Quark-gluon plasma: status 
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*Can we say that a QGP has been observed or is there only suggestive evidence?

*Is the idea that string theory, through the AdS/CFT correspondence, could help to understand this new state of matter still in the game?

*What can we hope from LHC and RIHC in the coming years?
 A: These are more questions than anybody can answer conclusively in a forum like this. I will nevertheless make an attempt at a brief comment.
1) The transition between the hadronic gas and quark gluon plasma is now known to be a smooth (but rapid) crossover, so there are no completely sharp observables that distinguish the two. There are, however, several observables that differ significantly between the two phases.
2) It was observed, for example, that the equation of state of state of the matter produced at RHIC and the LHC has a speed of sound (related to the equation of state) that approaches $c_s\simeq c/\sqrt{3}$, as expected for a QGP, and larger than that of a hadron gas. The measured temperature is higher than the limiting temperature of a hadron gas. The viscosity to entropy ratio $\eta/s$ is significantly below that of a hadron gas, and the energy loss parameter $\hat{q}$ is higher. 
3) The ongoing program at RHIC is aimed at finding direct evidence for a phase transition by pushing into the regime of finite baryon density, where the crossover is expected to turn into a first order phase transition, with an associated critical endpoint, There is also a program of finding more direct evidence for a chirally restored plasma by observing anomalous hydrodynamic effects. 
4) Both RHIC and LHC have ongoing programs of more accurately determining properties of the QGP, of characterizing the initial state and the equilibration mechanism, and of determining how far the thermo/hydrodynamic description can be pushed towards small systems. LHC will soon achieve the hottest plasma ever created, and this will help in mapping out the temperature dependence of observables like $\eta/s$.
5) Much ink has been spilled regarding the uses of AdS/CFT. Note that the holographic dual of QCD is not known, so the applications of gauge/gravity duality fall into the following categories:
i) elaborating the general formalism of non-equilibrium effects in strongly couped fluids
ii) predicting dimensionless quantities that might be universal properties of strongly coupled plasmas
iii) building AdS/CFT inspired models.
Activity i) has been very productive, activity ii) has one famous success story, the prediction $\eta/s\simeq 1/(4\pi)$, which continues to fit the RHIC data extremely well, and activity iii) has a more mixed and controversial record of achievements. 
