Isobar excitation in parity-nonconserving proton-proton scattering

A parity violating (PV) meson exchange model is applied in the coupled channels formalism to study the effect of isobar excitation on PV pp scattering. The meson-mixing model used to calculate PV forbids the mixing of S=0 mesons in particular pions, as well as isospin symmetry violation, so only the ${\mathit{pa}}_1$ contribution is considered in isobar excitation. We find a significant contribution to A${\mathrm{¯}}_{\mathit{L}}$ from the PV mixing of the states ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_2$ and ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ already at relatively low energies versus one (vector) meson exchange which gives predominantly ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_{0\mathrm{-}}^1$ ${\mathit{S}}_0$ mixing. At the energy of 230 MeV, where a forthcoming TRIUMF experiment aims to extract the parity violating ρpp coupling, our calculated effect from this source alone is of the same order as from the conventional single ρ exchange.