When a proton collides with a proton say at the LHC, is vacuum field theory used? It seems like you shouldn't have to include temperature effects since there are only two particles. But then again, all experiments take place at finite temperature, the ambient temperature of the room? When a heavy ion collides with a heavy ion, I assume you have to use thermal field theory. But what if you collide them really slowly? Also, at high temperatures, symmetry of a scalar field can be restored, i.e., the Higgs vacuum expectation value can be zero again. Does this mean the particles in heavy ion collisions can be massless? I guess I'm confused about ambient temperature versus collision temperature, how many particles are required to define a thermal system versus a vacuum system, and also about the Higgs field: if the temperature of one part of the universe is really really high, do particles in the vicinity lose mass?