Heavy Flavour Jets Production in Heavy Ion Collisions at the High Luminosity LHC with the ATLAS Detector

DOMAIN: Particle and Astroparticle Physics and associated scientific domains

SUPERVISOR: Helena Santos

CO-SUPERVISOR: Patricia Conde Muino

HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas

DEGREE INSTITUTION: Universidade de Lisboa


Context: Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) provide an unique opportunity to recreate the Quark Gluon Plasma (QGP) in the laboratory energy frontier. This plasma of quarks and gluons, which is known to behave as a nearly perfect liquid, was the prevailing state of the Universe shortly after the Big Bang. The capabilities of ATLAS, namely large acceptance and high granularity calorimeters, afford excellent handles for QGP studies. The ATLAS experiment is strongly committed with the HI program of LHC and great expectations on the capabilities of the Upgrade to bring deeply understanding on the nature of the QGP are raised. A major goal of the Heavy Ion Program of the LHC is the understanding of the effects of the QGP on jets, namely the study of the nature of the energy loss suffered by the quarks and gluons while crossing the QGP. The bottom quark, in particular, constitutes an excellent probe. Due to its large virtuality, Q, it has a formation time,∆t ≈1/Q ≈ 0.1 fm/c, much smaller than the formation time of the QGP at the LHC (≈10 fm/c). The understanding of the nature of the energy loss (either collisional or gluon radiative), by its turn, is crucial to infer the properties of the QGP. The HI ATLAS/LIP group is contributing with important developments preceding the b-jet physics analysis, namely on b-jet reconstruction, b- tagging and b-jet triggers in HI collisions. 

Objectives: The goal of the proposed thesis is the prospective study of the Heavy Flavour jets production in the HL-LHC phase (expected to start in 2027) benefiting from the 1 order of magnitude increased luminosity foreseen for the Pb+Pb runs. The most important ATLAS upgraded components for the proposed project are the calorimeters front-end electronics and the new tracker detector. Currently jets are reconstructed using the transverse energy of calorimeter towers (piled cells) as input signals, after subtracting the QCD underlying event. The new readout electronics of the calorimeters will provide support for a more sophisticated detector signal processing. The remaining part of jet reconstruction regards the identification of the b-jets, i.e. b-tagging. This technique aims the highest possible efficiency for tagging b-jets, with the largest possible rejection of light jets. In ATLAS the most used techniques take advantage of the relatively long lifetime of hadrons containing bottom quarks (ctau 450 mm), as well as of the hard fragmentation and the high mass of these hadrons. These properties lead to tracks in the ITk with large impact parameters (i.e., transverse and longitudinal distances of closest approach of the track to the primary and secondary vertices), on contrary to the tracks from light jets. Such a feature allows to disentangle heavy flavour jets from light jets, but it requires excellent impact parameter resolution. This is ensured by the ITk. Machine learning techniques using the properties of both the impact parameters and the secondary vertices have proven to increase significantly the b-tagging performance in pp collisions and the development in Pb+Pb is ongoing. Analysis of data taken in Run 2 and Run 3 of LHC will provide not only results on Heavy Flavour jets in HI collisions per se, but will also contribute preciously to the validation of the prospective study in the HL-LHC. 

Requirements: This is an experimental PhD program. The work will be developed at LIP – Laboratorio de Instrumentacao e Fisica Experimental de Particulas. The student should have solid computing skills, namely in C++ programming. Furthermore, she/he will concurrently participate in the technical activities in which the ATLAS/LIP group is involved, namely in the Tile calorimeter and/or in Trigger systems.

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