Observation of top-quark pair production in heavy-ion collisions in the ATLAS experiment at the LHC
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Abstract
Hard probes are expected to provide crucial input for nuclear parton distribution functions (nPDF), as well as to bring valuable insights into the quark-gluon plasma (QGP). In this thesis, measurements of heavy-ion collisions using top quarks in the ATLAS experiment at the Large Hadron Collider are presented. Analysed data from proton-lead (p+Pb) and lead-lead (Pb+Pb) collisions were collected with the ATLAS detector during Run 2 (2015-2018) at a nucleon-nucleon centre-of-mass energy of √sₙₙ = 8.16 TeV and √sₙₙ = 5.02 TeV, respectively.
Electrons play an important role in the top-quark pair (tt̄) decay modes, specifically in the ℓ+jets and dilepton channels, which involve electrons in the final state. Electron performance is evaluated in p+Pb and Pb+Pb collisions, using electrons from the Z→e⁺e⁻ resonance decay. Electron scale-factor corrections are derived in p+Pb collisions and applied in the measurement of tt̄ production. Moreover, electron identification is optimised for Pb+Pb collisions and currently serves as the baseline approach in Run 3 (2022-2026).
The tt̄ process is studied in the ℓ+jets and dilepton channels in p+Pb collisions. The inclusive tt̄ cross-section is measured with the total relative uncertainty of 9%, leading to the most precise tt̄ cross-section measurement in heavy-ion collisions achieved so far. The signal significance exceeds five standard deviations separately in the ℓ+jets and dilepton modes, resulting in the first observation of tt̄ production in the dilepton channel in p+Pb collisions. The nuclear modification factor for the tt̄ process is also extracted for the first time. The results are in agreement with theoretical predictions for various state-of-the-art nPDF sets.
The production of tt̄ pairs is also analysed in the dilepton decay mode in Pb+Pb collisions. The inclusive tt̄ cross-section is extracted with the total relative uncertainty of 31%, providing the most precise tt̄ cross-section measurement in Pb+Pb collisions to date. The observed signal significance amounts to 5.0 standard deviations, establishing the first observation of the tt̄ process in Pb+Pb collisions. The obtained result is consistent with the measurement by the CMS Collaboration and theoretical predictions based on the latest nPDF sets.
The conducted studies open a new path for further research on heavy-ion collisions at ultra-relativistic energies. The precise measurement of tt̄ production in p+Pb collisions provides valuable input for constraining nPDFs in the high Bjorken-x region. The observation of the tt̄ process in Pb+Pb collisions marks the start of the heavy-ion program with top quarks, and in particular, opens a possibility of exploring the time structure of the QGP in the future.