Complex sentence: That is why it makes sense to build circular accelerators for heavy particles--hadron colliders such as the LHC for protons or, alternatively, for lead nuclei; an electron-positron collider of the same size would never be able to achieve the same collision energies.
Split into simpler sentences: That is why it makes sense to build circular accelerators for heavy particles--hadron colliders such as the LHC for protons or, alternatively, for lead nuclei. An electron-positron collider of the same size would never be able to achieve the same collision energies.