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Reference : UMR9012-MARLOU-006
Workplace : ORSAY
Date of publication : Tuesday, January 26, 2021
Scientific Responsible name : MASSACRIER Laure
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
The aim of experiments with ultra-relativistic heavy-ions is the study of nuclear matter at high temperature and pressure, where Quantum Chromodynamics predicts the existence of a deconfined state of hadronic matter, the Quark Gluon Plasma (QGP). The ALICE experiment at LHC is devoted to the study of this QGP. In hadron-hadron collisions, heavy quarks are produced at the early stages of the collision, and then can interact with the formed medium, making them ideal probes of the QGP. The sequential suppression of charmonia (c and cbar quark bound states) in medium has indeed been proposed as a signature for the formation of a QGP. On the other hand, at LHC energies, another effect, predicting secondary charmonia production from uncorrelated charm-anticharm quarks in the deconfined medium, could also be at play. Recently, the ALICE collaboration reported the measurement of an excess in the yield of J/psi (the first excited state of the charmonia family) in Pb-Pb collisions , which cannot be explained by usual hadroproduction mechanisms nor by hot QGP medium effects. A plausible explanation is that this excess is caused by coherent photoproduction of charmonium. In this process, quasi-real photons produced by the strong electromagnetic field of one of the lead nucleus interact coherently with the other nucleus. Photoproduction of charmonium is a well-known process in Ultra-Peripheral Collisions (UPC) [2,3]. In UPC, nuclei are separated by impact parameters larger than the sum of their radii and therefore hadronic interactions are suppressed. Photoproduction of vector mesons, in such collisions, gives insight into the gluon distribution of the incoming Pb nuclei and therefore put stringent contrains on hadron structure, down to low Bjorken-x values (the fraction of the longitunal momentum of the hadron carried by the parton). The first indication of J/psi coherent photoproduction in Peripheral collisions with nuclear overlap, sheds new light on the study of gluon densities and collision mechanisms at play. In addition, studies of coherent J/psi photoproduction as a function of the collision geometry might become a new tool to probe the QGP [1,4]. The ALICE detector at the LHC offers unique capabilities to study quarkonium (a bound heavy quark-antiquark pair) production at low transverse momentum over a wide rapidity range, from mid- to forward rapidity in pp, pPb and PbPb collisions.
The thesis will focus on the analysis of ALICE data on quarkonium coherent photoproduction in Pb-Pb collisions with nuclear overlap. Such measurements impose great challenges for the existing coherent photoproduction theoretical models, especially for explaining how the broken nuclei satisfy the requirement of coherence. The ALICE upgrade combined with larger delivered luminosities will allow to perform extensive, new and precise quarkonium studies both at mid and forward rapidity. The Pb-Pb data foreseen in Run3 (2022-2024, Lint ~ 2 nb-1 expected), together with the already collected data in Run2 (2015-2018, Lint ~ 700 mub-1), will permit the measurement for the first time of the J/psi excess polarization aiming at confirming the photoproduction origin of the excess. The measurement of the J/psi excess as a function of centrality over a broad rapidity range can probe potential QGP-like effect on the photoproduced J/psi yield. An observation of a broadening of the J/psi excess transverse momentum distribution with increasing centrality would be an indication that the spectator nucleons (ie. the one not participating to the hadronic interaction) are responsible for the coherence condition . Such study is particularly relevant at mid-rapidity, thanks to the good transverse momentum resolution of the Time Projection Chamber. The Run3 data will also allow one to extend the earlier analyses to other quarkonia for the first time (Psi(2S), Upsilon(1S)).
A laptop from the laboratory will be provided to the PhD student to access all the resources from the ALICE Collaboration. He/She will benefit from the resources from the group to go to CERN for missions or to participate to international conferences.
The work of the PhD student will consist in one or several publications from the ALICE Collaboration to be published in peer reviewed journals. He/She will sign all the analysis notes on his work, which are internal to the ALICE Collaboration. The PhD student will present his/her studies during collaboration meeting, workshop and international conference. He/She will frequently travel to CERN (Switzerland). An excellent level of English and programmation skills are required. He/She will work inside the ALICE IJCLab group, whose expertise in J/psi production analyses is recognized. Our group played a leader role in several publications (http://cern.ch/go/Db7D). A participation to the data taking for the experiment at CERN, and the participation to a service task for the collaboration will be required. He/She will participate as well to outreach activities performed in the ALICE IJCLab group (CERN masterclasses).
The Irène Joliot-Curie Physics Laboratory of 2 Infinities (IJCLac) is a laboratory under the supervision of the CNRS, the University of Paris-Saclay and the University of Paris. The research axes of the laboratory are nuclear physics, high energy physics, astroparticles and cosmology, theoretical physics, accelerators and particle detection as well as research, development and applications for health, energy and environment.
The candidate will work in the ALICE group of IJCLab which is composed of four permanent researchers, one professor from Paris-Saclay University, two PhD students (among them one will defend his PhD in Oct. 2021), and one postdoc. L. Massacrier and C. Hadjidakis will co-supervise the PhD thesis. The candidate will work in an international collaboration and will present regularly (in English) his work in the group and in collaboration meetings. The participation to physics schools and the presentation of the results in international conferences is foreseen. The candidate will write a thesis manuscript about six months before the defense. The students in our group defend their PhD within three years maximum. The PHENIICS doctoral school will ensure a follow up of the thesis course.
Constraints and risks
Frequent travels (collaboration meetings, workshop, international conferences abroad) as well as on the CERN site (Switzerland) are expected. The student will participate in general service tasks for the ALICE Collaboration. The student will be involved also in the data taking at CERN for about 2 to 3 weeks per year (during those shifts the student can work during nights and week-ends).
The candidate must have obtained a master degree in particle/hadronic/nuclear physics at the starting date of the contract. In addition, the following skills are expected :
- Experience in programming (C++ language is a plus)
- Good understanding, speaking and writing skills in English
- Knowledge of French is a plus, although not mandatory
- Ability to work in team and inside a large collaboration of scientists
The applications must include a detailed CV ; at least two references (person which might be contacted) ; a cover letter of one page ; the marks obtained during the Master 1 and Master 2 or from the engineering school.
The deadline to send the application is the 31/03/2021.
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