-called internal force and pressure of these extended field-theoretic solitonic objects, which are derived from the energy-momentum tensor. Our method is slightly different from the so-called Laue's criterion for stability of nuclear matter, a local form of which had been proposed and applied in the earlier literature to the `t Hooft-Polyakov (HP) magnetic monopole, and found to be violated.By applying our method first to HP monopole, we also observe that, despite its topological stability, the total (finite) internal force (which has only radial components) is directed inwards, towards the centre of the monopole, which would imply instability. Thus this mechanical criterion for stability is arguably violated in the case of the HP monopole, as is the local version of Laue's criterion. The criterion is satisfied for the short-range part of the energy momentum tensor, in which the long-range part, due to the massless photon of the U(1) subgroup, is subtracted. Par contrast, the total internal force of the Cho-Maison (CM) electroweak monopole has both radial and angular components, which diverge at the origin, leading to rotational instabilities. Finally, by studying finite-energy extensions of the CM, either with non-minimal Higgs couplings with the hypercharge sector or hypercharge Born-Infeld type models, we find that the total force, integrated over space, is finite, but it has also angular components in the Born-Infeld case. The latter feature is interpreted as indicating that the Born-Infeld-CM monopole might be subject to rotations upon the action of perturbations, but it does not necessarily imply instabilities of the configuration.