Disoriented Chiral Condensate Formation in Heavy Ion Collisions?

One of the main aims of present and upcoming high energy heavy ion collision experiments is to study new phases of matter at extreme temperature and density. It is expected that a nontrivial classical pion field configuration can occasionally form during the out-of-equilibrium chiral phase transition. We have recently shown that, contrarily to what has been assumed so far, this configuration is not identical to the so-called disoriented chiral condensate (DCC), proposed in the early 1990’s. A detailed analysis reveals that a more realistic picture is that of an “unpolarized” DCC, where the Fourier modes of the field have completely independent orientations in isospin space instead of being aligned with each other as in the original DCC. This has important implications concerning the possible detection of the phenomenon. In particular, the main expected signature of the original DCC, which is used in most experimental searches, is absent in the unpolarized case. We point out that the fact that no evidence of DCC formation has been reported so far in nuclear collisions actually agrees with our present theoretical understanding. New experimental strategies should be designed to look for the unpolarized DCC in existing data from SPS as well as in future searches at RHIC and LHC. Talk at TH-2002, ’International Conference on Theoretical Physics’, Paris, France. email: [email protected] 1 The Disoriented Chiral Condensate Large number of pions with energies below or of the order of a few hundred MeV are commonly produced in high energy hadronic or nuclear collisions experiments. Very soon, it has been proposed to interpret this phenomenon as resulting from classical radiation, in analogy with the classical nature of electromagnetic waves corresponding to a large number of photons. This idea has received only a marginal attention until the early 1990’s where it has been rediscovered and further developed, on solid theoretical grounds [1, 2, 3]. In Ref. [2], Blaizot and Krzywicki studied the dynamics of a classical pion field configuration described by the non-linear sigma model, in the context of a simple idealized model of a high energy collision. They found a solution where the classical pion field oscillates coherently in a given direction in isospin space. In parallel, Bjorken proposed that, in such collisions, the chiral quark condensate could be momentarily misaligned from its vacuum orientation in isospin space, resulting in a non-trivial long wavelength configuration of the pion field [3]. This intuitive picture corresponds to Blaizot and Krzywicki’s solution. The “disoriented chiral condensate” (DCC) subsequently decays toward ordinary vacuum by coherent (classical) radiation of soft pions. The emitted pions can be though of as being in a coherent state [4], schematically:r