Laboratory link : http://irfu.cea.fr/dphn/Phocea/Vie_des_labos/Ast/ast_groupe.php?id_groupe=500
More : https://alice-collaboration.web.cern.ch
A few micro-seconds after the Big Bang, the Universe was in a quark gluon plasma (QGP) state. Such state is predicted by Quantum Chromodynamics, which is the theory of strong interactions, and should be reached at very high temperature or energy density. Such conditions are reproduced in ultra-relativistic heavy ion collisions at the LHC at CERN.
Among the various QGP observables, the study of hadrons with heavy-flavour quarks (charm c or beauty b) and quarkonia (c-cbar or b-bbar bound states) is particularly important to understand the properties of the QGP.
Quarkonia are rare and heavy particles which are produced in the initial stages of the collision even before the QGP is formed, mainly through gluon-fusion processes, and are therefore ideal probes of the QGP. As they traverse the QGP, the quark/anti-quarks pair will get screened by the many free quarks and gluons of the QGP. Quarkonia will then be suppressed by a colour screening mechanism in the QGP. Since the various quarkonium states have different binding energies, each state will have a different probability of being dissociated. This results in a sequential suppression pattern of the quarkonium states. Additionally, if the initial number of produced quark/anti-quark pairs is large and if heavy quarks do thermalise in the QGP, then new quarkonia could be produced in the QGP by recombination of heavy quarks. This mechanism is known as regeneration. At the LHC, Upsilon (b-bbar) and J/psi (c-cbar) are complementary. The former are thought to be more suited than to address the sequential suppression, while the latter should allow to study possible regeneration mechanisms. In addition, non-prompt J/psi, i.e. from weak decays hadrons containing one valance b quark, give access to the transport properties of b quarks in the QGP. More recently, photoproduction of J/psi has been observed in peripheral Pb-Pb collisions; J/psi are produced from the photon flux of the moving Pb ions mostly at very low transverse momenta. The characterization of these photoproduced quarkonia would allow to better constrain the initial state of the collisions as well as the properties of the QGP.
We propose to study the production of prompt and non-prompt quarkonia Pb-Pb collisions at a center-of-mass energy per nucleon pair (sqrt(sNN)) of 5 TeV at the LHC with the first data of Run 3 (2022-2024). An upgrade of the ALICE apparatus is ongoing with, in particular, the addition of silicon pixel tracker that will complement the ALICE forward spectrometer as well as new readout electronics for the latter. These upgrades will allow us to: Profit from the planned increase in luminosity of the LHC, thus tripling in one year the data collected in the full LHC Run 2 (2015-2018); Separate the prompt and non-prompt contributions thanks to the precise measurement of the quarkonium decay vertex into two muons.
The student will first develop the procedures to separate prompt and non-prompt quarkonia. In doing so, the student will thus contribute to the development of the new software for data reconstructions, simulation, calibration and analysis that the ALICE Collaboration is developing for Runs 3 and 4 of the LHC. Secondly, the student will study the production of prompt and non-prompt quarkonia in terms of production yields and azimuthal anisotropy. These studies could be performed as a function of the centrality of the collision and transverse momentum and rapidity of the quarkonia, for various types of quarkonia. Depending on the progress of the thesis work, these studies, which are a priority for quarkonia produced by the hadronic collision, could be extended to photoproduced quarkonia.