<Emphasis Type="Italic">J/Ψ</Emphasis>production

For more than 25 yearsJΨ production has helped to sharpen our understanding of QCD. In proton induced reaction some observations are rather well understood while others are still unclear. The current status of the theory ofJΨ production will be sketched, paying special attention to the issues of formation time andJΨ re-interaction in a nuclear medium.     

Plasma damping effects on the radiative energy loss of relativistic particles

The energy loss of a relativistic charge undergoing multiple scatterings while traversing an infinite, polarizable and absorptive plasma is investigated. Polarization and absorption mechanisms in the medium are phenomenologically modeled by a complex index of refraction. Apart from the known Ter-Mikaelian effect related to the dielectric polarization of matter, we find an additional, substantial reduction of the energy loss due to the damping of radiation. The observed effect is more prominent for larger damping and/or larger energy of the charge. A conceivable analog of this phenomenon in QCD could influence the study of jet quenching phenomena in ultrarelativistic heavy-ion collisions at RHIC and LHC.     

Importance of gluon-gluon and quark-antiquark contributions to charmonium production in proton-proton and antiproton-proton collisions

The importance of the gluon-gluon and quark-antiquark contributions to the production of charmonium states in proton-proton and antiproton-proton collisions is determined within the parton model in a way which does not rely on any assumption on the dynamics (leading diagrams, color neutralisation mechanism,h). It is shown that the combined analysis of total and differential inclusiveJ/ψ production cross-sections for both systems allows such a determination. The primordial contributions are also extracted at one energy. The implications of the numerical values are also discussed.     

Influence of the internal degrees of freedom onJ/ψ suppression

The influence of the internal degrees of freedom on theJ/ψ suppression in relativistic heavy ion collisions is studied in the frame of a quantum-mechanical model. The wave function for the internal motion of ac?c \(\bar c\) pair obeys a time-dependent Schrödinger equation with a potential reflecting the properties of the medium in which the pair is travelling. The initial wave function is evaluated theoretically. An imaginary potential is introduced to account for the loss of probability due to the coupling to theD?c \(\bar D\) channels. TheJ/ψ survival probability is estimated as a function of the time spent inside the plasma. The connection with semi-classical approximations based on the formation time concept is established. The quantum-mechanical effects are exhibited and shown to lead to a smooth perpendicular momentum dependence of theJ/ψ suppression, in agreement with the recent reanalysis of the NA38 data by Gupta and Satz. Several plasma scenarios, including or not the presence of a mixed phase are investigated and the effect of the quantum-mechanical treatment is analyzed for each of them. It is shown that the data do not constraint the plasma scenario very strongly, but indicate the possibility of having a mixed phase with a rather long lifetime.     

Preconditioned iterative methods for coupled discretizations of fluid flow problems

Computational fluid dynamics, where simulations require largecomputation times, is one of the areas of application of highperformance computing. Schemes such as the SIMPLE (semi-implicitmethod for pressure-linked equations) algorithm are often usedto solve the discrete Navier-Stokes equations. Generally theseschemes take a short time per iteration but require a largenumber of iterations. For simple geometries (or coarser grids)the overall CPU time is small. However, for finer grids or morecomplex geometries the increase in the number of iterationsmay be a drawback and the decoupling of the differential equationsinvolved implies a slow convergence of rotationally dominatedproblems that can be very time consuming for realistic applications.So we analyze here another approach, DIRECTO, that solves theequations in a coupled way. With recent advances in hardwaretechnology and software design, it became possible to solvecoupled Navier-Stokes systems, which are more robust but implyincreasing computational requirements (both in terms of memoryand CPU time). Two approaches are described here (band blockLU factorization and preconditioned GMRES) for the linear solverrequired by the DIRECTO algorithm that solves the fluid flowequations as a coupled system. Comparisons of the effectivenessof incomplete factorization preconditioners applied to the GMRES(generalized minimum residual) method are shown. Some numericalresults are presented showing that it is possible to minimizeconsiderably the CPU time of the coupled approach so that itcan be faster than the decoupled one.     

Charmonium cross section from p—A production

Several extractions of the ?-N cross section from ? production in γ+A and h+A reactions have been attempted. We discuss these and the issue of charmonium formation using the recent data obtained by E866/NuSea.     

Heavy quark energy loss in high multiplicity proton-proton collisions at the LHC

One of the most promising probes to study deconfined matter created in high energy nuclear collisions is the energy loss of (heavy) quarks. It has been shown in experiments at the Relativistic Heavy Ion Collider that even charm and bottom quarks, despite their high mass, experience a remarkable medium suppression in the quark gluon plasma. In this exploratory investigation we study the energy loss of heavy quarks in high multiplicity proton-proton collisions at LHC energies. Although the colliding systems are smaller than compared to those at the Relativistic Heavy Ion Collider (p+p vs Au+Au), the higher energy might lead to multiplicities comparable to Cu+Cu collisions at the Relativistic Heavy Ion Collider. The interaction of charm quarks with this environment gives rise to a non-negligible suppression of high momentum heavy quarks in elementary collisions.