A New Approach to Study Jet Production in Pb+Pb Collisions at the ATLAS-LHC

DOMAIN: Particle and Astroparticle Physics and associated scientific domains
SUPERVISOR: Helena Santos
HOST INSTITUTION: Laboratório de Instrumentação e Física Experimental de Partículas
DEGREE INSTITUTION: Universidade de Lisboa
ABSTRACT

Context: Ultra-relativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) provide a 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.

Jets, collimated sprays of particles produced in the LHC collisions, are crucial probes to infer the properties of the QGP if the mechanisms of the energy loss and substructure modification are fundamentally perceived. The main goal of this project is the development of new observables for quantifying jet modifications in the QGP, following the proposal in to measure average energy loss through the ratio of the transverse momentum (pT) in heavy-ion (HI) and proton-proton (pp) jets in the same quantile. This exploratory approach underlies any jet observable due to the relevance of selecting jets that were born alike. The standard R_AA (ratio between jets yields in HI and pp collisions at the same reconstructed pT bin), relying by definition on pT bin migration dueto energy loss, is highly mis-leading because the history of the jets cannot be traced back.

Objectives: The candidate will participate in Pb+Pb data acquisition at CERN during Run 3 (expected for the Fall of 2023) and will analyse the data. Beyond physics data analysis, the candidate will have the opportunity to acquire precious technical and computing know-how. Advanced machine learning techniques will be explored to separate signal jets from the huge background produced in HI collisions. This is also a significant challenge of the project, and the ATLAS LIP group has already contributed significantly to this effort.

Requirements: The work will be developed at LIP – Laboratório de Instrumentação e Física Experimental de Partículas and CERN – European Organisation for Nuclear Research. The candidate will concurrently participate in the technical activities in which the ATLAS/LIP group is involved, namely in the High Granularity Timing Detector, under development and targeting the HL-LHC physics programme. The candidate should have some knowledge on
Particle Physics and solid computing skills, namely in Python and C++ programming. Several abroad scientific stays are expected, including travelling to CERN 2 to 4 weeks in the year.