They call this a “quark-gluon plasma”, which I love to say ... “quark-gluon plasma”, that is. I remember years ago they used to talk about the “temperature of hell” as being the theoretically maximum possible temperature, because any attempt to make it hotter would just cause more particles to be created, which subtracts from the kinetic energy, thus cooling it.
The quark-gluon plasma is in this realm. They make graphs of the “chemical potential” ( a misnomer surely! ) of the quarks and gluons, which is a quantitative way of dealing with the “maximum temperature” idea.
It’s definitely a mind-blow, as we used to say.
As the velocity approaches c, the relativistic mass of each particle approaches infinity:
Which means the energy (and temperature) approach infinity. There is no such thing as a maximum theoretical temperature.