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. Indeed, heavy quarks are produced by hard scatterings between partons of the incoming nuclei in the early stage of the collision, and thus experience the full dynamics of the collision.
Thanks to the measurements of J/psi (c-cbar) production in Pb-Pb collisions of Runs 1 and 2 of the LHC, the ALICE collaboration showed the existence of the regeneration mechanism: when 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. Other mechanisms, such as colour screening, could affect the production of quarkonia. Bc mesons are composed of a b quark and an antiquark c. Their production is therefore strongly disfavored in proton-proton collisions. In Pb-Pb collisions, instead, their production could be largely increased due to the regeneration mechanism.
We propose to study the production of Bc mesons in Pb-Pb collisions at a center-of-mass energy per nucleon pair (sqrt(sNN)) of 5.36 TeV at the LHC with the data of Run 3 (2022-2025). The ALICE apparatus was upgraded in view of LHC Runs 3 and 4 with, in particular, the addition of a silicon pixel tracker (MFT) that complements 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); Measure with high precision secondary vertices of b-hadron decays. The Bc mesons will be measured at forward rapidity by reconstructing three secondary muons with the muon spectrometer and the MFT of ALICE.
The student will first contribute to the optimization and characterization of the muon spectrometer and MFT matching algorithm and the secondary vertex reconstruction. Secondly, the student will study the production of Bc mesons in Pb-Pb collisions. Finally, the results will be compared with other experimental results as well as various theory calculations.
During this work the student will become familiar with the grid computing tools and the simulation, reconstruction and data analysis software of the ALICE Collaboration.