::This is the energy of protons stored in the collider. Energy is energy total relativistic energy counts if you're in a frame to see it. In this case, particles trapped in a collider add "invariant mass" to the collider equal to their rest mass PLUS their kinetic energy. In this case where the net linear momentum is zero, the relativistic energy/c^2 IS the rest mass of the system. The collider is heavier (if you could weigh it) by not only the weight of the protons, but their kinetic energy also.