Sujet de thèse pour la rentrée 2017

Development of transport models : comparisons with experimental nuclear data and applications to astrophysical objects

Host Institution: Theory Group, Subatech, IMT Atlantique, Nantes

Supervisors: Dr Eric Bonnet and Virginia de la Mota

Research field: Transport models at Fermi energies and applications to astrophysics

Proposed starting date: September, 2017

Proposed ending date: August, 2020

The candidate will be involved in the development of transport models namely DYWAN and IQMD models. The complementary use of this two approaches will allow to progress and the overall comprehension of Heavy-Ion Collisions (HIC) at Fermi energies.

The comparison with the high quality experimental data from the INDRA collaboration and the forthcoming unique data collected within the FAZIA collaboration will allow to address different aspect like stopping and flow in HIC, fluctuations and correlations in the nuclei and the role of the symmetry energy in the equation of state of neutron rich nuclear matter.

Local context:
The candidate will be hosted in the Theory group of the Subatech laboratory. One main activity of the Theory group is the modelization of HIC from Fermi to Ultra-Relativistic energy scales with an active synergy with international experimental.

DYWAN is a one them and is dedicated to the the Fermi energy. It’s a microscopic model based on the e-TDHF approach. Briefly, a selfconsistent procedure is implemented to prepare nuclei on their ground states, from which the extracted nucleonic single particle wave functions are spanned in a coherent states basis. During the dynamical process of two heavy-ion collision the evolution of the system is given by the coherent states equations of motion. The later is ruled by the mean field, while the corresponding occupation numbers satisfy a master equation. The model is complemented by a cluster recognition algorithm and is being extended in order to incorporate higher order particle correlations.

Work plan:
In the continuation of its Master training, the candidate will work on the DYWAN model and will use it on different aspects :

1) Studying of the stopping in nuclear matter at Fermi energies :

Stopping is related to the sharing of initial longitudinal incident energy between perpendicular and longitudinal directions. Both experimental and theoretical works have shown that this quantity is related to different ingredients of transport models such as Nucleon-Nucleon cross section or Pauli blocking effect. These ingredients contribute together in the so-called collision term. In addition, the mean field plays also a role

by the choice of the effective nuclear force parametrization. At the end, to draw a coherent picture between experimental data and theoretical approaches, one has to test individual contribution by the mean of systematic calculations to study the stopping in a large range of incident energy and mass. Associated observables related to the stopping like fusion cross-section can bring complementary constrains on the ingredients mentioned above. The candidate will be in charge of this aspect.

2) Investigation on the dynamic of multifragmentation :

one specific process observed in Fermi energy domain is the multifragmentation. This phenomenon can be described as a spontaneous breaking of nuclear matter in many pieces. This process is associated to the more general question of the clustering of nuclear matter. Copious experimental and theoretical works have been published since the last 30 years. Anyway, one key issue remains largely debated and concerned the dynamics of clusterisation process. The role of fluctuations in transport models is a crucial point and general efforts are ongoing from the theoretical side. The candidate will be in charge of the implementation of the fluctuations in DYWAN. Detailed comparison with IQMD outputs and with experimental data on light cluster production and multifragmentation properties will be achieved.

3) Application to astrophysics and dynamical description of compact stellar objects :

the approach has been recently adapted to the description of the nuclear medium in the outermost layers of neutron stars. The purpose of this work is to investigate the properties of these extremely dense stars since their birth until their cooling down. The transport properties of the stellar matter and their corresponding sensibility to the nuclear equation of state will be studied.

International context:  
A world initiative aims to bring together a large community of researchers involved in the development of transport models to progress on the comprehension of nuclear collisions. The candidate will participate to this initiative bringing results from DYWAN model into the discussion. This is a unique opportunity to add the e-TDHF approach in the game respect to the Boltzmann-Langevin and the Molecular Dynamics approaches already highly represented.