Unveiling the space-time structure of jets
LIP - Laboratório de Instrumentação e Física Experimental de Partículas
Heavy-ion collisions are a unique laboratory to recreate the high energy and density conditions prevalent during the primordial moments of our Universe: the Quark-Gluon Plasma (QGP) . This new state of matter is made of the elementary building blocks (quarks and gluons) of Quantum Chromodynamics (QCD), a key ingredient of the Particle Physics Standard Model. Unveiling the properties of the QGP allows us to ascertain the QCD fundamental properties and understand the Cosmological evolution of the Universe.Over two decades of experimental data on heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) [2,3] and the Large Hadron Collider (LHC) [4-6] overwhelmingly support the picture of the QGP as a short-lived deconfined state of matter. The collective behaviour of this thermalised QCD medium is well described by almost ideal hydrodynamics, thus establishing the QGP as the most perfect fluid in nature . Together with the collective phenomena, the modifications experienced by high energy particles due to their interactions with the medium (usually known as jet quenching ) have become one of the most robust tools to assess QGP properties . Surprisingly, the recent discovery of similar collective, fluid-like phenomena in collisions of small systems  (proton-proton or proton-lead) hint at the formation of a droplet of QGP. However, jet quenching observations have been, so far, limited or inexistent [11,12], thus questioning the formation of a deconfined QCD matter phase in these type of collisions. For this reason, future jet quenching studies are crucial to understand small systems and the largely unknown mechanism which leads to the formation of a thermalised QGP.Typically, in hadronic and heavy-ion collisions, the high-energy quarks and gluons produced in the hard process concurrent with the collision result into parton shower fragmentation, usually referred to as jets . The fact that jets in proton-proton collisions are well understood has allowed the heavy-ion community to successfully use jets to extract the QGP's time-averaged properties [A4, A5]. However, jets are evolving multi-scale objects that witness the full space-time evolution of the system, so they may also be seen as a versatile and unique probe to analyse the strong time dependence of the QGP properties.This project aims at developing, for the first time, time-differential methods based on first-principle jet quenching calculations that will provide us with a full time-scan of the QGP at current colliders energies. Our approach lays the foundations for a complete novel strategy in heavy-ion collisions: using jet quenching to directly access the different time scales of the system. With the new lighter ions runs scheduled at the LHC, this project's execution is exceptionally timely.The methodology will be based on our team's preliminary results that have triggered QGP tomography with jets. We proposed top-initiated jets as time-delayed probes of the QGP [A1] and an analysis showing the potential of jet observables to constrain the initial stages of the system [A2]. We now aim to provide a complete set of experimental observables that will enable a complete time-scan of the system, anchored by a consistent analytical treatment of in-medium jets.The first step is the analytical development of state-of-the-art jet quenching theory. Current in-medium parton characterisation is formulated within restricted kinematic limits and assuming a static medium [A3, 23-25]. We plan to generalise existing formalisms to include a time-evolving medium while extending such descriptions to account for a full consistent definition of an in-medium modified jet. The next challenge will consist of moving from the existing QCD parton shower paradigm, done in the energy-momentum frame, to the first space-time description of QCD dynamics. Altogether, this will set the robust theoretical baseline for the subsequent phenomenological efforts of this project.The second goal will be to develop the phenomenological framework that will allow us to build a space-time picture of a jet to be linked to the QCD parton shower depicted in the first task. Taking advantage of recent results by the team members , we will explore the identification of "early-initiated jets" and "delayed jets" to correlate with the QGP evolution. The newly obtained observables will aim at the current LHC and RHIC collisions and thus require a feasibility study in a heavy-ion environment. In addition, we will also resort to innovative Machine-Learning-based techniques to explore jet substructure observables sensitive to different parton shower emission patterns.Finally, the obtained space-time picture can be used to build a visualiser of a QCD parton shower and resulting jet. We will fully exploit this visualisation tool for outreach and educational purposes in parallel with the above two tasks.
Reforçar a investigação, o desenvolvimento tecnológico e a inovação
Region of Intervention
Versão Extensa: Este trabalho é financiado por fundos nacionais através da FCT - Fundação para a Ciência e a Tecnologia, I.P., no âmbito do projeto EXPL/FIS-PAR/0905/2021
Versão Resumida: OE,FCT-Portugal, EXPL/FIS-PAR/0905/2021