Take this form of the QCD Phase Diagram for example:
This baryon density is a number density - i.e. number of baryons in some volume.
Why are baryon density and temperature regarded as independent parameters in the QCD Phase Diagram?
The following qualitative reasoning is the source of the trouble:
For conventional matter, the number density and temperature are clearly not independent. If you compress a fixed number of molecules of a gas, thereby increasing the number density, molecules would be closer together, more collisions and hence, higher temperature.
What is the physical reason/ justification for the same, in the context of QCD?
Is it related to the time-scale of a high energy collision (a.k.a. the lab for these sort of things) - i.e. produced particles can't diffuse out fast enough and hence, can be simultaneously hot and dense, at least for the time scale of the collision? (Clearly, this rough hand-waving argument can't be the true reason. So, what is it?)
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