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Reference : UMR5821-CHRVEL-094
Workplace : GRENOBLE
Date of publication : Monday, June 7, 2021
Scientific Responsible name : Julien FAIVRE / Gustavo CONESA BALBASTRE
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
Study of the Quark-Gluon Plasma with the ALICE experiment at LHC :
measurement of correlations between a hard probe and recoiling hadrons.
The so-called “heavy-ion” physics is a field which aims at exploring QCD (Quantum Chromodynamics, the theory of the strong interaction) and the strong interaction by studying the properties of the quark-gluon plasma (QGP) formed in ultra-relativistic nucleus-nucleus collisions. This state of matter is the strong interaction analog of the electromagnetic plasmas: the high energy density deposited in a finite volume results in a deconfinement of the color charges (the partons), which no longer form bound hadrons.
Such experiments, conducted on high energy accelerators like RHIC (BNL) and LHC (CERN), allow to test QCD in a non-perturbative regime, for a finite size deconfined medium: a situation which is unique. The discovery of the QGP, in the early 2000's, has been followed by a period during which its concept has been validated and new obervables developed. Much progress has been made since then, both on the theoretical and experimental sides. In the soft sector in particular, QCD-based calculations of the QGP equation of state or of its shear viscosity over entropy density ratio can be confronted with the measurements.
Other properties of the QGP are not as well determined yet. In particular, both perturbative and lattice QCD calculations of a key parameter which quantifies the interaction between an energetic parton and the QGP which it crosses, exist but are preliminary. Measurements constraining this parameter would thus stimulate calculations in this domain. Experimentally, this is done by analyzing how the properties (energy and direction) of the parton and of its hadron shower change with respect to in-vacuum production (p-p collisions), or as a function of the path length crossed in the medium. Examples of relevant observables are the modifications of the jet energy or substructure, of the fragmentation function, of the particles' momentum and angle with respect to the hard process direction, etc...
As the extracted information is more precise when the parton's properties before it crosses the QGP are known without bias, studying photon-jet events, where a parton recoils from a photon (color-neutral, thus not affected by the QGP), is of utmost importance. The price to pay, though, is a more than ten times lower cross-section of photon-jet with respect to the most common di-jet processes.
Such measurements of jets or hadrons correlated with a so-called direct photon have been made at RHIC (lower center-of-mass energies) around 10 GeV, and at LHC above ≃ 50 GeV. Yet, to constrain the models, it is also necessary to measure the properties of this parton energy loss and redistribution in the medium over a broad parton (thus photon) energy range. It is therefore important to use the ALICE detector, designed to focus on low energies, to push down the photon lower energy limit, where QGP effects on the parton are more pronounced.
Another reason to approach the 10 GeV domain is the preparation for the 2022-2029 LHC runs, where correlation measurements with heavy quarks will become possible in this range, thanks to the unique PID capabilities of ALICE. QCD calculations indeed indicate that gluon radiations at small angles are reduced for heavier quarks (“dead cone effect”), resulting in a mass dependence of the energy lost by the parton, an effect predicted to be best visible at the lowest process energies.
The ALICE group at LPSC-Grenoble has therefore investigated the feasibility of using photon-hadron correlations to obtain informations on the amount of energy lost by the parton recoiling off the photon and how it is redistributed to the medium. While the photon provides an energy and direction reference for the hard process, the information about the parton interaction with the QGP is given by the hadrons located on the opposite side from the photon in azimuth.
The identification of these direct photons is made difficult by the overwhelming decay and fragmentation photon background. A key technique to enhance the purity of the sample of candidates is the isolation: since direct photons directly come from the hard process, they should come alone in their angular region, apart from the unavoidable background coming from the underlying event (UE). The measurement of the amount of energy in a cone around the candidate particle allows to select those which appear isolated.
This identification technique has been shown to work down to 10 GeV in ALICE in p-p collisions, where the UE can be ignored, as well as in p-Pb, where the twice larger average UE must be evaluated and subtracted from the energy in the cone. In central (small impact parameter) Pb-Pb collisions, the UE can be more than an order of magnitude larger. The UE in Pb-Pb also has very different characteristics than in p-Pb collisions : it results from the hadronization of the “fireball”, and keeps the footprint both of the thermodynamical evolution and of the initial state of the collision. Well known techniques allow to measure its characteristics on an event-by-event basis, but its magnitude remains challenging.
The PhD which we propose aims at developing this analysis, starting from already existing preliminary studies, and testing it on the large 2015 and 2018 Pb-Pb datasets collected by the ALICE experiment. This important milestone will determine whether other studies are doable with the 2022-2029 LHC runs statistics, e.g. more differential measurements to better constrain the models, or the heavy quarks energy loss measurement to test the prediction of a quark mass dependent energy loss.
After an introductive period aiming at familiarizing with the ALICE software environment and preliminary studies, the PhD student will work on determining the parameters values and choices for the analysis of isolated photon- and pi0-hadron correlations, and will then be able to study the systematic uncertainties. 6 months of “service work to the collaboration” are due before the third year of the PhD.
Should unexpected difficulties show up, the student will have the possibility to switch to the study of the correlation measurements in the large p-p dataset, focussing on the collisions with highest number of particles (multiplicity) in the final state, in which collective effects have been discovered and of which the modified characteristics of the underlying event challenge the current event generators for Monte-Carlo.
The student who will join us will become a member of the ALICE collaboration, which has 1500 members belonging to 150 institutes from almost 40 countries. As the group has a planned budget for travels, this should allow the student to move regularly at CERN or to other relevant places, for collaborative work or conferences.
The Grenoble Laboratory of Subatomic Physics and Cosmology (LPSC) (http://lpsc.in2p3.fr) is a joint research unit involving CNRS-IN2P3, the University of Grenoble Alpes (UGA) and the Grenoble INP school, with an average staff of about 230 people.
The PhD student will be assigned to the ALICE group of the LPSC, composed of 4 agents, and will be placed under the direct hierarchical authority of the group leader.
His/her thesis supervisors will be Julien FAIVRE and Gustavo CONESA BALBASTRE.
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