Spontaneous Symmetry Breaking from an On-Shell Perspective

We show how the well known patterns of masses and interactions that arise from spontaneous symmetry breaking can be determined from an entirely on-shell perspective, that is, without reference to Lagrangians, gauge symmetries, or fields acquiring a vacuum expectation value. To do this, we review how consistent factorization of $2\rightarrow 2$ tree level scattering can lead to the familiar structures of Yang-Mills theories, and extend this to find structures of Yukawa theories. Considering only spins-$0$, $1/2$ and $1$ particles, we construct all the allowed on-shell UV amplitudes under a symmetry group $G$, and consider all the possible IR amplitudes. By demanding that on-shell IR amplitudes match onto on-shell UV amplitudes in the high energy limit, we reproduce the Higgs mechanism and generate masses for spins-$1/2$ and $1$, find that there is a subgroup $H \subseteq G$ in the IR, and other interesting relations. To highlight the results, we show the breaking pattern of the Standard Model $U(1)_{EM} \subset SU(2)_L \times U(1)_Y$, along with the generation of the masses and interactions of the particles.