Top-Quark Pair Production in Heavy-Ion Collisions in the ATLAS Experiment

TL;DR

Problem: constrain nuclear parton distribution functions and study the time evolution of the quark–gluon plasma using top-quark pair production in heavy-ion collisions. Approach: ATLAS analyzes $t\bar{t}$ in p+Pb and Pb+Pb collisions with leptonic channels, performing profile-likelihood fits across multiple signal regions and comparing results to four nPDF sets. Key findings: precise $σ_{t\bar{t}}^{p+\mathrm{Pb}}$ and $R_{p\mathrm{A}}$ measurements in p+Pb; first observation of $t\bar{t}$ in Pb+Pb with $σ_{t\bar{t}}^{Pb+Pb}$; results are consistent with CMS and predictions from TUJU21, nNNPDF3.0, nCTEQ15HQ, EPPS21. Significance: demonstrates that top-quark production is a powerful probe of nuclear matter and enables future nPDF constraints at high Bjorken-$x$ and investigations of QGP time evolution via top-quark decay products.

Abstract

Top-quark pair production in heavy-ion collisions provides a unique opportunity to probe nuclear parton distribution functions and study the time evolution of strongly interacting matter, including the quark-gluon plasma. This work presents the observation and measurement of top-quark pair production in both proton--lead (p+Pb) and lead--lead (Pb+Pb) collisions using the ATLAS experiment at the Large Hadron Collider (LHC). In p+Pb collisions at a centre-of-mass energy of 8.16 TeV, top-quark pair production is observed in the lepton+jets and dilepton channels, with significances exceeding 5 standard deviations in each channel. The nuclear modification factor, $R_{p\mathrm{A}}$, is measured for the first time in this process, providing new insights into nuclear parton distribution functions. In Pb+Pb collisions at a centre-of-mass energy of 5.02 TeV, top-quark pair production is studied in the ($eμ$) final state, using datasets recorded in 2015 and 2018 with an integrated luminosity of 1.9 nb$^{-1}$. The measurement achieves a significance of 5.0 standard deviations and is compared to theoretical predictions based on various nuclear PDF sets. These measurements establish top-quark pairs as valuable tools for investigating heavy-ion collisions, offering novel insights into the dynamics of the quark-gluon plasma and nuclear matter.

Figures (4)

• Figure 1: Post-fit plots representing the $H_\mathrm{T}^{\ell,j}$ variable in each of the six signal regions ($e$+jets: (a) $1\ell1b$ and (d) $1\ell2b\mathrm{incl}$, $\mu$+jets: (b) $1\ell1b$ and (e) $1\ell2b\mathrm{incl}$, dilepton: (c) $2\ell1b$ and (f) $2\ell2b\mathrm{incl}$), with total uncertainties represented by the hatched area. The bottom panels show ratios of data and a sum of predictions bib:ttbarpPb.
• Figure 2: Comparison of the (a) cross section and (b) nuclear modification factor for $t\bar{t}$ production with theoretical models and other measurements. The dashed line at $R_{p\mathrm{A}}=1$ indicates the geometric expectation based on $pp$ collisions bib:ttbarpPb.
• Figure 3: Post-fit plots representing the $m_{e\mu}$ variable in (a) SR$_1$ and (b) SR$_2$, with total uncertainties represented by the hatched area. The bottom panels show ratios of data and a sum of predictions bib:ttbarPbPb.
• Figure 4: Comparison of the $t\bar{t}$ cross section with other measurements and theoretical models based on four nPDF sets bib:ttbarPbPb.