2 sujets /DPhN/LQGP

Dernière mise à jour :


 

Bc meson production in the Pb-Pb collisions at 5.36 TeV of the LHC Run 3

SL-DRF-24-0364

Research field : Particle physics
Location :

Service de Physique Nucléaire (DPhN)

Laboratoire plasma de quarks et gluons (LQGP) (LQGP)

Saclay

Contact :

Javier CASTILLO

Starting date : 01-10-2024

Contact :

Javier CASTILLO
CEA - DRF/IRFU/DPhN/LQGP

+33 169087255

Thesis supervisor :

Javier CASTILLO
CEA - DRF/IRFU/DPhN/LQGP

+33 169087255

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. 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.
Drell-Yan production measurement in proton-proton collisions and preequilibrium dilepton production in heavy-ion collisions with the LHCb experiment at the LHC

SL-DRF-24-0277

Research field : Particle physics
Location :

Service de Physique Nucléaire (DPhN)

Laboratoire plasma de quarks et gluons (LQGP) (LQGP)

Saclay

Contact :

Michael Winn

Starting date : 01-10-2024

Contact :

Michael Winn
CEA - DRF/IRFU/DPhN/ALICE

+33 1 69 08 55 86

Thesis supervisor :

Michael Winn
CEA - DRF/IRFU/DPhN/ALICE

+33 1 69 08 55 86

At the Large Hadron Collider (LHC) at Geneva, collisions of lead nuclei are used to create a thermodynamic system described by fluid dynamics under extreme conditions. The temperature of the short-lived system is sufficiently large in order to release the building blocks of matter at a subnucleonic scale, quarks and gluons. This state of matter is commonly called Quark Gluon Plasma (QGP). The space-time evolution of heavy-ion collisions at the LHC is described by close-to-ideal hydrodynamics after a short lapse of time. However, key features of the early stages of these collisions are largely unknown. These characteristics are crucial to understand the applicability limits of hydrodynamics and to understand thermalisation of a strongly interacting system.
In recent publications, it was pointed out that the dilepton production in the intermediate mass scale between 1.5 and 5 GeV/c² is highly sensitive to the ´thermalisation´ time scale towards the equilibrium QGP.

In addition, the LHC provides highly energetic proton and heavy-ion beams. They allow us to access the hadronic structure of the projectiles at very small fractional longitudinal momenta and at the same time still relatively large four momentum transfers. This configuration enables hence for perturbative calculations allowing the extraction of hadron structure information at very small fractional longitudinal momenta.
The theoretically best understood process in hadronic collisions is the production of dilepton pairs, the so-called Drell-Yan process. However, so far, no measurement down to 3 GeV/c² at a hadron collider has been published despite its theoretical motivation to test the lowest fractional momenta. In fact, at masses below around 30 GeV/c², semileptonic decays from heavy-flavour hadron decays start to dominate the dilepton production. This process has obscured any attempt to extract dilepton production in this kinematic domain.

The first goal of the thesis is the first measurement of Drell-Yan dimuons at low invariant masses at the LHC in proton-proton collisions that will be taken in 2024. This measurement will be based on novel background rejection techniques exploiting the forward geometry of LHCb. In a second part, the feasibility of the measurement in heavy-ion collisions will be investigated in the present and the future LHCb set-up. Depending on the outcome of the studies, a measurement in heavy-ion collisions will be conducted.

• Particle physics

 

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