Séminaires

Maxime Guilbaud (CMS-CERN) How small a Quark-gluon plasma can be? — Toward a unified paradigm to describe high energy hadronic collisions

Europe/Paris
Amphi Charpak (Amphi Charpak)

Amphi Charpak

Amphi Charpak

Description
Few microseconds after the Big Bang, the Universe was in a quark-gluon plasma (QGP) state. In this extremely hot and strongly interacting state of matter, quarks and gluons are not bound to hadrons 
anymore. The QGP can also be created and studied using high-energy colliders and heavy-ion (A-A) collisions such as what is done at the Large Hadron Collider (LHC). The formation of a QGP in heavy ion collisions and the collective/hydrodynamic expansion of the created medium are well established and reasonably well understood. In particular, looking at anisotropy of the final-state distribution of particles produced in A-A collisions, it is now established that the QGP behaves like a nearly perfect fluid with a shear viscosity close to the KSS bound. This state of nuclear matter was not expected to be produced in reactions involving smaller colliding systems, such as the proton-nucleus (p-A) and proton-proton (p-p) collisions. Nevertheless, a wealth of experimental evidence in recent years has suggested the presence of collective phenomena and a possible QGP medium being formed also in high-multiplicity p-A and p-p collisions. A detailed investigation is needed to establish the cause of the observed collective behavior and to determine if, indeed, a QGP medium is being created or if another mechanism is responsible. Over the past year, the CMS experiment at the LHC has recorded a large amount of Pb-Pb, p-Pb and p-p collisions, opening new opportunities in the understanding of collective phenomena in high-multiplicity hadronic final state. Upon reviewing the experimental data from the CMS experiment and confronting them with theoretical models, a unified paradigm in describing the observed collectivity accross all hadronic collision system is emerging. Potential future paths toward addressing key open questions, especially on collectivity in small colliding systems (p-A and p-p) and new opportunities to study emergent Quantum Chromodynamics phenomena under extreme conditions, will be discussed.