http://arxiv.org/PS_cache/hep-.....5066v1.pdf

Start to read the new section 2 at the bottom. It says that everyone since the 1970s until recently, everyone assumed that QGP was a free gas of dressed quarks and gluons, and even when it looked inconsistent with some observations, people were still hoping.

All the papers that argue otherwise appeared in the 21st century.

Although it is true that asymptotic freedom implies that particles get free at high energies, it was rather clear from the beginning that the rigid gas approximation for QG plasma at RHIC will fail, because characteristic gluon occupation numbers after ion collision will be very large.

Cheers,
Dmitry.

In gases, all particles propagate independently – which also means in all directions. Only when interactions between the “molecules” become strong enough, one can have liquids or solids with preferred directions and clustering.

I don’t know exactly why the people expected a gas but I guess it’s roughly because the typical energies were rather high (at least the total ones), and they though that at high energies, quarks are free etc.

However, a liquid was observed, with some viscosity etc. This behavior is captured well (even quantitatively, to some extent) by AdS/QCD methods.