A composite massless vector boson

Abstract

In a non-perturbative gauge-invariant formulation of grand-unified theories all low energy vector states need to be composite with respect to the high-scale gauge group, including the photon. We investigate this by using lattice methods to spectroscopically analyze the vector channel in a toy grand-unified theory, an SU(2) adjoint Higgs model. Our results support indeed the existence of a massless composite vector particle.

The spectrum of GUT-like gauge-scalar models

“…Further, we confirm previous results in other theories [11,27] that in the 'BEH-like' case the lightest state is a vector, while otherwise it is a scalar particle. In the elementary spectrum, this is not necessary.…”

“…So far, previous investigations [24] suggest that this is not caused by nonperturbative, strong interactions in the unbroken SU(2) group. Ungapped physical spectra are, however, also possible and have been observed in other theories with a BEH effect but otherwise the same obstructions as for the present one [27].…”

“…A parallel ongoing project is to repeat the spectroscopy for a similar model with the scalar in the adjoint representation. The adjoint case is phenomenologically interesting due to the multiple possible breaking patterns of the gauge group [3], but is computationally much more expensive due to the presence of massless modes and the need for finite-momentum states and improved discretisation of operators [10,27]. In the adjoint case, the operators no longer map as cleanly onto elementary states, instead corresponding to more complex Wilson loops and glueballs with insertions of scalars at different points.9 Such operators are expected to be even noisier than those for the fundamental case.…”