This is the 3rd post in our series in which we have asked a number of leading scientists in our field to identify the 3-5 most important challenges which the field of hot and dense QCD matter theory has to address (click for the previous posts by Larry McLerran and Carsten Greiner).
Nu Xu (Lawrence Berkeley Lab & spokesperson of the STAR collaboration):
The Holy Grail for high-energy nuclear collisions is the creation of a new form of matter with partonic degrees of freedom. In the past seven years, experimental results have demonstrated that a hot and dense system has been created in heavy ion collisions at RHIC. The system also shows a strong collectivity that has developed at the early partonic stage of the collision:
Strictly speaking, the term MATTER signifies a state in equilibrium. However, in my view, the question of local thermalization in high-energy nuclear collisions has not yet been answered. We do not know when (and how) the system reaches equilibrium nor how long the system remains in equilibrium during its evolution. This question remains an important challenge to both theory and experiment in the era of LHC and RHC II. The future development of theory depends on the answer to this question. Here I give two examples:
- In LGT calculations (see e.g. here), the equilibration is an intrinsic assumption in the calculation. In order to perform a comparison of the data with the lattice results, one must have an adequate understanding of the equilibrium of the system;
- recently, there have been intense discussions on the application of the AdS/CFT duality to QCD and high-energy nuclear collisions (see .e.g here). Again, in these calculations, thermal equilibrium of the medium is assumed. Without a clear knowledge about the nature of the equilibrium of the medium created in high-energy nuclear collisions, the comparison between data and the results from above calculations are meaningless.
The progress of experiments also depends on the answer to the question of thermalization. At both SPS (with the NA61 experiment) and RHIC, there are experimental plans to search for the tri-critical point and/or the phase transition in the QCD phase diagram. The whole concept of phase implies the underlying equilibration of the matter. The understanding of the QCD phase diagram will not be complete if one does not have the control over thermalization in heavy ion collisions.
The second challenge is the process of hadronization in high-energy nuclear collisions. This question address the issues related to how the system, may it be may or not be in equilibrium, is transferred from a parton dominated to a hadron dominated state. Quark coalescence or recombination models have been proposed in analyzing the RHIC data (see the Parton Recombination Primer). However, the microscopic dynamics of the model are not fully understood. For example, we do not know when (and how) the gluon degrees of freedom disappear in the hadronization process. We also do not know when this approach should stop and the ‘normal’ fragmentation should take over at the high transverse momentum region.
I think these are the two most important issues in our field. Many other questions are all linked to these two. In the near future, experimentally, the heavy quark upgrade and Energy Scan Program at RHIC and the LHC heavy ion results will shed lights on both questions.