Pol-Bernard Gossiaux

Charmonium suppression in p-A collisions

The new high precision data on charmonium production in proton-nucleus collisions by the E866/NuSea Collaboration at Fermilab allow us---together with older data at lower energies---to fix a unique set of parameters for the standard production and absorption scenario of charmonium in a proton-nucleus reaction. In this scenario the

Heavy quarkonium hadron cross-section in QCD at leading twist

c\overline{c}

Retardation Effect for Collisional Energy Loss of Hard Partons Produced in a QGP

pair is formed in an octet state, emits a gluon, and continues its radial expansion in a singlet state until it has reached the charmonium radius. In all three phases it can interact with the nuclear environment. We find that the lifetime of the octet state is much shorter than acceptable on physical grounds. This challenges the physical reality of the first phase in the standard scenario.

The multifragmentation of spectator matter

We compute the total cross section of a heavy quarkonium on a hadron target in leading twist QCD, including target mass corrections. Our method relies on the analytical continuation of the operator product expansion of the scattering amplitude, obtained long ago by Bhanot and Peskin. The cross section has a simple partonic form, which allows us to investigate the phenomenology of J/psi and Upsilon dissociation by both pions and protons.

Learning by Doing: A Teaching Method for Active Learning in Scientific Graduate Education.

We study the collisional energy loss suffered by an energetic parton travelling the distance L in a high temperature quark-gluon plasma and initially produced in the medium. We find that the medium-induced collisional loss ??E(L) is strongly suppressed compared to previous estimates which assumed the collisional energy loss rate ?dE/dx to be constant. The large L linear asymptotic behaviour of ??E(L) sets in only after a quite large retardation time. The suppression of ??E(L) is partly due to the fact that gluon bremsstrahlung arising from the initial acceleration of the energetic parton is reduced in the medium compared to vacuum. The latter radiation spectrum is sensitive to the plasmon modes of the quark-gluon plasma and has a rich angular structure.

Dynamical collisional energy loss and transport properties of on- and off-shell heavy quarks in vacuum and in the Quark Gluon Plasma

Abstract We present the first microscopic calculation of the spectator fragmentation observed in heavy-ion reactions at relativistic energies which reproduces the slope of the kinetic energy spectra of the fragments as well as their multiplicity, both measured by the ALADIN collaboration. In the past both have been explained in thermal models, however with vastly different assumptions about the excitation energy and the density of the system. We show that both observables are dominated by dynamical processes and that the system does not pass a state of thermal equilibrium. These findings question the recent conjecture that in these collisions a phase transition of first order, similar to that between water and vapor, can be observed.

Energy loss of a heavy quark produced in a finite size medium

This article describes an active learning method for the teaching of physical sciences and mathematics to engineers. After defining the challenges involved in the training of engineers, we shall describe the answers provided by our method, ‘learning by doing’ (named ‘Apprentissage Par l’Action’ in French), by introducing four key points: real-life simulation, the management of non-success, the result requirement and the different roles of the teachers. An assessment of this experience is carried out which emphasizes the factors paramount in the success of this pedagogical innovation. Similarities between our experience and other well-known methods such as problem-based learning, problem solving and, more generally, the concept of learning by doing coined by John Dewey in his philosophy of education, are mentioned.

Transport coefficients of heavy quarks around

In this study we evaluate the dynamical collisional energy loss of heavy quarks, their interaction rate, as well as the different transport coefficients (drag and diffusion coefficients, q , etc). We calculate these different quantities for (i) perturbative partons (on-shell particles in the vacuum with fixed and running coupling) and (ii) for dynamical quasi-particles (off-shell particles in the QGP medium at finite temperature T with a running coupling in temperature as described by the dynamical quasiparticles model). We use the perturbative elastic [ q ( g ) Q → q ( g ) Q ] cross section for the first case and the infrared enhanced hard thermal loop cross sections for the second. The results obtained in this work demonstrate the effects of a finite parton mass and width on the heavy-quark transport properties and provide the basic ingredients for an explicit study of the microscopic dynamics of heavy flavors in the QGP–as formed in relativistic heavy-ion collisions–within transport approaches developed previously by the authors.

T_c

We study the medium-induced energy loss –ΔE0(Lp) suffered by a heavy quark produced at initial time in a quark-gluon plasma, and escaping the plasma after travelling the distance Lp. The heavy quark is treated classically, and within the same framework –ΔE0(Lp) consistently includes: the loss from standard collisional processes, initial bremsstrahlung due to the sudden acceleration of the quark, and transition radiation. The radiative loss induced by rescatterings –ΔErad(Lp) is not included in our study. For a ultrarelativistic heavy quark with momentum p 10 GeV, and for a finite plasma with Lp 5 fm, the loss –ΔE0(Lp) is strongly suppressed compared to the stationary collisional contribution –ΔEcoll(Lp) ∝ Lp. Our results support that –ΔErad is the dominant contribution to the heavy quark energy loss (at least for Lp 5 fm), as indeed assumed in most of jet-quenching analyses. However they might raise some question concerning the RHIC data on large p⊥ electron spectra.

at finite quark chemical potential

The interactions of heavy quarks with the partonic environment at finite temperature