Madeleine Soyeur

Hydrodynamics of ultra-relativistic heavy ion collisions

Abstract The central rapidity regime in a central collision of two heavy ions at ultra-relativistic energies is expected, according to Bjorkens recent space-time picture of this event, to undergo a longitudinal hydrodynamic expansion with accompanying transverse rarefaction. This paper describes in detail the hydrodynamic equations governing this behavior, the reduction of their solutions to that of the central rapidity slice, certain analytic solutions, and for the case of constant sound velocity, numerical and approximate analytic solutions for the full expansion. We compare the numerical results with previous calculations within the Landau hydrodynamic model of a spherically expanding collision volume, and find that matter in the present cylindrical geometry undergoes a more rapid cooling in the interior; a quark-gluon plasma formed in the collision will thus rapidly hadronize. Effects of the hadronization transition on the hydrodynamic evolution and the possibility of shock formation, as well as effects of transverse expansion on expected transverse momentum distributions, are briefly discussed.

Photoproduction of vector mesons off nucleons near threshold

We propose a simple meson-exchange model of the photoproduction of ϱ- and ω-mesons off protons near threshold (Eγ ≲ 2 GeV). This model provides a good description of the available data and implies a large ϱ-nucleon interaction in the scalar channel (σ-exchange). We use this phenomenological interaction to estimate the leading contribution to the self-energy of ϱ-mesons in matter. The implications of our calculation for experimental studies of the ϱ-meson mass in nuclei are discussed.

In-medium ω-meson broadening and s-wave pion annihilation into e+e− pairs

Abstract An ω-meson in motion with respect to a nuclear medium can couple to a σ-meson through a particle-hole excitation. This coupling is large. We investigate its consequences for the width of ω-mesons in matter and for the s-wave annihilation of pions into lepton pairs which can take place in relativistic heavy ion collisions. We find that the two pion decay of ω-mesons, resulting from the ω-σ transition and the subsequent 2π decay of the σ-meson, leads to a substantial broadening of ω-mesons in matter and possibly to an observable effect in experiments measuring the e + e − decay of vecor mesons produced in nuclei and in relativistic heavy-ion collisions. The inverse process, the s-wave annihilation of pions into ω-mesons decaying into e + e − pairs, has in general a much smaller cross section than the corresponding p-wave annihilation through ϱ-mesons and is expected to contribute rather little to the total e + e − pair production in relativistic heavy ion collisions.

A meson-exchange model of the associated photoproduction of vector mesons and N∗(1440) resonances

Abstract We discuss the photoproduction of ω- and ρ0-mesons off protons in the particular channel where the target proton is excited to a Roper resonance N ∗ (1440). We propose a simple meson-exchange model for these processes in order to evaluate their cross sections near threshold and at low momentum transfers. It is suggested in particular that the differential cross section for the associated photoproduction of a ρ0-meson and a Roper resonance in these kinematic conditions could provide direct information on the strength of the scalar–isoscalar excitation of the N ∗ (1440) and hence on the magnitude of an effective σNN ∗ (1440) coupling. The latter quantity is poorly known and of much interest for the nuclear many-body problem.

Collective spin-isospin vibrations and δ-excitation in heavy-ion collisions

Abstract A coherent mechanism for the excitation of spin-isospin modes in heavy ion reactions is presented and discussed in two kinematic regimes. The spin-isospin response of nuclei in the particle-hole and in the Δ-hole channels is first studied in charge exchange reactions induced by relativistic heavy ions. Recent data on the reaction 12 C[ 12 C, 12 N(1 + ; GS)] at 900 and 1100 MeV per nucleon are particularly suited to study these modes and can be well described by the exchange of effective phonons between the projectile and the target; these effective phonons are built up from the summed interactions between the individual nucleons of the colliding ions. This mechanism in the Δ-hole channel is then used to derive an estimate of the cross section for subthreshold pion production in the 12 C[ 12 C, 12 C(1 + ; T = 1; 15.11 MeV)] reaction at 95 MeV per nucleon.

The associated photoproduction of positive kaons and π0Λ or πΣ pairs in the region of the Σ(1385) and Λ(1405) resonances

The

Quantum interference of rho0- and omega-mesons in the pi N -> e+e- N reaction

gamma p to K^+ pi^0 Lambda

Meson Photoproduction Near Threshold

and

Phases, quantum interferences and effective vector meson masses in nuclei

gamma p to K^+ pi Sigma

Quantum interference in the e^+e^- decays of rho^0 and omega-mesons produced in pi^- p reactions

reactions are studied in the kinematic region where the