I'm familiar with the formalisms for both zero temperature and finite temperature field theory, but (somewhat embarrassingly) I don't actually have a good physical intuition for when physical scenarios call for one set of methods vs. the other.
For instance, at say the LHC it's my impression that despite the huge energies involved during collisions, zero temperature field theory is appropriate for all relevant calculations. One the other hand (I could certainly be wrong here, again), it's my impression that heavy ion collisions such as those performed at RHIC are suited to finite temperature calculations instead.
So my questions are as follows:
1) Generically, how does one know when to use finite vs. zero temperature field theory?
2) Can one a priori determine whether finite temperature corrections will be important?
3) If my impression are correct about the LHC and RHIC, is there a quick, back-of-the-envelope calculation which demonstrates that zero temperature and finite temperature field theory should be used in these two scenarios, respectively?