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تسجيل مستخدم جديدHeavy Ion Collisions: Achievements and Challenges
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تأليف
Edward Shuryak
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A decade ago brief summary of the field could be formulated as a discovery of strongly-coupled Quark-Gluon-Plasma, sQGP, making a very good liquid with surprisingly small viscosity. Since 2010 we have LHC program, which added a lot to our understanding, and now there seems to be a need to consolidate what we learned and formulate a list of issues to be studied next. Hydrodynamical perturbations, leading to higher harmonics of angular correlations, are identified as long-lived sound waves. Recently studied reactions involving sounds include phonon decays into two (loop viscosity), phonon+magnetic field into photons/dileptons (sono-magneto-luminescence), and two phonons into a gravity wave, a penetrating probe of the Big Bang. The mainstream issues in the field now include a quest to study transition between $pp,pA$ and heavy ion $AA$ collisions, with an aim to locate the smallest drops of the sQGP displaying collective/hydrodynamics behavior. The issues related to out-of-equilibrium stage of the collisions, and mechanisms of the equilibration, in weak and strong coupling, are also hotly debated.
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There are interesting parallels between the physics of heavy ion collisions and cosmology. Both systems are out-of-equilibrium and relativistic fluid dynamics plays an important role for their theoretical description. From a comparison one can draw i
nteresting conclusions for both sides. For heavy ion physics it could be rewarding to attempt a theoretical description of fluid perturbations similar to cosmological perturbation theory. In the context of late time cosmology, it could be interesting to study dissipative properties such as shear and bulk viscosity and corresponding relaxation times in more detail. Knowledge and experience from heavy ion physics could help to constrain the microscopic properties of dark matter from observational knowledge of the cosmological fluid properties.
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The shapes of invariant differential cross section for charged particle production as function of transverse momentum measured in heavy-ion collisions are analyzed. The data measured at RHIC and LHC are treated as function of energy density according
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quarks in the Quark-Gluon Plasma and here extended to include a modeling of their hadronization in the presence of a medium. Hadronization is described as occurring via the fragmentation of strings with endpoints given by the heavy (anti-)quark Q(Qbar) and a thermal parton qbar(q) from the medium. The flow of the light quarks is shown to affect significantly the R_AA and v_2 of the final D mesons, leading to a better agreement with the experimental data. The approach allows also predictions for the angular correlation between heavy-flavour hadrons (or their decay electrons) and the charged particles produced in the fragmentation of the heavy-quark strings.
Heavy flavor supplies a chance to constrain and improve the hadronization mechanism. We have established a sequential coalescence model with charm conservation and applied it to the charmed hadron production in heavy ion collisions. The charm conserv
ation enhances the earlier hadron production and suppresses the later production. This relative enhancement (suppression) changes significantly the ratios between charmed hadrons in heavy ion collisions.
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