Research overview

Key words: theoretical physics, heavy quarks, quark-gluon plasma, open quantum systems

Introduction

-> Our universe, 13.8 billions years ago.
Up to the first micro-seconds after the Big Bang (the « birth » of our universe), space was filled with a strange form of matter : the Quark-Gluon Plasma (QGP). The latter was an extremely hot (more than one trillion degrees !) and dense soup of elementary particles, which gave later the matter as we know it on cooling.

History of universe and QGP 2The QGP stage in the universe history.

Since the 80s, the physics community has been able to reproduce this primordial form of matter in high energy particle colliders (e.g. in the CERN Large Hadron Collider at the French-Swiss border).

-> What is a « quark » in quark-gluon plasma ?
A quark is an elementary particle, like the electron. There are actually six quarks and antiquarks of different masses and they are, in particular, the fundamental bricks of the protons and neutrons, the components of atomic nuclei.

AtomToQuarkFrom atoms to quarks and gluons.

A proton (or a neutron) is a particular system of three light quarks confined together thanks to the strong interaction occurring between them. This strong interaction is one of the four fundamental forces of Nature and is mediated via the exchange of gluons (which are other elementary particles). The quarks and antiquarks can form other systems than the protons and neutrons, generally called hadrons. Systems implying heavy quarks are for instance the quarkonia (bound states of a heavy quark and a heavy antiquark) and the open heavy mesons (bound states of a heavy quark and a light antiquark, or vice versa). These systems can also be produced in high energy particle colliders alongside the QGP.

-> How do we produce a Quark-Gluon Plasma in high energy particle colliders ?
The extrem conditions of temperature and density required to form a QGP can be reached by colliding heavy ions (e.g. the lead Pb) at very high speeds in particle colliders.

IonsQGP3 Formation of a QGP by colliding heavy ions at very high energies, followed by its cooling.

In these experiments, the QGP life-time (~10-21 s) and size (~ 10-14 m) are really small: proving its existence and describing its properties are therefore quite a challenge !

Project 1: quarkonia, QGP & open quantum systems

One of the possible probes of the QGP properties (temperature…) is the suppression of the quarkonia. The suppression of the quarkonia is a characteristic decrease of the detected amount of quarkonia in heavy ion collisions in comparison to proton-proton collisions, in which no QGP production is possible. This suppression has indeed been observed experimentally, but is still poorly understood. Treating the heavy quark/antiquark pairs as open quantum systems, we study their fate inside a hydrodynamic QGP, by considering their binding potential and energy exchanges with the QGP. Two main formalisms have been developed: the stochastic shrodinger equations and the quantum master equations. We showed the limits of the historical models and the importance of considering both the thermal effects and the quantum dynamics.

Proj1_fig2_siteThe quarkonia « Q » propagation in a QGP can be used as a probe of its properties.

Project 2: open heavy flavors, QGP & event-by-event engineering

Some other interesting probes of the QGP properties are the suppression and flow of the open heavy mesons. The flow describes the collectivity of their motion in the plane transverse to the collision axis. The simultaneous description of these obervables in a realistic situation has revealed to be a long standing puzzle. The project consists in building a state-of-the-art numerical simulation that includes and tests the many possible ingredients participating to the dynamics. We also carried out the first QGP size scan with open heavy mesons and showed the interest of such an analysis.

Proj2_fig2_site_2Example of models vs. experimental data comparison for the suppression « RAA » of an open heavy meson as a function of its transverse momentum « pT ».

 

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