Final answer:
The particle produced in the principal decay mode of the sigma zero (Σ°) into lambda zero (Λ°) and a photon (γ) is the lambda zero (Λ°). This decay indicates that the sigma-zero is an excited state of the lambda-zero due to their quark composition.
Step-by-step explanation:
In the principal decay mode of the sigma zero, which is represented by the equation Σ°→Λ°+γ, the particle produced along with the photon (gamma ray) is the lambda zero (Λ°). This decay process does not involve the production of protons, neutrons, muons, or neutrinos. Instead, it involves a transition of a strange baryon (Σ°) to a less strange baryon (Λ°). The presence of the photon in the final state indicates that this is an electromagnetic decay process. Considering the quark structure of these baryons, the decay suggests that the sigma-zero particle is indeed an excited state of the lambda-zero particle. Both particles are composed of quarks, and the decay involves a change in their internal energy levels without changing the overall quark composition.